LECTURE VIII. ARC LAMPS AND ARC LIGHTING. Volt- Ampere Characteristics of the Arc. 62. The voltage consumed by an arc, at constant current, increases with increase of arc length, and very closely propor- tional thereto. Plotting the arc voltage, e, as function of the 190 180 170 160 150 140 130 120 110 100 00 80 70 60 50 '40 30 20 10 I.fi6 0[5 25 1 0 FIG. 45. arc length, I, we get tor every value of current, i, a practically straight line, as shown for the magnetite arc in Fig. 45, for values of current of 1, 2, 4 and 8 amperes. These lines are steeper 137 138 RADIATION, LIGHT, AND ILLUMINATION. for smaller currents, that is, low-current arcs consume a higher voltage for the same length than high-current arcs, the in- crease being greater the longer the arc. These lines in Fig. 45 intersect in a point which lies at I = — 0.125 cm. = — 0.05 in. and e = 30 volts; that is, the voltage consumed by the arc consists of a part, e0 = 30 (for the magnetite arc), which is con- F;G. 46. stant, that is, independent of the arc length and of the cur- rent in the arc, but different for different materials, and a part, ev which is proportional to the arc length, Z, or rather to the arc length plus a small quantity, 1L= 0.125 (for the magne- tite arc): e^ = \(l + 0.125), and depends upon the current, being the larger the smaller the current. Plotting the arc voltage, e, as function of the current, i, we get curves which increase with decrease of current, the increase being greater the longer the arc, as shown in Fig. 46, for the ARC LAMPS AND ARC LIGHTING. 139 magnetite arc, for I = 0.3, 1.25, 2.5, 3.75 cm. = 0.125, 0.5, 1 and 1.5 in. Subtracting from the voltage, 6, in Fig. 46, the con- stant part, e0 = 30 volts, which apparently represents the terminal drop of voltage, that is, the voltage which supplies the energy used in producing the conducting vapor stream at the negative, and the heat at the positive terminal, leaves the voltage, el = e — eQJ as the voltage consumed in the arc stream. The curves of arc-stream voltage, ev as function of the cur- rent, ij in Fig. 46, can with approximation be expressed by k cubic hyperbolas: e^i = kz2; or, el =— £j and since we find for Vi constant value of current: el = k^ (I + 0.12), as function of arc length and current, i, the voltage of the arc stream is ex- pressed by : k (I + I) ei = TT1-' (1 and the total arc voltage by : , *(*+*,: (2) where e0, k and Zt are constants of the terminal material (k, how- ever, varies with the gas pressure in the space in which the arc exists). This equation (2) represents the arc characteristics with good approximation, except for long low-current arcs, which usually require a higher voltage than calculated, as might be expected from the unsteady nature of such long thin arcs. The equation (2) can be derived from theoretical reasoning as follows: Assuming the amount of arc vapor, that is, the volume of the conducting vapor stream, as proportional to the current, and the heat produced at the positive terminal also as proportional to the current, the power p0 required to produce the vapor stream and the heating of the positive terminal is proportional to the current, i\ and, as the power is p0 = eQi, it follows that the voltage, e0, consumed at the arc terminals is constant. The power consumed in the arc stream : pl = ej,, is given off, by heat conduction, convection, and by radiation, from the sur- 140 RADIATION, LIGHT, AND ILLUMINATION. face of the arc stream, and thus, as the temperature of the arc stream is constant, and is that of the boiling point of the arc vapor, the power pl consumed in the arc stream is proportional to its surface, that is, to the product of arc diameter ld and arc length I, or rather the arc length I increases by a small quantity lv which allows for the heat carried away to the electrodes. As the diameter ld is proportional to the square root of the section of the arc stream, and the section of the arc stream, or the volume of the arc vapor, was assumed as proportional to the current, i, the arc diameter is proportional to the square root of the current, and the power pt consumed in the arc stream thus is proportional to the square root of the current, i, and to (I + IJ ; thatis' p^kVid + lJ; and since pl = ej, which is equation (1), and herefrom, since e = e0 + eir follows equation (2). 63. Since e0 represents the power consumed in producing the vapor stream and the heating of the positive terminal, and k the power dissipated from the arc stream, e0 and k are different for different materials, and in general higher for materials of higher boiling point and thus higher arc temperatures. It is, approximately, e0 = 13 volts for mercury, = 16 volts for zinc and cadmium, = 30 volts for magnetite, = 36 volts for carbon, k = 31 for magnetite (123 in inch measure), = 35 for carbon (130 in inch measure). The magnetite arc, of which the characteristics are shown in Figs. 45 and 46, thus can be represented by: ' (3) The least agreement with the theoretical curve (2) is shown by the carbon arc. This may be expected from the exceptional character of the carbon arc, as discussed in Lecture VI. Plot- ARC LAMPS AND ARC LIGHTING. 141 ting, in Fig. 47, the voltage, e, consumed by a carbon arc, at constant values of current i, as function of the arc length Z, — as done for the magnetite arc in Fig. 45, — when using only the observation for arc length of 0.25 in. and over, we get fairly satisfactory straight lines, which intersect at the point, giving e0 = 36 volts, but I, = - 0.8 cm. = - 0.33 in.; that is, a value much greater than for any other arc. For short arc VOLTS 150- •100 2 LE -20- 25 3 "5 cn 1J5 IN. FIG. 47. lengths, however, the observed values of voltage drop below the straight line, as shown in Fig. 47, and converge towards a point, at zero arc length, or e0' = 28 volts. This looks as if, of the potential drop of e0 = 36 volts of the carbon arc, only a part, e0' = 28 volts, occurs at the surface of the terminals, and the remaining part, e" = 8 volts, occurs in the space within a short distance from the terminal surface. If then the arc length is decreased to less than the distance within which the terminal drop e" occurs, the arc meets only a part of this ter- minal drop e", and, for very short arc length, only the terminal drop e0' occurs. Possibly the voltage e0' = 28 is consumed at the negative terminal in producing the conducting vapor stream, 142 RADIATION, LIGHT, AND ILLUMINATION. while the voltage e" = 8 is consumed by the moving vapor stream in penetrating a layer of dead carbon vapor formed by heat evaporation from the positive terminal, and surrounding this terminal. Stability Curves of the Arc. 64. From the volt-ampere characteristic of the arc, as rep- resented by equation (2) and reproduced in Fig. 48 as Curve I, for a magnetite arc of 1.8 cm. (about 0.75 in.) length, it follows that the arc is unstable on constant potential supply, as the, voltage consumed by the arc decreases with increase of current and, inversely, a momentary increase of current decreases the consumed voltage, and, on constant voltage supply, thereby increases the current, still further decreases the arc voltage and increases the current, and the arc thus short circuits; or a momentary decrease of current increases the required voltage and, at constant supply voltage, continues to decrease the cur- rent and thus increase still further the required voltage, that is, the arc goes out. On constant voltage supply only such apparatus can operate under stable conditions in which an increase of current requires an increase, and a decrease of current a decrease of voltage, and thus checks itself. Inserting in series with the arc, curve I, in Fig. 48, a constant resistance of 10 ohms, the voltage consumed by this resistance, e = ir, is proportional to the current, and given by the straight line II. Adding this voltage to the arc voltage curve I, gives the total voltage consumed by the arc and its series resistance, as curve III. In curve III, the voltage decreases with increase of current, for values of current below i0 = 2.9 amperes, and the arc thus is unstable for these low currents, while for values of current larger than i0 = 2.9 amperes, the voltage increases with increase of current. The point i0 = 2.9 amperes thus separates the unstable lower part of the curve III from the stable upper part. With a series resistance of r = 10 ohms, a 1.8-cm. mag- netite arc thus requires at least e = 117 volts supply voltage, and i0 = 2.9 amperes for steady operation. With a larger series resistance, as r = 20 ohms, represented by curve II' and III', a larger supply voltage is required, but smaller currents can be operated ; with a lower series resistance, r = 5 ohms, curves ARC LAMPS AND ARC LIGHTING. 143 II" and III", larger currents are required for stable operation, but a lower supply voltage is sufficient. When attempting to operate an arc close to the stability limit, i0, where a small variation of voltage causes a large variation of current, the operation of the arc is unsatisfactory, that is, the FIG. current drifts; small variations of the resistance of the arc stream, and thereby of the voltage consumed by the arc, cause excessive fluctuations of the current. These pulsations of cur- rent can be essentially reduced by using a large inductance in series with the arc, and an arc can be operated very much closer to its stability limit if its series resistance is constructed highly inductive, that is, wound on an iron core. Obviously, 144 RADIATION, LIGHT, AND ILLUMINATION. no series inductance can extend stable operation beyond the stability point %. At the stability limit iQJ the resultant characteristic III in Fig. 48 is horizontal, that is, the slope of the resistance curve ef II, r = - i is equal but opposite to the slope of the arc char- de acteristic I, — ; that is, at the stability limit, ii+r=°-> ; <*> and, substituting equation (2) in (4), gives and the total voltage consumed by the arc of current i and length I and the series resistance r required to just reach sta- bility is E = e + ir, , fe (* + Z.) , * (Z + Z.) . — eo ~r - — ; r - — 7: — , \/i 2 Vt that is, /. VTV or, E^e+ti. (6) This curve is called the stability curve of the arc. It is shown as IV in Fig. 48. It is of the same form as the arc characteristic I, and derived therefrom by adding 50 per cent of the voltage consumed in the arc stream. Thus, in an arc requiring 80 volts, of which e0 = 30 volts are consumed at the terminals, ^ = 50 volts in the arc stream, for stable operation, a supply voltage of more than E = e + -^ = 80 + 25 = 105 volts is required. ARC LAMPS AND ARC LIGHTING. 145 The stability limit, on constant potential, thus lies at an ex- cess of the supply voltage over the arc voltage by 50 per cent of the voltage, ev consumed in the arc stream. In general, to get reasonable steadiness of the current, and absence of drifting, a supply voltage is used which exceeds the arc voltage by from 75 per cent to 100 per cent or more of the voltage, ev of the arc stream. 65. The preceding consideration applies only to those arcs in which the gas pressure in the space surrounding the arc, and thereby the arc vapor pressure and temperature, are constant and independent of the current, as is the case with arcs in air (even " enclosed" arcs, as the enclosure cannot be absolutely air- tight), as it is based on the assumption that the section of the vapor stream is proportional to the current. With arcs in which the vapor pressure and temperature vary with the current, as with vacuum arcs, as the mercury arc, the reasoning has to be correspondingly modified. Thus in the mercury arc in a glass tube, if the current is sufficiently large to fill the entire tube, and not so large that condensation of the mercury vapor cannot freely occur in the condensing chamber, the power pl dissi- pated by radiation, etc., may be assumed as proportional to the length Z of the tube, and to the current i: pi = 6li = kli, (7) thus gives e1 = klf or independent of the current; and e = e0 + ev = e0 + A*; (8) that is, the voltage consumed by a mercury arc, within a cer- tain range of current, is constant and independent of the cur- rent, and consists of a constant part, the terminal drop e0, and a part which is proportional to the length and to the diameter of the tube. Approximately it is for the mercury arc in a vacuum: 1.4 e0 = 13 volts ; k = -y- Id hence, , Calculating approximately the increase of vapor pressure and thereby of arc-stream resistance at high currents, and the 146 RADIATION, LIGHT, AND ILLUMINATION. increase of resistance at low current, due to the arc stream not completely rilling the vapor tube, gives for the vacuum arc the approximate equation: I e = e« + df°> 7 7 • d ald -01 r- ^ where ld = diameter of arc tube, cm., I = length of arc, cm., i = current. For the mercury arc, it is : e0 = 13 volts, a = 1.68, 6 = 0.29 for mercury anode, = 0.167 for graphite or metal anode, c = 0.52. Arc Length and Efficiency. 66. The arc most frequently employed for illumination is the plain carbon arc. In this the arc flame of the vapor stream gives no useful light, but the light is given by the black-body radiation of the incandescent carbon terminal, mainly the positive terminal, which is hottest, and is given at high efficiency due to the very high temperature of the radiator. The light of the carbon arc thus is incandescent light, and not lumines- cence. In the alternating carbon arc, alternately, the two ter- minals are positive and negative, and, as relatively little heat is produced at the negative terminal, the average temperature of the carbon terminals of an alternating arc is lower, and the efficiency of light production therefore less. Thus, while direct- current carbon arcs reach efficiencies corresponding to specific consumptions of from 1 to 1.5 watts per mean spherical candle power, alternating carbon arcs show only from 2.5 to 3 watts per candle power, or even still higher specific consumption. Thus, the only excuse for the use of the alternating carbon arc is the much greater simplicity and convenience of the electric generating apparatus, the stationary transformer, compared to the arc machine with the direct-current arc, and with the de- velopment of the constant-current mercury-arc rectifier; this ARC LAMPS AND ARC LIGHTING. 147 difference in the simplicity of generation of the arc current has largely disappeared. In the direct-current carbon arc, the light comes mainly from the positive terminal; in the alternating carbon arc equally from both terminals, and the distribution curve of the light thus is different. Since in the carbon arc no useful light comes from the arc flame, the voltage and therefore the power consumed in the arc flame is wasted, and in general, therefore, the efficiency of light production of the carbon arc is the higher the shorter the arc. Thus comparing in Fig. 49 a 1-in. carbon arc A with a FIG. 49. 0.5-in. carbon arc B, the former requires, at 5 amperes, 112 volts and 560 watts, the latter only 84 volts and 420 watts, but radiates the same amount of light from the incandescent tip of the positive carbon. As the 1-in. arc requires 33 per cent more power, and only produces the same amount of light, it is less efficient than the latter. Thus, the shorter we make the arc and the less power we therefore consume in it the more efficient seems the light production, as we produce the same amount of light radiation from the positive terminal in either case. When we come to short arc lengths, however, while the same amount of light is produced at the positive terminal, we do not get the same amount of light from the lamp, as an increasing part of the light is intercepted by the negative ter- minal. Thus with the 1-in. arc, A, in Fig. 49, the light escapes 148 RADIATION, LIGHT, AND ILLUMINATION. freely from the incandescent positive only in the space above the lines m, while at m the shadow of the negative terminal begins to obstruct the light more and more, and does so com- pletely vertically below the arc. In the 0.5-in. arc B the area covered by the shadow of the negative terminals is somewhat increased, but in both arcs A and B the obstruction of the light by the shadow of the negative is still so small that the saving in power far more than makes up for it. In the 0.125-in. arc, however, C in Fig. 49, the shadow of the negative ter- minal m has crept up greatly, and thus, when decreasing the arc length, a point is reached where the increasing shadow of the negative terminal reduces the light more than the de- creasing arc length reduces the power supply. Maximum efficiency of light production thus is reached in the carbon arc at a certain definite arc length (which depends on the size of the electrodes and on the current), at which the change of power consumption just balances the change of radiated light, which results from a change of arc length and thereby of shadow of negative terminal. With the high-current (9 to 10 amperes) open arcs, the maxi- mum efficiency point is at about J-in. arc length, giving a voltage consumption of about 45 to 50 volts. Such arcs require daily trimming, and therefore are no longer used in American cities, except in a few places. The open or short-burning arc has been practically entirely superseded by the enclosed or long-burning arc lamp, in which the arc is enclosed by an almost air-tight globe, the combustion of carbon is greatly decreased, and the life of the carbons thus increased about tenfold. In the open arc of large current, the carbon terminals burn off into a rounded shape, but in the enclosed arc, the current being less and combustion greatly reduced, the carbon terminals burn off to more flat shape, and thus obstruct the light more; and, furthermore, since at the lower current the size of the incandescent spot of the positive terminal is less, the maximum efficiency of light production in the enclosed arc lamp is reached at a much greater arc length, about f in. As the result thereof, the enclosed arc lamp, with 5, 6.5 or 7.5 amperes in the arc, consumes from 70 to 75 volts. 67. Entirely different are the conditions in the luminous ARC LAMPS AND ARC LIGHTING. 149 arc, as the magnetite arc. In this, the light is given by the vapor stream, and not by the terminals, and the voltage e0 and power consumed by the terminal drop thus is wasted, and the voltage el and power pt consumed by the arc stream is useful for light production. The greater, therefore, the voltage el of the arc stream is, compared with the terminal drop e0, or in other words the longer the arc, the higher is the efficiency of light production. Thus a 4-amp. magnetite arc of 0.125-in. length requires 41 volts, while a 0.5-in. 4-amp. arc requires 64 volts ; that is, only 56 per cent more voltage and thus power, but gives about four times the light. A 1-in. arc requires 95 volts or 48 per cent more power than the 0.5-in. arc, and gives twice the light. The greater, thus, the arc length of the luminous arc, with the same current, the higher is the efficiency. How- ever, at the same current, the longer the arc, the greater is the power consumption. In the design of the arc lamp the power consumption is given, and the problem is to select the most efficient arc length for a given and constant power consumption. As an increase of arc length increases the arc voltage for the same power consumption, the current has to be decreased, and the efficiency of the arc conductor decreases with decrease of current. Thus, with increasing arc length at constant power consumption in the luminous arc, a point is reached where the decrease of current required by the increase of arc length and thus arc voltage decreases the efficiency more than the increase of arc length increases it. Thus with the luminous arc, for a given power consumption, a definite arc length exists, which gives maximum efficiency. Assuming the light given by the arc to be proportional to the arc length and the current in the arc, L = k'li, (9) if the power p shall be consumed in the arc, ei = p] (10) however, by (2), e=e0 + ^ (11) Vi (neglecting the small quantity lv as the calculation can obviously be approximate only). 150 RADIATION, LIGHT, AND ILLUMINATION. From (10) and (11) follows: ~ (12) fe and, substituting this in (9), gives: k L = - (pVi - ejVi), (13) and the maximum amount of light produced by power p is given by: dL This gives (14) 3e0 hence, by (13): 0 z, (15) and herefrom, by (12) and (11), the values of the arc length Z and the arc voltage e. Assuming p = 300 watts, and the constants of the magnetite arc: eQ = 30, k = 31, gives: i = 3.33 amperes, e = 90 volts, I = 2.21 cm. = 0.885 in. Near the maximum efficiency, where the efficiency curve is horizontal, the efficiency does not vary much for moderate changes of current and of arc length. Thus, in above instance, practically the same efficiency is reached for currents from 3 amperes to 4 amperes. Larger currents and shorter arc lengths, however, are pref- erable in an arc lamp. (1) Because the shorter and thicker arc is less affected by minor air currents, etc., than the thin long arc, hence, is steadier. (2) The shorter arc gives lower voltage, and this, in constant- current arc lighting, permits with the same total circuit voltage the use of more arc lamps in series. Thus in the magnetite arc lamp a current of 4 amp. has been chosen. i = 4: e = 75 volts, and I + l^ = 0.73 in., or about f-in. arc length. ARC LAMPS AND ARC LIGHTING. 151 In general, obviously the maximum efficiency points of lumi- nous arcs occur at much greater arc lengths than in the plain carbon arc. Since the lower efficiency of the alternating carbon arc is due to the lower temperature of the terminals, which are heated during one half-wave only, and in the luminous arc the tem- perature of the terminals does not determine the light pro- duction, but the light is produced by the vapor stream, no essential difference exists in the efficiency of a luminous arc, and practically no difference in the efficiency of the flame- carbon arc, whether operated on alternating or on direct current; that is, the alternating luminous or flame-carbon arc, with the same luminescent material, has the same efficiency as the direct- current luminous or flame-carbon arc, but the alternating plain carbon arc is much less efficient than the direct-current carbon arc. Arc Lamps. 68. The apparatus designed for the industrial production of light by arc conduction, or the arc lamp, in general comprises four elements : (1) The current-limiting or steadying device. (2) The starting device. (3) The feeding device. (4) The shunt protective device. (1) From the volt-ampere characteristic of the arc as given by equation (2) and curves, Fig. 46, it follows that an arc can- not be operated directly on constant voltage supply, but in series thereto a steadying device must be inserted; that is, a device in which the voltage increases with the current so that the total voltage consumed by the arc and the steadying device increases with increase of current, and pulsations of current thus limit themselves. All arc lamps for use on constant voltage supply thus contain a sufficiently high steadying resistance, or, in alternating-current circuits, a steadying reactance. Arc lamps for use on constant-current circuits, that is, cir- cuits in which the current is kept constant by the source of power supply, as the constant-current transformer or the arc machine, require no steadying resistance or reactance. 152 RADIATION, LIGHT, AND ILLUMINATION. Where several lamps are operated in series on constant poten- tial mains, as two flame-carbon arcs in series in a 110-volt cir- cuit, or five enclosed arc lamps in a 550-volt railway circuit, either each lamp may have its own steadying resistance, or a single steadying resistance or reactance of sufficient size may be used for all lamps which are in series on the constant poten- tial mains. (2) Since the arc does not start itself, but has to be started by forming the conducting vapor bridge between the terminals, all arc lamps must have a starting device. This consists of a mechanism which brings the terminals into contact with each other and then separates them, and hereby forms the vapor conductor, that is, starts the arc. (3) As the arc terminals consume very rapidly in some arcs, as the open carbon arc, and very slowly in others, as the enclosed carbon arcs or the luminous arcs, some mechanism must be provided which moves the terminals towards each other at the rate at which they are consumed, and thereby main- tains constant arc length and thus constant voltage and power consumption. With arcs in which the electrodes consume very slowly, as the magnetite arc, the feeding may occur only at long intervals, every quarter or half hour, or even less frequently, while in arcs with rapidly consuming electrodes, as the flame arcs, practically continuous feeding is required. (4) The circuit between the arc electrodes may accidentally open, as by a breakage of one electrode, or by the consumption of the electrodes if the lamp-trimmer has forgotten to replace them, or one of the electrodes may stick and fail to feed, and the arc thus indefinitely lengthen. In such cases, either the entire circuit would open, and thus all the lamps in series in this circuit go out, or, if the circuit voltage is sufficiently high, as in a constant-current series system, the arc lamp would be consumed and the circuit damaged by a destructive arc. Thus a device is necessary which closes a shunt circuit between the lamp terminals in case the lamp voltage becomes excessive by a failure of proper operation of the lamp. Where only one lamp is operated, on constant potential low- voltage supply, no such protective device is needed. If with two or more lamps in series on constant potential supply, no ARC LAMPS AND ARC LIGHTING. 153 objection exists, in case of the failure of one lamp, to have the others go out also, the shunt protective device may also be omitted, except if the circuit voltage is so high that it may damage the inoperative lamp, as is the case with 550 volts. When operating a number of lamps in series on constant potential supply, as two flame lamps on 110 volts, the shunt circuit, which is closed in case of the failure of one lamp to operate, must have such a resistance, or reactance with alter- nating currents, that the remaining lamp still receives its proper voltage, even if the other lamp fails and its shunt circuit closes. With alternating-current lamps, this does not require a reactance of such size that the potential difference across the reactance equals that across the lamp, which it replaces, but the reactance must be larger; that is, give a higher potential difference at its terminals, than the lamp which it replaces, to leave the normal operating voltage for the remaining lamp, since the voltage consumed by the reactance is out of phase with the voltage consumed by the lamp. 69. For illustration, the operating mechanism of a constant direct-current arc lamp is shown diagrammatically in Fig. 50 : The lower electrode A is held in fixed position. The upper electrode B slides loose in a holder C, and thus, if there is no current through the lamp, drops down into contact with the lower carbon, as shown in Fig. 50. When there is a current through the arc circuit, its path is from terminal 1 through electromagnet S, holder C, upper electrode B, lower electrode A to terminal 2. The electromagnet S is designed so as to give a long stroke. When energized by the current, it pulls up its armature, the lever DD', which is pivoted at E. Through the rod F, the lever D pulls up the clutch G. This clutch and its operation are shown in larger scale in Fig. 51, A and B', it con- sists of a metal piece G, which has a hole somewhat larger than the upper electrode B. This electrode slides freely through the hole, if the clutch G is in horizontal position, as shown in Fig. 51a. When the rod F pulls the clutch G up, and thereby inclines the piece G, as shown in Fig. 516, the edges p and q of the hole in the piece G catch the electrode B, and, in the further upward motion of D and F, raise the other electrode, B, from contact with the lower carbon, A, and thereby start the arc. An elec- tromagnet of many turns of fine wire, and of high resistance, P, 154 RADIATION, LIGHT, AND ILLUMINATION. FIG. 50. FIG. 51a. FIG. 51b. connected in shunt between the lamp terminals 1 and 2, acts upon the side D' of the lever DD', opposite from the side D, on which the series magnet S acts. With the carbons in contact with each other, and practically no voltage between the lamp terminals 1 and 2, the coil P receives no current, and exerts no pull. When by the action of the series magnet S the lever D pulls up, and the arc starts and lengthens, its voltage increases, ARC LAMPS AND ARC LIGHTING. 155 a branch current is established through the shunt magnet P, and this shunt magnet thus opposes the series magnet S. With increasing arc length and thus arc voltage, a point is reached, where the shunt magnet P counterbalances the pull of the series magnet S, and the lever D and thereby the electrodes B come to rest; the arc has reached its full length, that is, the starting operation is over. As soon as, by the combustion of the electrodes, the arc length and thereby the arc voltage be- gins to rise, the current in the shunt magnet P, and thus its pull, increases, while that of the series magnet S, being ener- gized by the constant main current, remains constant; the lever D' thus pulls up, and lowers D, and thereby, through rod F and clutch G, the upper electrode B, and thus maintains con- stant arc length. During the combustion of the electrodes, by the operation of the shunt magnet P the clutch G and thereby the upper electrode B are gradually lowered, and the arc length thus maintained constant. Ultimately, however, the clutch G hereby approaches the horizontal position, shown in Fig. 51 A, so far, that the edges of the hole, p and q, cease to engage, and the electrode B is free and drops down on the lower carbon A. While dropping, however, the arc shortens, the arc voltage, and thereby the current in the shunt magnet P, decreases, the pull of this magnet decreases correspondingly, and the series magnet S pulls the clutch G up again, thereby catches the electrode B -usually before it has dropped quite down into contact with the lower carbon A — and again increases the arc to its proper length, and the same cycle of operation repeats: a gradual feeding down of the upper electrode B by the shunt magnet until it slips, and is pulled up again by the series magnet S. From the same lever D is supported, by rod L, a contact- maker K. If then the upper electrode B should stick, and thus does not slip, when by the shunt magnet P the clutch G has been brought into horizontal position, or, if B has been entirely con- sumed, etc., the arc continues to lengthen and the pull of the shunt magnet P to rise, and D thereby goes still further down until contact-maker K closes the contacts MN, and thereby closes a shunt circuit from terminal 1 over resistance R, con- tacts MKN to terminal 2. In the same manner, if, by the breaking of one electrode or any other cause, the arc should be interrupted, for a moment the full current passes through shunt 156 RADIATION, LIGHT, AND ILLUMINATION. magnet P, it pulls up its armature D' to its full extent, and thereby closes the shunt circuit around the lamp. When the current is taken off the circuit, armature D drops down, and thereby K closes the shunt circuit, and clutch G releases the electrode B, and it drops down into contact with carbon A. In starting the lamp, two paths thus are available: over series magnet S, and electrodes B and A, or over resistance R and contacts MN. While the resistance of the former path is very low, it is not entirely negligible. Therefore a sufficient resistance R must be inserted in the by-path MN, so that in starting practically all the current passes over S and the elec- trodes, as otherwise the lamp would not start. During the pulling up of the armature D by the series magnet S, in start- ing, the contact K opens, before the clutch G has caught the electrode B', that is, while the electrodes are still in contact with each other, and the opening of contact K therefore breaks no appreciable voltage or current, hence is sparkless. In this lamp, no steadying resistance is used, as it is intended for operation on a constant-current circuit. If used on con- stant-potential circuit, as, for instance, a number in series on 550 volts, a steadying resistance R0 would be inserted, as indi- cated at R0 in Fig. 50. The starting of the arc is accomplished by series magnet S and clutch G; the feeding by shunt magnet P; the protective device is the contact MKN. Such an arc lamp is called a differential lamp, as it is con- trolled by the differential action of a shunt and a series magnet. It contains a floating system of control; that is, the upper electrode is suspended by the balance of two forces, exerted by the series and the shunt magnet; that is, by the current and the voltage; the upper carbon therefore is almost continuously moving slightly in following the pulsation of the arc resistance which occurs during operation. Since, with the plain carbon arc, the arc flame gives no light, this pulsation of the arc length is not objectionable; and, since the lamp regulates very closely and rapidly for constant terminal voltage, it is very easy on the circuit, that is, does not tend to produce surging of current and voltage in the circuit. The floating system of control is, there- fore, used in all carbon arc lamps. ARC LAMPS AND ARC LIGHTING. 157 Where a single lamp is operated on a constant-potential circuit, the mechanism can be simplified by omitting the pro- tective shunt circuit RMN, and omitting the shunt magnet P, as, with a change of arc length, the main current and thereby the pull of the series magnet S varies, and the control thus can be done by the series magnet. Such a lamp then is called a series lamp. An alternating-current series lamp is shown diagrammatically in Fig. 52. In starting, the series magnet S pulls up the electrode B by the clutch G, in the same manner as in Fig. 50. With increasing arc length and thus increasing voltage consumed by the arc, the current in the arc and thus in the series magnet S decreases, and thereby the pull of this magnet, until it just counterbalances the weight of the armature, and the motion stop. With the consumption of the carbons, the armature D, clutch G and elec- trode B gradually move down, until the clutch lets the carbon slip, the arc shortens, the current rises, and the magnet S pulls up again, the same as in Fig. 50. A reactance x in series with the lamp, as steadying device, limits the current. This reactance usually is arranged with different terminals, so that more or less reactance can be connected into circuit, and the lamp thereby operated, with the same arc voltage, on supply circuits of different voltage, usually from 110 to 125 volts. Obviously, such a series lamp can be used only as single lamp on constant potential supply, as it regulates by the varia- tion of current, and, with several lamps in series, the current would vary in the same manner in all lamps. One lamp would then take all the voltage, draw an arc of destructive length, while the other lamps would drop their electrodes together and go out. 70. With the luminous arc, in which the light is proportional FIG. 52. 158 RADIATION, LIGHT, AND ILLUMINATION. to the arc length, pulsations of the arc length, if appreciable, give pulsations of the light, and the floating system of control, which maintains constant voltage by varying the arc length in correspondence with the pulsation of arc resistance, thus is undesirable, and a mechanism maintaining fixed arc length is required. Such a mechanism, that of the magnetite arc lamp, is diagrammatically illustrated in Fig. 53. As, during operation, a melted pool forms on the sur- face of the electrode, the elec- trodes are left separated from each other when taking the power off the circuit, since when letting them drop to- gether when taking off the power — as in the carbon arc — they may weld together and the lamp thus fail to start again. A represents the lower or negative magnetite terminal which is movable, B, the non- consuming upper positive elec- trode, consisting of a piece of copper, with heat-radiating wings, W, which is a station- ary and fixed part of the lamp. C is the chimney required to FlG- 53- carry off the smoke. When starting, the circuit, beginning at terminal 1, passes contacts MN, through a powerful electromagnet or solenoid 0 and resistance R, to terminal 2. The solenoid 0 pulls up its core D, and by the clutch G raises the lower electrode A into contact with the upper electrode, B, and thereby closes the circuit from 1 over series coil S, electrodes B and A, to terminal 2. The series coil S pulls up the core, and thereby opens the contact MN, thus cuts out the shunt circuit OR. The solen- oid 0 thus loses its excitation, and drops the clutch G, and the lower terminal A drops away from the upper terminal B by a ARC LAMPS AND ARC LIGHTING. 159 distance which is fixed by an adjustable clutch, Q, and thus starts an arc of definite length. When during the consumption of this electrode A the arc length and thereby the arc voltage rises, the shunt magnet P increases in strength, and ultimately pulls the core F away from the series magnet S, closes the contact MN of the shunt circuit OR, and thereby energizes the solenoid 0. This again raises, by the clutch G, the lower electrode A into contact with the upper electrode B, and so repeats the cycle of operation. If the arc between A and B opens, the solenoid S loses its excitation, the coil F drops and closes the contact MN of the shunt circuit OR. If the arc resistance were perfectly constant, such a mechan- ism would not operate satisfactorily, as the arc would have to lengthen considerably to have the shunt coil P overpower the series coil S sufficiently to pull the core F down, close the con- tact MN, and thereby feed. The resistance of an arc, and thereby, at constant length, its voltage, pulsates, however, continuously, about as shown diagrammatically in Fig. 54, that is, peaks of voltage of vari- isol 110 1005 \r FIG. 54. ous height follow each other. Thus with an average arc volt- age of 75, momentary peaks of 85 volts will probably be reached every few seconds, peaks of 100 volts every few minutes, of 110 volts every half hour. Adjusting the shunt magnet P so as to operate at 105 volts, a voltage peak above 105, which causes the lamp to feed, would be reached about every 20 minutes. During 20 minutes, however, the arc length does not appre- 160 RADIATION, LIGHT, AND ILLUMINATION. ciably increase by the consumption of the electrode. Due to the character of the arc as a pulsating resistance, such a con- trolling mechanism thus maintains constant arc length by a potential magnet set for a voltage considerably above the aver- age arc voltage. Such a mechanism, controlling for constant arc length, does not operate for constant voltage at the lamp terminals, but allows the pulsation of the arc resistance to appear as pulsation of the terminal voltage. In a constant-current circuit, with many lamps in series, these voltage pulsations of the individual arcs overlap and have no effect on the circuit. When operating, however, a lamp of such a mechanism on a low-voltage constant potential circuit, a highly inductive steadying resistance is de- sirable, to take care of the pulsations of arc voltage. 71. The open or short-burning carbon arcs of former times — which have survived only in a few cities — were operated on constant direct-current circuits of 9.6 and 6.6 amp. with 40 to 45 volts per lamp. The present enclosed or long-burning carbon arcs are oper- ated on constant-current circuits of 5 amp. and 6.6 amp. direct current, or of 6.5 and 7.5 amp. alternating current, with about 72 volts at the lamp terminals. They are operated as single lamps, of 5 to 9 amp. on direct- or alternating-current constant potential circuits of 110 to 125 volts, or two lamps in series on circuits of 220 to 250 volts. The flame-carbon arcs, as short-burning open arcs, are usually operated two in series on constant-potential circuits of 110 to 125 volts, or four in series on circuits of 220 to 250 volts, with 10 to 15 amp. in the arc. The luminous arcs are operated on 4- and 6.6-amp. constant direct-current circuits (magnetite lamp), with 75 volts per lamp, and on 3- and 4.5-amp. constant alternating-current cir- cuits (titanium-carbide arc), with 80 volts per lamp. Arc Circuits. 72. Arc lamps are built for, and operated on, constant-poten- tial supply, and on constant-current supply. In general, the constant-potential arc lamp is less efficient, as voltage and thereby power is consumed in the steadying resistance which is required to limit the current, that is, to give an approximate ARC LAMPS AND ARC LIGHTING. 161 constant-current effect, as discussed above. In alternating- current circuits, reactance may, and usually is, employed in- stead of the steadying resistance, and the waste of power thereby greatly decreased. Voltage, however, is still consumed and the power factor lowered. An additional waste of energy generally occurs in constant- potential arc-lamp circuits, due to the standard distribution voltages of low-potential circuits being higher than necessary for the operation of a single lamp, but too low for the operation of two lamps in series. Thus with an enclosed 5-amp. carbon arc lamp, with about 70 volts at the arc, a supply voltage of 95 to 100 volts would be sufficiently high above the stability curve of the arc (Fig. 46) to give steady operation. Distri- bution voltages, however, vary between 110 to 130 volts, and the difference thus must be consumed in resistance, giving an additional waste. (Except in those rare cases, where as steady- ing resistance some useful devices, as incandescent lamps, can be employed.) With an enclosed arc lamp on a 125- volt cir- cuit, only 36 volts, or 29 per cent, are usefully employed in heating the carbon terminals and thereby producing the light, while the remaining 71 per cent is wasted in the resistance and in the non-luminous arc flame. Somewhat better are the conditions when operating two high-current open arcs, as two flame lamps, in series on such a circuit. However, at the lower distribution voltage, as 110 volts, the supply voltage may be already so close to the stability limit of the arc that the arcs are not as steady as desirable. The low-current, long luminous arcs of electro-conduction, as the magnetite arc, can in general be operated only with diffi- culty on circuits of 110 to 125 volts, and therefore are, for constant-potential service, frequently designed for operation as single lamps on 220 to 250 volts supply, with extra great arc length. For indoor illumination, constant-potential arc lamps must obviously be used, as safety does not permit the introduction of the high-voltage series arc circuits into houses. For out- door illumination, as street lighting, in the United States the constant-current arc lamp is, with the exception of the interior of a few very large cities, as New York, used exclusively, due to its greater efficiency, and the greater distance to which the cur- 162 RADIATION, LIGHT, AND ILLUMINATION. rent can be sent at the high voltage of the constant-current circuit. In American towns and cities, where arc lamps are used for street lighting, practically always the entire city up to the farthest suburbs is lighted by arc lamps, and frequently arc lamps installed even beyond the reach of the high-potential primary alternating-current supply. To reach such distances with low-voltage constant-potential supply, is impossible, and thus the constant-current series system becomes necessary. In European cities, where a prejudice exists against high-voltage constant-current circuits, and people are satisfied to have arc lamps only in the interior of the city, and leave the lighting of the suburbs to gas lamps, constant-potential street lighting is generally employed, and is eminently satisfactory within the limitations with which European cities are satisfied, but would be impossible in the average American city. Where plain carbon arcs are used in American cities the enclosed arc lamp is exclusively installed, and open arc lamps have survived only in a few exceptional cases, mainly where political reasons have not yet permitted their replacement by modern lamps. The lesser attention required by the enclosed arc lamp — weekly trimming instead of daily with the open arc lamp — has been found to make it more economical than the open arc lamp of old. In Europe, where labor is cheaper, and the daily attention not considered objectionable, the open or short-burning arc lamp has maintained its hold, and the en- closed arc lamp has never been used to any great extent. With the development of the flame carbon, the flame arc, therefore, has found a rapid introduction in Europe, while in the United States it has been excluded from use in street lighting, due to its short-burning feature, which requires daily trimming, and is used only for decorative purposes, for advertising, etc., and for low-grade interior lighting, as foundries, etc. In spite of the lower efficiency of the alternating carbon arc, the constant-current circuits used for arc lighting are generally alternating, due to the greater convenience of generation of alternating current, and constant direct-current arc circuits used only where the city or the electric light company lays stress on the efficiency of light production. While the development of the constant-current mercury arc rectifier has made the generation of constant direct current ARC LAMPS AND ARC LIGHTING. 163 almost as simple and convenient as that of alternating current, this has very little increased the use of the direct-current en- closed arc lamp, but when changing to direct current sup- ply, usually the arc lamp is also changed to the luminous arc, the magnetite lamp, which gives more light and consumes less power. In constant-current arc circuits, usually from 50 to 100 lamps are operated in series on one circuit, with circuit volt- ages of 4000 to 8000 volts. Seventy-five-lamp circuits, of 6000 volts, probably are the most common. 73. Constant direct current was produced by so-called "arc machines" or "constant-current generators." Of these only the Brush machine has survived, and is now also beginning to disappear before the mercury arc rectifier, which changes the alternating current of the constant-current transformer to direct current without requiring moving machinery. The Brush machine in its principle essentially is a quarter- phase constant-current alternator with rectifying commutator. An alternator of low armature reaction and strong magnetic field regulates for constant potential: the change of armature reaction, resulting from a change of load, has little effect on the field and thereby on the terminal voltage, if the armature reaction is low. An alternator of very high armature reaction and weak field, however, regulates for constant current: if the m.m.f., that is, the ampere-turns required in the field coil to produce the magnetic flux, are small compared with the field ampere-turns required to take care of the armature reaction, and the resultant or magnetism-producing field ampere-turns thus the small difference between total field excitation and armature reaction, a moderate increase of armature current and thereby of armature reaction makes it equal to the field excitation, and leaves no ampere-turns for producing the mag- netism; that is, the magnetic flux and thereby the machine voltage disappear. Thus, in such a machine, the current out- put at constant field excitation rises very little, from full volt- age down to short circuit, or, in other words, the machine regulates for approximately constant current. Perfect constant- current regulation is produced by a resistance shunted across the field, which is varied by an electromagnet in the machine circuit, and lowered — that is, more current shunted through it, and 164 RADIATION, LIGHT, AND ILLUMINATION. thereby the field excitation decreased — if the machine cur- rent tends to rise by a decrease of the required circuit voltage, and inversely. The constant-current regulation of the arc machine thus is not produced by its so-called " regulator/' but approximate constant-current regulation is inherent in the machine design, and the regulator merely makes the regulation perfect. A more explicit discussion of the phenomena in the arc machine, and especially its rectification, is given in Chapter III of Section II of "Theory and Calculation of Transient Electric Phenomena and Oscillations." In alternating-current circuits, approximate constant-current regulation is produced by a large reactance, that is, by self- induction, in the circuit. In transformers, the self-induction is the stray field, or the leakage flux between primary coil and secondary coil. In the constant-current transformer, which is most generally used for constant alternating-current supply from constant alternating voltage, the primary turns and the secondary turns are massed together so as to give a high mag- netic stray flux between the coils. Such a transformer of high internal self-induction, or high stray flux, regulates approxi- mately for constant current. Perfect constant-current regula- tion is produced by arranging the secondary and the primary coils movable with regard to each other, so that, when low cir- cuit voltage is required, the coils move apart, and the stray flux, that is, the reactance, increases, and inversely. The motion of the coils is made automatic by balancing the magnetic repulsion between the coils by a counter- weight. A discussion of the constant-current transformer and its mode of operation is given in "Theory and Calculation of Alternating Current Phenomena," Fourth Edition, page 85. In the so-called "constant-current reactance," the two coils of the constant-current transformer are wound for the same current, and connected in opposition with each other, and in series to the arc circuit into the constant-potential mains. With the coils close to each other, the reactance is a minimum, and it is a maximum with the coils their maximum distance apart. The constant-current reactance has the advantage of greater cheapness, but also has the serious disadvantage that it connects ARC LAMPS AND ARC LIGHTING. 165 all the arc circuits electrically with the constant-potential alter- nating-current system, and any ground in an arc circuit is a ground on the constant-potential supply system. As grounds are more liable to occur in arc circuits, the constant-current reactance is therefore very little used, and generally the con- stant-current transformer preferred, as safer. In the constant direct-current mercury-arc rectifier system, the constant-potential alternating-current supply is changed to constant alternating current by a constant-current transformer, and the constant alternating current then changed to constant direct current by the mercury-arc rectifier. An explicit dis- cussion of the phenomena of the constant-current mercury arc rectifier is given in Chapter IV of Section II of " Theory and Calculation of Transient Electric Phenomena and Oscillations." If the constant-current arc circuit accidentally opens, with a Brush machine as source of supply, the voltage practically vanishes, as the machine has series field excitation, and thus loses its field on open circuit. The constant-current trans- former, however, maintains its voltage, and gives maximum voltage on open circuit. The mercury arc rectifier, when sep- arately excited by a small exciting transformer, also maintains its voltage on open circuit. If, however, after starting, the excitation is taken off, that is, the exciting circuit opened, as is permissible in a steady arc circuit, the voltage in the arc circuit disappears if the arc circuit is opened. Inversely, when connecting an arc circuit to a Brush arc machine, an appre- ciable time elapses while the voltage of the machine builds up, while with the constant-current transformer and thus also with the constant-current mercury arc rectifier system, full volt- age exists even before the circuit is closed.