CHAPTER V MAGNETISM Magnetic Constants 47. With the exception of a few ferromagnetic substances, the magnetic permeability of all materials, conductors and dielectrics, gases, liquids and solids, is practically unity for all industrial purposes. Even liquid oxygen, which has the highest permea- bility, differs only by a fraction of a per cent, from non-magnetic materials. Thus the permeability of neodymium, which is one of the most paramagnetic metals, is /x = 1.003; the permeability of bismuth, which is very strongly diamagnetic, is /* = 1 — 0.00017 = 0.99983. The magnetic elements are iron, cobalt, nickel, manganese and chromium. It is interesting to note that they are in atomic weight adjoining each other, in the latter part of the first half of the first large series of the periodic system: Ti V Cr Mn Fe Co Ni Cu Zn Atomic weight 48 51 52 55 56 58 59 61 65 The most characteristic, because relatively most constant, is the metallic magnetic saturation, S, or its reciprocal, the satura- tion coefficient, cr, in the reluctivity equation. The saturation density seems to be little if any affected by the physical condition of the material. By the chemical composition, such as by the presence of impurities, it is affected only in so far as it is reduced approximately in proportion to the volume occupied by the non- ^Q-gnetic materials, except in those cases where new compounds result. It seems, that the saturation value is an absolute limit of the elenxent, and in any mixture, alloy or compound, the saturation valu^ reduced to the volume of the magnetic metal contained therein, can not exceed that of the magnetic metal, but may be ^^er, if the magnetic metal partly or wholly enters a compound ^^ lower intrinsic saturation value. Thus, if S = 21 X 10' is ^^^ saturation value of iron, an alloy or compoimd containing 77 78 ELECTRIC CIRCUITS 72 per cent, by volume of iron can have a maximum saturation value ofS = 0.72 X 21 X 10« = 15.1 X 10* only, or a still lower saturation value. The only known exception herefrom seems to be an iron-cobalt alloy, which is alleged to have a saturation value about 10 per cent, higher than that of iron, though cobalt is lower than iron. The coefficient of magnetic hardness, a, however, and the co- efficient of hysteresis, 77, vary with the chemical, and more still with the physical characteristic of the magnetic material, over an enormous range. Thus, a special high-silicon steel, and the chilled glass hard tool steel in the following tables, have about the same percentage of non-magnetic constituents, 4 per cent., and about the same saturation value, S = 19.2 X 10^, but the coefficient of hardness of chilled tool steel, a = 8 X 10~^, is 200 times that of the special silicon steel, a = 0.04 X 10"^, and the coefficient of hysteresis of the chilled tool steel, 17 = 75 X 10"', is 125 times that of the sili- con steel, 77 = 0.6 X 10"'. Hardness and hysteresis loss seem to depend in general on the physical characteristics of the material, and on the chemical constitution only as far as it affects the phys- ical characteristics. Chemical compounds of magnetic metals are in general not ferromagnetic, except a few compounds as magnetite, which are ferromagnetic. With increasing temperature, the magnetic hardness a, decreases, that is, the material becomes magnetically softer, and the satura- tion density, S, also slowly decreases, until a certain critical temperature is reached (about 760°C. with iron), at which th^ material suddenly ceases to be magnetizable or ferromagnetic a but usually remains slightly paramagnetic. As the result of the increasing magnetic softness and decreasini saturation density, with increasing temperature the density Bj at low field intensities, jff, increases, at high field intensiti^^fi decreases. Such B-temperature curves at constant H, howev^:^', have little significance, as they combine the effect of two chang^cii the increase of softness, which predominates at low Hy and ffce decrease of saturation, which predominates at high H. Heat treatment, such as annealing, cooling, etc., very gtesirtly changes the magnetic constants, especially a and 17— more or less in correspondence with the change of the physical constants brought about by the heat treatment. MAGNETISM 79 Very extended exposure to moderate temperature — 100 to 200°C. — increases hardness and hysteresis loss with some mate- rials, by what is called ageing, while other materials are almost free of ageing. 48. The most important, and therefore most completely in- vestigated magnetic metal is iron. Its saturation value is probably between S = 21.0 X 10^ and S = 21.5 X 10^, the saturation coefficient thus a = 0.047. As all industrially used iron contains some impurities, carbon, silicon, manganese, phosphorus, sulphur, etc., usually saturation values between 20 X 10' and 21 X 10' are found on sheet steel or cast steel, etc., lower values, 19 to 19.5 X 10*, in silicon steels containing several per cent, of Si, and still much lower values, 12 to 15 X 10', in very impure materials, such as cast iron. Two types of iron alloys seem to exist: 1. Those in which the alloying material does not directly afifect the magnetic qualities, but only indirectly, by reducing the vol- ume of the iron and thereby the saturation value, and by chang- ing the physical characteristics and thereby the hardness and hysteresis loss. Such apparently are the alloys with carbon, silicon, titanium, chromium, molybdenum and tungsten, etc., as oast iron, silicon steel, magnet steel, etc. 2. Those in which the alloying material changes the magnetic, characteristics. Such apparently are the alloys with nickel, manganese, mercury, copper, cobalt, etc. In this class also belong the chemical compounds of the mag- netic materials. Thus, a manganese content of 10 to 15 per cent, makes the iron Pi'actically non-magnetic, lowers the permeability to /x = 1.4. However, even here it is not certain whether this is not an ^^reme case of magnetic hardness, and at extremely high Magnetic fields the normal saturation value of the iron would be approached. Some nickel steels (25 per cent. Ni) may be either magnetic, or ^On-magnetic. However, pure iron, when heated to high incan- ^^cence, becomes non-magnetic at a certain definite temperature, ^^d when cooling down, becomes magnetizable again at another definite, though lower temperature, and between these two tem- 80 ELECTRIC CIRCUITS peratures, iron may be magnetic or unmagnetic, depending whether it has reached this temperature from lower, or from higher temperatures. Apparently, for these nickel steels, the critical temperature range, within which they can be magnetic or un- magnetic, is within the range of atmospheric temperature, and thus, after heating, they become non-magnetic, after cooling to sufficiently low temperature, they become magnetizable again. Thus, a steel containing 17 per cent, nickel, 4.5 per cent, chro- miiun, 3 per cent, manganese, has permeability 1.004, that is, is almost completely unmagnetic. Heterogeneous mixtures, such as powdered iron incorporated in resin, or iron filings in air, seem to give saturation densities not far different from those corresponding to their volume per- centage of iron, but give an enormous increase of hardness, a, and hysteresis, 77, as is to be expected. Most chemical compounds of iron are non-magnetic. Fer- romagnetic is only magnetite, which is the intermediate oxide and may be considered as ferrous ferrite. There also is an alleged magnetic sulphide of iron, though I have never seen it, magnetkies, FeTSg or FegSg. As magnetite, Fe304, contains 72 per cent, of Fe, by weight, and has the specific weight 5.1, its volume per cent, of iron would be 48 per cent., and the saturation density 5^ = 10 X 10^. Observations on the magnetic constants of magnetite give a saturation density of 4.7 X 10^ to 5.91 X 10^, so that magnet- ite would fall in the second class of iron compoimds, those in which the saturation density is affected, and lowered, by the composition. Not only m^neUte, which may be considered as ferrous ferrite, but numerous other ferrites, that is, salts of the acid Fe204H2, are to some extent ferromagnetic, such as copper and cobalt fer- rite, calciiun ferrite, etc. 49. Cobalty next adjoining to iron in the periodic system of ele- ments, is the magnetic metal which has been least investigated. Its saturation value probably is between S = 12 X 10^ and S = 14 X 10^, and its magnetic characteristic looks very similar to that of cast iron. Partly this is due to the similar saturation value, partly probably due to the feature that most of the available data were taken on cast cobalt. It is interesting to note that Cobalt retains its magnetizability MAGNETISM 81 up to much higher temperatures than iron or any other material, so that above 800 degrees C, Cobalt is the only magnetic material. More information is available on nickel, the metal next ad- joining to cobalt in the periodic system of elements. Its satura- tion density is the lowest of the magnetic metals, probably be- tween S = 6 X 10^ and /S = 7 X lO^. Some data on nickel and nickel alloys are given in the following table. In general, nickel seems to show characteristics very simi- lar to those of iron, except that all the magnetic densities are re- duced in proportion to the lower saturation density; but the effect of the physical characteristics on the magnetic constants appears to be the same. Interesting is, that nickel seems to be least sen- sitive to impurities in their effect on the reluctivity curve. Ifickel ceases to be magnetizable already below red heat. The next metal beyond nickel, in the periodic system of ele- ments, is copper, and this is non-magnetic, as far as known. On the other side of iron, in the periodic system, is manganese. This is very interesting in so far as it has never been observed in a strongly magnetic state, but many of the alloys of manganese are more or less strongly magnetic, and estimating from the satu- ration values of manganese alloys, the saturation value of man- ganese as pure metal should be about S = 30 X 10^. This would make it the most magnetic metal. In favor of manganese as magnetic metal also is the unusual behavior of its alloys with iron: the alloys of nickel, and of cobalt with iron also show imusual characteristics, and this seems to be a characteristic of alloys between magnetic metals. The best known magnetic manganese alloys are the Heusler s-lloys, of manganese with copper and almninum, and the char- acteristics of three such alloys are given in the following table. The most magnetic shows about the same saturation value as Diagnetite, but higher saturation values, equal to those of nickel, have been observed. A curious feature of some Heusler alloys is, that when slowly cooled from high temperatures, they are very little magnetic, ^ and have low saturation values. The quicker they are cooled, the higher their permeability and their saturation value, and the j> best values have been reached by dropping the molten alloy into ^ater, so suddenly chilling it. Ill general, the Heusler alloys are especially sensitive to heat treatment, and some of them show the ageing in a most pro- 82 ELECTRIC CIRCUITS nounced degree, so that maintaining the alloy for a considerable time at moderate temperature, increases hardness and hysteresis loss more than tenfold. Magnetic alloys of manganese also are known with antimony, arsenic, phosphorus, bismuth, boron, with zinc and with tin, etc. Usually, the best results are given by alloys containing 20 to 30 per cent, of manganese. Little is known of these magnetic al- loys, except that they may be in a magnetic state, or in an unmagnetic stage. They are most conveniently produced by dissolving manganese metal in the superheated alloying metal, or in this metal with the addition of some powerful reducing metal, as sodium or aluminum, but the alloy is only sometimes magnetic, sometimes practically unmagnetic, and the conditions of the formation of the magnetic state are unknown. Apparently, there also exists an intermediary oxide of mangan- ese, or a compound oxide of manganese with that of the other metal, which is strongly magnetic. The black slag, appearing in the fusion of manganese with other metals such as antimony, zinc, tin, without flux, often is strongly magnetic, more so than the alloy itseK. A mixture of about 25 per cent, powdered manganese metal, and 75 per cent, powdered antimony metal, heated together to a moderate temperature — in a test-tube — gives a strongly mag- netic black powder, which can be used like iron filings, to show the lines of forces of the magnetic field, but has not further beeo investigated. A considerable number of such magnetic manganese alloys have been investigated by Heusler and others, and their constants are given in the following table. It is supposed that these magnetic manganese alloys are chem- ical compounds, similar as magnetite or magnetkies. Thus the copper-aluminum-manganese alloy of Heusler is a compound of 1 atom of aluminum with 3 atoms of copper or manganese: Al- (Mn or Cu)3, usually AlMnCu2. Other magnetic manganese compounds then are: With antimony MnSb and MnsSb With bismuth MnBi With arsenic MnAs With boron MnB With phosphorus MnP With tin Mn4Sn a&d MniSn MAGNETISM 83 Next adjacent to manganese in the periodic system of elements is chromium. Neither the metal, nor any of its alloys (except those with magnetic metals) have ever been observed in the mag- netic state. There is, however, an intermediary oxide of chro- mium, alleged to be CrsOo (a basic chromic chromate?) which is strongly magnetic. It forms, in black scales, in a narrow range of temperature, by passing Cr02Cl2 with hydrogen through a heated tube. A second strongly magnetic chromium oxide is Cr409 (a basic chromic bichromate?). It is easily produced by rapidly heat- ing CrOs, but the product is not always the same. Their magnetic characteristics have never been investigated, and they are the only indication which would point to chromium having potentially magnetic qualities. The metal next to chromium in the periodic system of elements, vanadiimi, is non-magnetic, as far as known. 50. On attached tables are given the magnetic constants of the better known magnetic materials, metals, alloys, mixtures and compounds: The first tables give the saturation density, S, and the demag- netization temperature, that is, temperature at which the ma- terial ceases to be ferromagnetic, and its specific gravity. It is interesting to note that with some magnetic materials the demagnetization temperature is very close to, or within the range of, atmospheric temperature. The second table gives more complete data of those materials, of which such data are available. It gives: S = saturation density, or value of B — H for infinitely high H; a = coefficient of magnetic hardness; •!« 12- 4 II 18:8S? 8-15 8:i7! 7:00 0:70 Wedckind if is, if 13::e, Manga Mania. Ch CrrfJt, •:t.o.. J^.- isaa naaneu. HcUBkr .^Ibya: hromlB chconislo 88 ELECTRIC CIRCUITS < O o O » o < 5,c3 d 02 ii I |4 « s fl S.2 • .a u ♦» 00 a 08 hi O 1^ 0) to a 08 08 ♦» CC c8 00 I c8 0) a 08 hi to 3 0) to a c8 05 I . hi a 5o 08 ^ CO 08 2 b 'P 00 h go 08 S is s ♦» .2 u V c8 o I o o X I o X I o X I o X CQ jaqinna o X S CO o o CO to eo I CI eo I 10 w "*§ »o op oo o o eo oo ^ eo o s to 2 - X o oo »o>o ooeo»o 00 00 CO »o»o« ooo odd oco •lOCSI ddd eo 00 >0 n •o lo ooeo CO oo oooo OOiO I I C4 OOOtO • • • • ^«i-ieo C4iOeOC4 CI o »o co^ ^ 1^ 0>O lO 0 O OOOOOO d dddddd lO I CO eo i^o^^o>oo d dodddd 8g AA ^ eo 0)00 OO dd dd dd e^c4 « to c8 hi 08 i 10*3 O c8 o'v • « C) OQ ■■ ■♦* « O C8 OS 08 O a a 08 »o »o d lO 1-1 »o o eo •o .-hO OJ hi V 0. lO Ji "flS a a 08 •O 00 CI d CI (D 0) CI v Soo »H«0«0 AAA «oeOi-i ddd CI OCIO .-•i-ici «oco«o ddoo CldfH oapq 9i u "^ « ® *•*» o 00 ^^^-a .^ ea 00 * ^ ^ d*> £ St> " '-' ■*^^^ ^^ 8© ^ "^ 08 -; S >oo ®.fl 08^ aiaa CQ g8 JS ^ C3 > >>s s 08^ O O d «-»*+» o ao o GQCQCQ OB o »o CI o o CI^OO 2§g • • • ooo CI n o o CO CI O 00 • • ^ CI o 00 A to CI o »o" A A CO CO CIO) oo CO O i-iO fHOC^iH 1-1 ilO »HC|i-lCI • • • « a • • o oo 00>H0 lO o »H coo OOOOft CI coo eo coo< >00 d ooo oocod-^*oio 5^- t«,d 'Si to «o 11 |22 •sis* d o •♦* . *H ^M « ► ► ^S -Sdd |§*:s§s 9 > OB § d^. 2|8i + « 3 h«. AA •o A I CO lO CO o ot^ot^ «ot* .... . . X oooo oo I o ^*ooo 00 CO "CO CO oo 2 •-•cO'Hco ^^ • • • • X i-ieOi-ico ^ 1-4 1-4 1^ iH CO to o CI II 08 o d 9i o ♦» CO II .OiQO 08 d*-" . ^ o o -^ !>. 1-1 lO O CO 2 •o c^ -^ to CI •o to 00 to CO O) o' o o -^ A A o "* A o t •o to CO O A QOtOiO cocat»r« ooo xco 1^1^ CO s to ^ 1-1 ^ GO > CO ^ CO 1-" to CI N CO CO "^COd-^ fHdCO'* •O to CO 1-1 CO CO* o GO oooo cooocio r^oooc^ CS| to w N CO CO 0> CO too to ooo1-l^* CO CI CO f-4i^ r«cocoo nil- OOmO > 0) d^ " d CO O ..OS <»s go o d^. J '■i . 0, . to3( high .2 ; 00^ •k N ® 08 00 . D a Xi » •k *> . CQ^ n 3 : i8 d o : O h s o o o . -a d d . S : -^ V ^ ; •k OB V d •a d O o a 1^ 04 d 5 .§ GO •< a d • • d § o V d o> CO GO 28 f-4 C< ^^^* S 90 ELECTRIC CIRCUITS Such extremely high fields, as to reach complete magnetic saturation, are produced only between the conical pole faces of a very powerful large electromagnet. The area of the field then is very small, and it is difficult to get perfect uniformity of the field. The tendency is to imderestimate the field, and this gives too high values of S, Thus, in the following table those values of Sy which appear questionable for this case, have been marked by the interrogation sign. The indirect method, from the straight-line reluctivity curve, gives more accurate values of iS, as /S is derived from a complete curve branch, and this method thus is preferable. However, the value derived in this manner is based on the assumption that there is no further critical point in the reluctivity curve beyond the observed range. This is correct with iron, as the best tests by the direct method check. With cobalt, there may be a critical point in the reluctivity curve beyond the observed range, as there are several observations by the direct method, which give very much higher, though erratic, values of saturation, S. The value of the magnetic hardness, a, also is difficult to de- termine for very soft materials, especially where the method of observation requires correction for joints, etc., and the extremely high values of permeability — over 15^000 — therefore appear questionable.