New volumes of the Encyclopædia Britannica > Volume 30, K-MOR

MAGNETISM
447
Metal.
Percentage Gain or Loss
of Moment at -186° C.
First Effect. Cyclic Effect.
Carbon steel, hard .
,, ,, medium
,, ,, annealed .
Chromium steels (four samples)
Aluminium steels (three samples)
Nickel steels, up to 7'65 per cent.
,, ,, ,, 19'64 per cent.
,, ,, ,, 29 per cent.
Pure nickel
Silicon steel, 2‘67 per cent.
Iron, soft
,, hard
Tungsten steel, 15 per cent.
. ,, ,, 7'5 per cent.
,, ,, 1 per cent.
-6
Decrease
-33
Increase
-2
Small
-50
-20
Decrease
99
None
Decrease
+ 12
+ 22
+ 33
+ 12
+ 10
+ 10
-25
-10
+ 3
+ 4
+ 2-5
+ 10
+ 6
+ 10
+ 12
Other experiments relating to the effect of temperature upon
permanent magnets have been carried out by Ashworth,1 who
showed that the temperature coefficient of permanent magnets
might be reduced to zero (for moderate ranges of temperature) by
suitable adjustment of temper and dimension ratio; also by Pictet2
and by Durward.3
Alloys of Nickel and Iron.—A most remarkable effect
of temperature was discovered by Hopkinson (fProc. Roy.
Soc. vol. xlvii. (1890), p. 23; vol. xlviii. (1891), p. 1) in
1889. An alloy containing about 3 parts of iron and 1
of nickel—both strongly magnetic metals—is under ordi¬
nary conditions practically non-magnetizable (//.= !’4 for
any value of H). If, however, this non-magnetic sub¬
stance is cooled to a temperature a few degrees below
freezing-point, it becomes as strongly magnetic as average
cast-iron (/x = 62 for H = 40), and retains its magnetic
properties indefinitely at ordinary temperatures. But if
the alloy is heated up to 580° C., it loses its susceptibility
—rather suddenly when H is weak, more gradually when
H is strong—-and remains non-magnetizable till it is once
more cooled down below the freezing-point. This material
can therefore exist in either of two perfectly stable con¬
ditions, in one of which it is magnetizable, while in the
other it is not. When magnetizable, it is a hard steel,
having a specific electrical resistance of 0’000052; when
non-magnetizable, it is an extremely soft, mild steel, and
its specific resistance is 0’000072. Alloys containing
different proportions of nickel were found to exhibit the
phenomenon, but the two critical temperatures were less
widely separated. The following approximate figures are
deduced from Hopkinson’s curves :—
Percentage of
Nickel.
0-97
4’7
4-7
24’5
30-0
33’0
73-0
Susceptibility lost
at temp. C.
890
820
780
680
140
207
202
Susceptibility gained
at temp. C.
660
600
-10
125
193
202
Guillaume4 has further investigated the properties of nickel-
steels, and found that the temperature at which magnetic suscepti¬
bility is recovered is lowered by the presence of chromium; a
certain nickel-chromium alloy was not rendered magnetic even by
immersion in liquid air. Experiments on the subject have also
been carried out by Dumont5 and Osmond.6
Alloys and Compounds of Iron.
In 1885 Hopkinson (Phil. Trans, vol. clxxvi. (1885),
p. 455) employed his yoke method to test the magnetic
properties of thirty-five samples of iron and steel, among
1 Proc. Roy. Soc. vol. Ixh. (1898), p. 210.
2 C. R. vol. cxx. (1895), p. 263.
3 Amer. Jour. Scien. vol. v. (1898), p. 245.
4 O. R. vol. cxxiv. (1897), p. 1515 ; Journ. de Phys. vol. vii.
(1898), p. 262.
5 C. R. vol. cxxvi. (1898), p. 741.
6 Ibid. vol. cxxviii. (1899), pp. 304, 1395.
which were steels containing substantial proportions of
manganese, silicon, chromium, and tungsten. The results,
together with the chemical analysis of each sample, are
given in a table contained in his paper, some of them
being also represented graphically. The most striking
phenomenon which they bring into prominence is the
effect of any considerable quantity of manganese in an¬
nihilating the magnetic property of iron. A sample of
Hadfield’s manufacture, containing 12’36 per cent, of
manganese, differed hardly at all from a non-magnetic
substance, its permeability being only 1’27. According
to Hopkinson’s calculation, this sample behaved as if 91
per cent, of the iron contained in it had completely lost
its magnetic property.7 Another point to which attention
is directed is the exceptionally great effect which harden¬
ing has upon the magnetic properties of chrome steel;
one specimen had a coercive force of 9 when annealed,
and of no less than 38 when oil-hardened. The effect
of the addition of tungsten in increasing the coercive
force is very clearly shown; in two specimens contain¬
ing respectively 3’44 and 2’35 per cent, of tungsten the
coercive force was 64’5 and 70’7. These high values
render hardened tungsten-steel particularly suitable for
the manufacture of permanent magnets. Hopkinson
(Proc. Roy. Soc. vol. xlviii. (1890), p. 1) also noticed
some peculiarities of an unexpected nature in the magnetic
properties of the nickel-steel alloys already referred to.
The permeability of the alloys containing from 1 to 4’7
per cent, of nickel, though less than that of good soft
iron for magnetizing forces up to about 20 or 30, was
greater for higher forces, the induction reached in a field
of 240 being nearly 21,700. The induction for consider¬
able forces was found to be greater in a steel containing
73 per cent, of nickel than in one with only 33 per cent.,
though the permeability of pure nickel is much less than
that of iron.
The magnetic qualities of various alloys of iron have
been submitted to a very complete examination by
Barrett, Brown, and Hadfield (Trans. Roy. Dub. Soc.
vol. vii. (1900), p. 67; Journ. Inst. Elec. Eng.,
1902).8 More than fifty different specimens were
tested, most of which contained a known proportion
of manganese, nickel, tungsten, aluminium, chromium,
copper, or silicon; in some samples two of the sub¬
stances named were present. Of the very numerous
results published, a few of the most characteristic are
collected in the following table. The first column contains
the symbols of the various elements which were
added to the iron, and the second the percentage pro¬
portion in which each element was present; the sample
containing 0’03 per cent, of carbon was a specimen of the
best commercial iron, the values obtained for it being
given for comparison. All the metals were annealed.
Element.
C
Cu
Mn
Mn
Ni
Ni
Ni
W
A1
Cr
Si
Si
Per Cent.
0’03
2’5
2’25
15’2
3’82
19’64
31’4
7’5
2’25
3’25
2’5
5’5
B
for H=45.
16800
14300
14720
0
16190
7770
4460
15230
16900
16420
15980
B
residual.
9770
10410
10460
9320
4770
1720
13280
10500
4080
3430
1625
1080
1375
90
357
500
1700
1680
1630
Coercive
Force.
1’66
5’4
6’0
2’76
20’0
0’5
9’02
10
12’25
0-9
0-85
7 See also Hopkinson, Journ. Inst. Elect. Eng. vol. xix. p. 20,
and Ewing, Phil. Trans, vol. clxxx. (1889), p. 239.
8 Many of the figures which, through an error, were inaccurately
stated in the first paper are corrected in the second.