1912-13. J Application of the Law of Torsional Oscillation. 191 
the form of wires, of length 10 cms. and diameter 1 mm. It was found 
that any casting outside the limits 92 per cent, copper with 8 per cent, 
aluminium on the one hand, and 88 per cent, copper with 12 per cent, 
aluminium on the other, was useless for drawing purposes ; but that within 
these limits wires could be fairly easily got with careful management. In 
all cases hut that of alloy (1) one value of a was found sufficient to group the 
points on one line, whilst a uniformly decreasing average value of a and n 
was got with decreasing percentage of copper, these percentages being 
determined only roughly. The values were : 
(1) 92 per cent. Cu- 8 per cent. A1 : a= 6 and 30 : n = 75 and T70 
(2) 90 „ Cu - 10 „ A1 : a = 20 : n = - 95 
(3) 89 „ Cu - 11 „ A1 : a=10 :« = *6 5 
(4) 88 „ Cu - 12 „ A1 : a = 5 : n=- 58. 
A specimen of Heusler alloy — of composition 62 per cent, copper, 25 per 
cent, manganese, and 13 per cent, aluminium — was also cast, but the alloy 
was found to be far too brittle to draw. 
Diagram III. shows the nature of the curve got with alloy (2). 
Effects of Magnetic Field on Constants for Iron. 
To induce magnetism in the wire it was held within a long coil through 
which a current was run, and a magnetic field thus established in the 
neighbourhood of the wire. After removal from the coil the wire under- 
went a process of “ ageing,” in order that a steady magnetic state should be 
reached. The plan adopted was to torsionally oscillate it in the torsion 
apparatus formerly described.* Preliminary experiments were made to see 
if this “ fatigue ” in itself would alter the constants. The specimen of iron 
wire taken gave originally two sets of values : 
A. a= 2 and B a =12 
n=l'7 5 n=' 70 
6 = 430 6=102-8. 
This wire was then subjected to ninety minutes torsional fatigue and 
again tested, giving the values : 
A. a = 0 B. a = 30 
n = 1*85 n= *60 
6= 169 6 = 92-5. 
The only constant seen to change much is the b of line A, i.e. for the large 
oscillations. Repeated experiments showed this large increase of b to be real. 
* Proc. Roy. Soc. Edin ., vol. xxxi. p. 440. 
