416 Mr. H. E. Wimperis on 



the three 3 -ohm coils to the bridge is different. It is easy to 

 do this thus: — Let the resistance between each of the adjacent 

 cups, such as I to A, A to B, B to C &c. be y ohms. Then 

 the resistances of the 3-ohm coils in the multiple arc position 

 are 3+a + 2y, 3+/3+<ly, and + 7+63/ respectively, and 

 neglecting squares of small quantities as before, we have 



B = l + J-(« + /3 + y + 12y). 



While the series value of the coils is easily seen to be 



B'=9 + «+£ + 7 + % 

 = 9B-3y. 



The value of y is found by connecting I. and II. to the 

 bridge, short-circuiting the other gap of the bridge and then 

 short-circuiting in turn AA', BB', OC/, and finding a series 

 of balance-points. The difference of the readings thus found 

 gives a. In the case of the apparatus used the mean of a 

 number of readings gave 



c( = *4 bridge-wire divisions ='00002 ohms. 



So that instead of putting B' = 9B in reducing the results it 

 would have been more accurate to write B' = 9B — '00006. 

 The correction is too small to be of importance. I also found 

 that at a temperature of 16° the value of the 3-ohm coils 

 were respectively 3'0038, 3'0026, and 3'0027. 



XL I. Experiments on the Elasticity of Wires. By H. E. 

 Wimperis, B.A., Whitworth Scholar, Late Salomons and 

 Foundation Scholar of Caius College, Cambridge, Demon- 

 strator in the University Engineering Laboratory*. 



IN the Philosophical Magazine for February 1900 Mr. G. 

 F. C. Searle described an apparatus he had used' to deter- 

 mine some elastic constants for wires of different metals. One 

 very curious result which he obtained was that the value of 

 Poisson^s ratio found .for certain metals was greater than \, 

 notably in the case of copper whether hardened or annealed. 



Mr. SeaiuVs deduction was that " wires of these four sub- 

 stances (copper, nickel, platinoid, and german-siiver) are so 

 far from being isotropic, that it is improper to apply the 

 theory of isotropic solids to them." 



It was suggested to the author by Professor Ewing that it 



* Communicated bv the Author. 



