1811 



1894.] Experimental Determination of Poissons Ratio. 373 



This gives H = 0'1952. t = 286. 



Hit = 6-83 x 10-*. 



This the last, and, I believe, the best, result, is almost exactly 

 equal to d- for the solution. 



With the last apparatus and a solution of nitrate of copper, for 

 ; which was measured and found = 6*14, a perfect balance in both 

 directions was obtained 



with C = 245 potentiometer 



R = 1-05 

 S = 11-695 



H = 0-1764. * = 288. 

 Hft = 6-1 x 10- 4 . 



I should have liked to do a few more solutions, but something went 

 wrong with the insulation of the bobbins, and I had no time to repair 

 them. However, these two results appear to be enough to enable us- 

 to say that the equation 



dE[dt = Hft 



is true for the junctions of the kind under examination, and that 

 these thermo-electric phenomena are reversible. 



VII. " Experimental Determination of Poisson's .Ratio." By 

 C. E. STROMEYER. Communicated by LORD KELVIN, P.R.S. 

 Received April 12, 1894. 



The experiments with which this paper deals were carried out 

 between the years 1883 and 1886 by Professor Kennedy and the 

 author, with an instrument which the latter had originally designed 

 for measuring local strains in metal structures, but which proved 

 itself to be so exceedingly sensitive that it seemed capable of being 

 applied to the measuring of the cross contraction of test pieces while 

 these were subjected to a longitudinal pull, thus providing the means 

 for measuring Poisson's ratio direct. In its original form the instru- 

 ment consisted of two small frames, which were secured to each 

 other by means of two flat springs, in such a manner, that any relative 

 motion was a perfectly parallel one. One of these frames carried a 

 small piece of dark glass, and close to it, but on the other frame> a 

 right-angled reflecting glass prism was secured. The two glass sur- 

 faces, which faced each other, were then carefully adjusted, so as to- 



2 D 2 



