Ifu^trument for Measurinu the Contraction of Tie-hars. 417 



to a certain point, and then to diminish. This behaviour of 

 the spark suggests a resonance phenomenon. 



From these experiments it will be evident that for this 

 kind of a mercury-break any equation representing the 

 optimum capacity as a function of E, Rp, Rj, Re, and L will 

 be quite complicated. 



Summarizing these results we may make the following 

 statements with regard to the optimum capacity for the 

 particular kind of mercury-break used. 



It depends upon the sign of the breaking-pole. 



It is, in general, proportional to a power of the primary 

 current, greater than the square and less than the cube. 



It depends very markedly upon the resistances of the 

 connexions leading both to the break and to the condenser. 



It is a function of the inductance of the primary, but for 

 large inductances becomes practically independent of it. 



In conclusion, I wish to express my indebtedness to Mr. 

 Gordon Farnham, of the University of Cincinnati, for the 

 ^reat assistance which he has rendered me in this research. 



University of Cincinnati, 

 June 1902. 



LY. On an Instrument for Measuring the Lateral Contrac- 

 tion of Tie-bars, and on the Determination of Poisson's 

 Ratio. By J. MoREOW, M.Sc. (Vict.), Lecturer in Engi- 

 neering, University College, Bristol *. 



THE ratio of the lateral to the longitudinal strain in a bar 

 under the action of a simple pull or push in the direc- 

 tion of its axis has been the subject of much controversy and 

 a considerable amount cf research. Some French mathema- 

 ticians, including Poisson and Navier, held that this ratio 

 was 5- for all isotropic substances, their arguments were based 

 on a theory of the constancy of the ratio of the elastic co- 

 efficients for all materials. 



AA ertheim endeavoured to show that this theory was 

 false. He filled hollow prisms with liquid and subjected 

 them to a tensile force. The interior of the prism commu- 

 nicated with a capillary tube, and the change of volume of 

 the interior was measured by the distance the fluid advanced 

 or receded in the tube. If Poisson's ratio be denoted by a, 

 change of unit volume = tensile strain (1 — icr) very approxi- 

 mately. He maintained that the ratio <t should really be 



* Communicated by the Physical Society : read May 22, 1903. 

 Phil. Mag. S. 6. Vol. 6. No. 34. Oct. 1903. 2 E 



