On the Controversy concerning VoltcCs Contact Force. 351 

 Put T=0 when L = l, 



But w = ~Lm, 



Lm v ' 

 and 



JL_ T_£/L-Z\ 

 L 2 "™ - ^ L / 



Also for a vibrating string 



1 /T 



V L 



The value of this expression increases very slowly with 

 increase of L if the latter is several times /. This appears to 

 be the explanation of the remarkable constancy in pitch of 

 the note given by the vibration of a rubber cord under in- 

 creasing tension *. 



King Edward's School, Birmingham. 



XXXIII. On the Controversy concerning Volta's Contact 

 Force. By Professor Oliver Lodge, B,Sc, LL.D., F.R.S.'t 



Chapter I. 



Thermodynamic Arguments. 



rpHOSE who take what I may call a metallic view of the 

 X Volta contact-force are accustomed to deny that the 

 Peltier evolution of heat measures the local E.M.F. existing 

 at a junction ; they assert instead that it measures the rate at 

 which that same E.M.F. varies with temperature : — 



II-T— 



Let us examine this assertion. 



* Since this was written, the author's attention has been directed to a 

 paper on the same subject by von Lang- (Wied. Ann. vol. lxviii.). The 

 constancy of pitch is briefly mentioned as due to an observed propor- 

 tionality between the tension and the stretched length of the rubber cord. 

 The paper deals chiefly with small discrepancies between the frequencies 

 observed and those calculated from Taylor's formula. 



f Communicated by the Physical Society, being the Presidential 

 Address for 1900. 



