FAILURE OF OHM's LAW AT HIGH CURRENT DENSITIES. 165 



Rayleigh's formula for a conductor of circular section to a conductor 

 of the same area of section as that of the specimens actually used. 

 At the upper limit of frequencies used here the skin effect for such a 

 circular conductor is only one part in 10^-, and is hence absolutely 

 not to be considered. 



TJw Electrostriction Effect. The pinch effect is well known in con- 

 ductors carrying heavy currents, and it might be expected that under 

 the high current densities of these experiments there might be mechani- 



FiGtJRE 12. Collected results for silver 2X10"* cm. thick. Abscissae, 

 current density in 10" amp/cm^; ordinates, extrapolated difference between 

 A.C. and D.C. resistance in per cent of D.C. resistance. The numbers in 

 the body of the diagram show the breadth of the various samples in mm. 



cal strains set up in virtue of the magnetic action of the current ele- 

 ments on each other which would alter the resistance. But it is to be 

 noticed that the pinch effect depends on the total current carried|by 

 the conductor and not on its density. The currents carried here are 

 only of the order of 1 ampere, and numerical calculation shows that 

 any such effect is beyond the possibility of detection. 



Variation of Resistance with Frequency. The electron theory pre- 

 dicts a change of resistance at high frequencies independent of the skin 



