of a Mayneto or Induction- Coil. 



493 



metal terminals a gradually increasing current i and 

 measure the potential difference V between the terminals. 

 Then we shall obtain a relation of the form shown by the 

 full line in fig. 2. In the first stage, V increases with i, 



Ficr 2. 























A i 









C 





E L*S / 





0-4 

 C-3 

 0-2 

 0-1 



/" 









, * 







( 







s 













| 





s 

 s 



s 













J 



y 



s 















y 



s 

















20 30 



ifamp.) 



4-0 



it then becomes independent of i over a considerable range, 

 and later begins to increase once more. The values of i x 

 and i 2} corresponding to the points A and B on the curve r 

 and the constant value of V (V ) between A and B, vary 

 greatly with the nature of the terminals and the pressure 

 between them ; but the form of the curve is the same in all 

 cases investigated. 



But now suppose that, having increased i until the 

 point 0, lying between A and B, is reached, we begin to 

 reduce i again, taking great care not to break or shake 

 violently the contact ; then the values of V obtained will 

 not lie on the full line but on the straight dotted line passing- 

 through the origin. That is to say, the contact has now a 

 definite ohmic resistance, V /i c , determined by V and i c the 

 maximum value of the current which has been passed 

 through the contact. If the contact is broken momentarily 

 the relation between Y and i becomes once more that given 

 by the full line. 



No explanation of these observations is offered. It bus 

 been recorded before that the resistance of a contact depends 

 on the current which has been passed through it, and also 



