1883.] On the Electrical Resistance of Carbon Contacts. 



9 



9. Permanent Effect of Current upon Resistance. 



Under certain circumstances the current produces a permanent and 

 not merely temporary effect upon the resistance ; that is to say, the 

 resistance of the contacts (so long as they are undisturbed by vibra- 

 tion or otherwise,) is found to be lower or higher after the current 

 has ceased, than it was at first. 



Thus, for example, with a pressure of - 5 grm. at the point of 

 contact, the resistance measured with a current of one or two mil- 

 liamperes was found to be 10'5 ohms. A current of - 15 ampere was 

 then passed through the carbons for 10 seconds ; and on again testing 

 with the weak current, the resistance was found to have fallen to 5*8 

 ohms. With a pressure of 2 - 5 grms., the same current reduced the 

 resistance from 6'6 to 4*3 ohms. With 5 grms., the resistance was. 

 permanently reduced from 4*7 to 3'4 ohms ; and with 25 grms. 

 from 2*4 to 2'0 ohms. All these effects were produced by a current 

 of "15 ampere. Stronger currents (up to a certain point) effect 

 greater reductions. 



Table VIII. 



Smallest current 



ressure. with, which 



rise occurred. 



Grrms. Ampere. 



•05 -02 



•1 '05 



•15 10 



•2 -19 



•3 17 



•4 -29 



•5 -37 



# -40 



1-5 43 



2 -47 



3 -57 



4 -51 



5 -63 



But when the proportion of the current strength to the pressure 

 exceeds a certain limit, the effect of the current is apparently to pro- 

 duce a permanent increase in the resistance, and this increase is 

 generally very considerable. The carbon contacts, a tangent galvano- 

 meter, a box of resistance coils, and a battery of ten Leclanche cells 

 are connected in simple circuit, and the strength of the current 

 gradually increased by plugging out resistance in the box. As the 



