1883.] On the Electrical Resistance of Carbon Contacts. 5 



It is clear therefore that the diminution of resistance which occurs 

 Avhen the pressure between carbon contacts is increased is not, under 

 ordinary circumstances, due solely to the direct effect of pressure. 

 For the diminished resistance which results from the increased 

 pressure causes an increase in the strength of the current, the effect; 

 of which is a still further fall in the resistance. Thus the total 

 diminution of resistance is due partly to increase of pressure, and 

 partly to increased strength of current. 



5. Variation of Resistance with Varying Pressure and Constant Current. 



Table IV and the curves in fig. 3 show the results when special 

 means were taken to maintain constant currents of '1 and '001 

 ampere respectively, the pressure at the contacts increasing from '05 

 to 25 grms. 



Table IV. — Effect of Pressure upon Resistance of Carbon Contacts 

 with Constant Currents. 



Pressure. 

 Grms. 



Eesistance in olirns with 

 current of 



"1 ampere. 



•05 11 



•25 9 



•5 8 



1 6 



15 5 



2 5 



2 5 4 



3 4 



3 5 4 



4 3 



4-5 3 



5 3 



7 5 2 



10 2 



15 2 



20 1 



25 1 



•ooi 



ampere. 



02 68-00 



•27 25 50 



•45 17 75 



•56 11-75 



•53 9 75 



•34 7-50 



•57 6-50 



•15 5 85 



•03 5-70 



•63 5-70 



•41 4-95 



•31 4-95 



•95 3 65 



•51 3 15 



10 2 45 



•89 2-10 



•67 1-75 



It will be seen that with small pressures the resistance is largely 

 dependent upon the strength of the current, but when the pressure is 

 considerable, the resistance with weak and strong currents is nearly 

 the same. 



