COMMUTATION 



179 



The curves, Figs. 68 and 69, give respectively the contact re- 

 sistance and drop of potential for different current densities. 

 Two curves are plotted in each case: the one referring to a hard 



,0.05 



10.04 



.0.02 



0.01 



2.2 



L8 





1>4 



- 

 w 



- 8 



0.6 

 0.4 

 0.2 



20 80 40 50 60 70 



Current Density- Amperes per.Square Inch 



FIG. 68. Contact resistance, carbon brushes. 



Carbon 



10 



20 30 40 50 60 



Current Density Amperes per Square Inch 



70 



FIG. 69. Pressure drop at contact surfaces, carbon brushes. 



quality of carbon, and the other to the much softer electro- 

 graphitic carbon which has a lower resistance and may be worked 

 at a higher current density. An average pressure of 1.5 Ib. 



