1883.] On the Electrical Resistance of Carbon Contacts. 7 



7. The Effect of Pressure on the Resistance of Carbon is Temporary. 



In order to ascertain whether the diminution of resistance under 

 ■pressure was entirely of a temporary nature, or whether it continued 

 in any degree after the pressure had been removed (the carbons of 

 course being undisturbed) a special arrangement was made by means 

 of which a 5 grm. weight could be placed in or taken out of the scale- 

 pan without causing any appreciable oscillation. The weight was 

 attached by a thread to one end of a horizontal lever, the lever being 

 so supported that when the other end of it was depressed the weight 

 was raised about a centimetre above the pan; this could be done 

 without in any material degree disturbing the carbon contacts. 



A weight of 1 grm. being in the scale-pan, the resistance of the 

 contact was measured with the Wheatstone's bridge, and one 

 Leclanche cell ; the 5 grm. weight was then lowered into the pan, and 

 a measurement made of the diminished resistance. The 5 grm. 

 weight was again gently raised, and the resistance with a single 

 gramme once more measured. In every case it was found to have 

 returned almost exactly to its original value. Table VI gives six 

 series of measurements thus made. 



Table VI. — One Leclanche. Proportional Coils 10 and 1,000 ohms. 









Resistance. 







Pressure. 















Grins. 

















Ohms. 



Oil ms. 



Ohms. 



Ohms. 



Ohms. 



Ohms. 



'5 



19 



20 



20 



19-6 



193 



19-2 



3 



25 



15 



11 



13 -9 



13-7 



13 6 



'5 



20 



20 



20 



19 9 



19'5 



20-0 



The experiment was repeated with a resistance of 300 ohms between 



