1883.] On the Electrical Resistance of Carbon Contacts. 17 



The points of contact were, of course, changed for each series of 

 observations, and it should be noted that in the case of the last three 

 (which were not made on the same day as the others) I was less 

 successful in arranging the lever so as to secure perfect freedom from 

 oscillation when the weight was changed. 



16. Summary. 



The following appear to be the most important results of the 

 investigation. 



(1.) Carbon Contacts. 



Changes of pressure produce proportionately greater changes of 

 resistance when the initial pressure is small than when it is great. 



Changes of pressure produce proportionately greater changes of 

 resistance with weak currents than with strong currents. 



Changes of current produce proportionately greater changes of 

 resistance with small currents than with large currents ; and with 

 light pressures than with heavy pressures. 



When the resistance of a carbon contact has been reduced by an 

 increase of pressure, it will, on the removal of the added pressure, rise 

 to approximately its original value. 



The passage of a current whose strength does not exceed a certain 

 limit, depending upon the pressure, causes a permanent diminution 

 in the resistance, and the stronger the current the greater will be 

 such diminution. 



When the strength of the current exceeds a certain limit the 

 resistance is greatly and permanently increased. The greater the 

 pressure the higher will be such limit. 



Unless special means are adopted for preserving a constant current, 

 the fall in the resistance which attends increased pressure is greater 

 than that which is due to increased pressure alone ; being partly due 

 also to increased strength of current. 



It is not proved that the diminished resistance which follows an 

 increase of current is an effect of temperature. 



(2.) Metallic Contacts. 



In the case of bismuth and probably of other metals — 



With a given pressure the weaker the current the higher will be 

 the resistance. This effect is most marked when the pressure is small. 



Increase in the strength of the current is accompanied by a fall in 

 the resistance, and if the current be again reduced to its original 

 strength, the resulting change in the resistance will be small, and it 

 will in no case return to its original value. 



Diminution in the strength of the current is followed by a small 



VOL. XXXV. 



