Edison Effect in Glow Lamps. 71 



§ 9. Experiment 6. — In order to confirm the results ob- 

 tained by the condenser method and to eliminate all the 

 conditions which necessarily exist when we attempt to 

 measure potential-difference galvanometrically, an electro- 

 static method of measuring the potential-difference at any 

 instant between the metal plate and the positive electrode of 

 the lamp was next used. For this purpose a Kelvin multi- 

 cellular electrostatic voltmeter was employed to determine 

 the potential-difference between the positive and negative 

 electrodes of the lamp and between the positive electrode of 

 the lamp and the middle plate, with the following results : — 



A lamp (No. 4) having the plate fixed between the carbon 

 legs was raised to various working voltages and the potential- 

 differences above mentioned taken. 



Table No. 7. Lamp No. 4. Kelvin Electrostatic 

 Voltmeter. 



Working volts of the lamp. 



Static potential- difference in 



volts between middle plate 



and positive electrode of 



the lamp. 



41 



58-1 



61-0 



41 



58-7 



61-5 



These observations confirm conclusively the previous re- 

 sults. The insulated metal middle plate is in this case brought 

 to the same potential as the base of the negative Jeg of the 

 carbon ; and hence, on measuring electrostatically the poten- 

 tial-difference between that metal plate and the positive 

 electrode of the lamp, we find it to be the same as the poten- 

 tial-difference between the two electrodes of the lamp. 



§ 10. Experiment 7. — In order to see if this was the case 

 when the metal collecting-plate had a very small surface 

 placed at some distance from the negative electrode of the 

 lamp, the lamp called No. 1 was employed. In this lamp a 

 platinum wire threaded through the turns of a double spiral 

 100 volt carbon lamp (see § 11). The lamp was raised to 

 various working voltages, and the electrostatic voltmeter 

 employed to measure at the same time the static potential- 

 difference between the positive electrode of the lamp and the 

 platinum wire, with the following results : — 



