Edison Effect In Glow Lamps. 85 



Experiment 15. — The lamp was placed on a circuit so that 

 the leg shielded by the aluminium cylinder was the positive 

 leg. The milamperemeter was then connected between the 

 positive electrode of the lamp and the middle plate, and the 

 usual measurements made. It was found that the current 

 u jumped" a good deal, and that high and low values of the 



Fm. 14. 



galvanometer current occurred, even when the terminal 

 voltage of the lamp was kept perfectly constant. 



The lamp was then reversed on the circuit so that the 

 shielded leg was the negative one, all other arrangements 

 remaining the same. The current now between the positive 

 electrode and the middle plate was practically zero, at any 

 rate too small to be measured with this galvanometer. 

 Hence we see that shielding the negative leg, whether by 

 glass or a metallic cylinder, entirely cuts off the production 

 of a current between the positive lamp electrode and the 

 middle plate. 



§ 18. Experiment 16. — Another series of experiments was 

 made with the lamp No. 10 in which the galvanometer was 

 connected between the positive electrode of the lamp and the 

 aluminium cylinder (see fig. 15), the leg inside the cylinder 

 being either the positive or the negative leg. In this case 

 the middle plate remained unused and insulated and acted as 

 a shield between the cylinder and the carbon leg which was 

 not contained in the cylinder. 



It was found that when the cylinder surrounds the negative 

 leg and its surface is, therefore, as much exposed to it as 

 possible the current is a, maximum, but that when it includes 



