Edison Effect in Glow Lamps. I 99 



electrode of the lamp and the cylinder Y embracing the top 

 part of the negative leg of the carbon (see fig. 22) ; but in the 

 last case the steady current sent by the cell across the vacuous 

 space between the cylinder and the hot carbon was Only about 

 a quarter as great in magnitude as when the cylinder X was 

 employed. There was the same kind of "kick" of the galvano- 

 meter on breaking the lamp-circuit. These experiments evi- 

 dently showed that the highly vacuous space between the hot 

 carbon traversed by its own current, which rendered it incan- 

 descent, and the insulated cylinder possessed a sort of unilateral 

 conductivity, negative electricity from a separate source of 

 small electromotive force being able to be forced through it 

 from the hot carbon surface to the cooler metal surface, but 

 not in an opposite direction. 



§ 29. In the above-recorded experiments the carbon con- 

 ductor was rendered incandescent by a unidirectional or con- 

 tinuous current in a highly perfect vacuum. In seeking for 

 an hypothesis to connect them together, it became essential to 

 ascertain how the effects would be modified if the vacuum 

 was imperfect and if the current was alternating instead of 

 continuous. 



Experiment 27. — The fundamental experiment was therefore 

 repeated with the normal type of lamp (No. 4), having a middle 

 metal plate placed symmetrically between the legs of the 

 carbon. A lamp of this type was set in action by an alternating 

 current of suitable strength and of which the frequency was 

 some 80 to 100 per second. On connecting the milampere- 

 meter between either of the electrodes of the lamp and the 

 middle plate, a continuous electric current was found flowing 

 through the galvanometer. The direction of this current was 

 such that positive electricity was found to flow from either 

 lamp terminal to the middle plate of the lamp. In other words, 

 a continuous current of negative electricity flowed out of the 

 middle plate to one or other of the two terminals of the lamp, 

 viz. to that terminal to which the other extremity of the gal- 

 vanometer was joined. Hence, since in this case each leg of 

 the carbon becomes in rapid succession positive and negative 

 when the lamp is- operated with an alternating current, the 

 unilateral effect observed of a current flowing' between the 

 middle plate and the positive leg, when the current through 

 the carbon is a continuous current, is here found to exist 

 equally between the middle plate and both terminals of the 

 lamp. This is only what might have been expected. The 

 potential of the middle plate is then not the same as that of 

 the base of either leg of the carbon, but something between 

 the two depending upon the position of the plate. 



H 2 



