38 



Professor J. A. Fleming 



[Feb. 14, 



the position of the plate with respect to the carbon loop. There is a 

 distance of half-an-inch, or in some cases many inches, between either 

 leg of the carbon and this middle plate. Setting the lamp in action, 

 I connect a sensitive galvanometer between the middle plate and the 



Fig. 4. 



Fig. 5. 



Sensitive galvanometer connected be- 

 tween the middle plate and positive 

 electrode of a glow-lamp, showing 

 current flowing through it when the 

 lamp is in action (" Edison eflect "). 



Mode of connection of galvanometer G 

 to middle plate if and carbon horse- 

 shoe' shaped conductor Cin the ex- 

 periment of the " Edison effect." 



negative terminal of the lamp, and you see that there is no current 

 passing through the instrument. If, however, I connect the terminals 

 of my galvanometer to the middle plate and to the positive electrode of 

 the lamp, we find a current of some milliamperes is passing through 

 it. The diagrams in Fig. 5 show the mode of connection of the 

 galvanometer in the two cases. This effect, which is often spoken of 

 as the " Edison effect," clearly indicates that an insulated plate so 

 placed in the vacuum of a lamp in action is brought down to the 

 same potential or electrical state as the negative electrode of the 

 carbon loop. On examining the direction of the current through 

 the galvanometer we find that it is equivalent to a flow of negative 

 electricity taking place through it from the middle plate to the posi- 

 tive electrode of the lamp. A consideration of this fact shows us that 

 there must be some way by which negative electricity gets across the 

 vacuous space from the negative leg of the carbon to the metal plate, 

 whilst at the same time a negative charge cannot pass from the metal 

 plate across to the positive leg. Before I pass away from this initial 

 experiment, I should like to call your attention to a curious effect at 

 the moment when the lamp is extinguished. Connecting the galvano- 

 meter as at first, between the middle plate and the negative electrode 



