u 



Professor J. A. Fleming 



[Feb. 14, 



the battery must exceed a certain value. Here, however, we have 

 indication that if the negative electrode by which that current seeks 

 to enter the vacuous space is made incandescent the current will pass 

 at a very much lower electromotive force than if the electrode is not 

 "^o heated. 



Fig. 11. 



Current from Clark cell Ck being sent 

 across vacuous space between negative 

 leg of carbon and middle plate M. Posi- 

 tive pole of cell in connection with plate 

 M thjough galvanometer G. 



Fig. 12. 



Experiment showing that when the 

 " middle plate " is a carbon loop 

 rendered incandescent by insulated 

 battery B, a current of negative elec- 

 tricity flows from M to the positive 

 leg of main carbon C across the 

 vacuum. 



A little consideration of the foregoing experiments led to the 

 conclusion that in the original experiment, as devised by Mr. Edison, 

 if we could by any means render the middle plate very hot, we should 

 get a current flowing through a galvanometer when it is connected 

 between the middle i)late and the negative electrode of the carbon. 

 This experiment can be tried in the manner now to be shown. Here 

 is a bulb (Fig. 12) having in it two carbon loops ; one of these is of 

 ordinary size, and will be rendered incandescent by the current from 

 the mains. The other loop is very small, and will be heated by a 

 well-insulated secondary battery. This smaller incandescent loop 

 shall be employed just as if it were a middle metal plate. It is, in 

 fact, simply an incandescent middle conductor. On repeating the 

 typical experiment with this arrangement, we find that the galvano- 

 meter indicates a current when connected between the middle loop 

 and either the positive or the negative terminal of the main carbon. 

 I have little doubt but that if we could render the platinum plate in 

 our first-used lamp incandescent by concentrating on it from outside 

 a powerful beam of radiant heat we should get the same result. 



