40 



Professor J. A. Fleming 



[Feb. 14, 



galvanometer current is very much less tlian when the shield is shaken 

 on one side and the plate exposed fully to the negative leg. 



At this stage it will perhaps be most convenient to outline briefly 

 the beginnings of a theory proposed to reconcile these facts, and 



leave you to judge how far the sub- 

 PjQ 7 sequent experiments confirm this 



hypothesis. The theory very briefly 

 is as follows: — From all parts of 

 the incandescent carbon loop, but 

 chiefly from the negative leg, car- 

 bon molecules are being projected 

 which carry with them, or are 

 charged with, negative electricity. 

 I will in a few moments make a 



Glow-lamp having mica shield S 

 interposable between middle plate 

 M and negative leg of carbon, 

 thereby diminishiug the "Edison 

 effect." 



suggestion to you which may point 

 to a possible hypothesis on the 

 manner in which the molecules ac- 

 quire this negative charge. Sup- 

 posing this, however, to be the case, 

 A Q— ^ and that the bulb is filled with 



7 \^ these negatively-charged mole- 



^— H-\ cules, what would be the result of 



introducing into their midst a con- 

 ductor such as this middle metal 

 plate which is charged positively ? 

 Obviously, they would all be 

 attracted to it and discharge against 

 it. Suppose the positive charge 

 of this conductor to be continually renewed, and the negatively- 

 charged molecules continually supplied, which conditions can be 

 obtained by connecting the middle plate to the positive electrode of 

 the lamp, the obvious result will be to produce a current of electricity 

 flowing through the wire or galvanometer, by means of which this 

 middle plate is connected to the positive electrode of the lamp. If, 

 however, the middle plate is connected to the negative electrode of 

 the lamp, the negatively-charged molecules can give up no charge to 

 it, and produce no current in the interpolated galvanometer. We see 

 that on this assumption the effect must necessarily be diminished by 

 any arrangement which prevents these negatively-charged molecules 

 from being shot oif the negative leg or from striking against the 

 middle plate. Another obvious corollary from this theory is that 

 the " Edison effect " should be annihilated if the metal collecting 

 plate is placed at a distance from the negative leg much greater than 

 the mean free path of the molecules. 



Here are some experiments which confirm this deduction. In 

 this bulb (Fig. 8) the metal collecting plate, which is to be connected 

 through the galvanometer with the positive terminal of the lamp, is 

 placed at the end of a long tube opening out of and forming part of 



