1890.] on Problems in the Physics of an Electric Lamp, 



45 



A similar set of results can be arrived at by experiments with a 

 bulb constructed like an ordinary vacuum tube, and having small 

 carbon loops at each end instead of the usual platinum or aluminium 

 wires. Such a tube is now before you (see Fig. 13), and will not 



Fig. 13. 



M^^ 



^A 



Vacuum tube having carbon loop electrodes, cc, at each end rendered incandescent 

 by insulated batteries B^ B^, showing current from Clark cell, Ck, passing 

 through the high vacuum when the electrodes are incandescent. 



allow the current from a few cells of a secondary battery to pass 

 through it when the carbon loops are cold. If, however, by means 

 of well insulated secondary batteries we render both of the carbon 

 loop electrodes highly incandescent, a single cell of a battery is 

 sufficient to pass a very considerable current across that vacuous 

 space provided the resistance of the rest of the circuit is not large. 

 We may embrace the foregoing facts by saying that if the electrodes, 

 but especially the negative electrode, which form the means of 

 ingress and egress of a current into a vacuous space are capable of 

 being rendered highly incandescent, and if at that high temperature 

 they are made to differ in electrical potential by the application of a 

 very small electromotive force, we may get under these circum- 

 stances a very sensible current through the rarefied gas. If the 

 electrodes are cold a very much higher electromotive force will be 

 necessary to begin the discharge or current through the space. 

 These facts have been made the subject of elaborate investigation by 

 Hittorf and Goldstein, and more recently by Elster and Geitel. It 

 is to Hittorf that I believe we are indebted for the discovery of the 

 fact that by heating the negative electrode we greatly reduce the 

 apparent resistance of a vacuum. 



Permit me now to pave the way by some other experiments for a 

 little more detailed outline of the manner in which I shall venture to 

 suggest these negative molecular charges are bestowed. This is 

 really the important matter to examine. In seeking for some probable 

 explanation of the manner in which these wandering molecules of 

 carbon in the glow-lamp bulb obtain their negative charges, I fall 

 back for assistance upon some facts discovered by the late Prof. 

 Guthrie. He showed some years ago new experiments on the relative 

 powers of incandescent bodies for retaining positive and negative 



