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



43 



Fig. 10. 



Charged condenser C discharged by middle 

 plate 3f, when the positively charged side 

 of condenser is in connection with the 

 plate and other side to earth e. 



periment with the condenser discharged by the lamp may be then 

 looked upon as an arrangement in which the plates of a charged con- 

 denser are connected respec- 

 tively to an incandescent carbon 

 loop and to a cool metal plate, 

 both being enclosed in a highly 

 vacuous space, and it appears 

 that when the incandescent con- 

 ductor is the negative electrode 

 of this arrangement the dis- 

 charge takes place, but not when 

 the cooler metal plate is the 

 negative electrode of the charged 

 condenser. The negative charge 

 of the condenser can be carried 

 across the vacuous space from 

 the hot carbon to the colder 

 metal plate, but not in the re- 

 verse direction. 



This experimental result led 

 me to examine the condition of 

 the vacuous space between the 

 middle metal plate and the nega- 

 tive leg of the carbon loop in the 

 case of the lamp employed in our 



first experiment. Let us return for a moment to that lamp. I join the 

 galvanometer between the middle plate and the negative terminal of 

 the lamp, and find, as before, no indication of a current. The metal 

 plate and the negative terminal of the lamp are at the same electrical 

 potential. In the circuit of the galvanometer we will insert a single 

 galvanic cell having an electromotive force of rather over one volt. 

 In the first place let that cell be so inserted that its negative pole is 

 in connection with the middle plate, and its positive pole in connec- 

 tion through the galvanometer with the negative terminal of the 

 lamp (see Fig. 11). Regarding the circuit of that cell alone, we find 

 that it consists of the cell itself, the galvanometer wire, and that 

 half-inch of highly vacuous space between the hot carbon conductor 

 and the middle plate. In that circuit the cell cannot send any 

 sensible current at all, as it is at the present moment connected up. 

 But if we reverse the direction of the cell so that its positive pole is 

 in connection with the middle plate, the galvanometer at once gives 

 indications of a very sensible current. This highly vacuous space, 

 lying between the middle metal plate on the one hand, and the 

 incandescent carbon on the other, possesses a kind of unilateral con- 

 ductivity, in that it will allow the current from a single galvanic cell 

 to pass one way but not the other. It is a very old and familiar fact 

 that in order to send a current from a battery through a highly 

 rarefied gas by means of metal electrodes, the electromotive force of 



