90 



Prof. J. A. Fleming on the 



nated by rendering its carbon incandescent. It is a very- 

 striking experiment to see a condenser charged with this 

 amount (250 microcoulombs) of electricity instantly discharged 

 when its positive coating is brought into connexion with the 

 middle plate of such a lamp. The discharge may be brought 



Fte. 17. 



70 ' SEcY. BATTERY 



To EARTH 



about either by joining up the positive side of the condenser 

 to the middle plate first, and then rendering the carbon of the 

 lamp incandescent by switching on the lamp, or the lamp 

 may be first of all illuminated and then the junction of the 

 condenser effected. In both cases the middle plate when 

 positively electrified is instantly discharged. 



It was found that if the lamp carbon is rendered incandescent 

 bva highly insulated secondary battery then in order to produce 

 the discharge, the plate of the condenser not in connexion 

 with the middle plate, and which is negatively charged, must 

 be somewhere connected with the battery circuit. It does 

 not matter, however, whether the wire from the negative 

 side of the condenser is in connexion with the positive or the 

 negative pole of the secondary battery actuating the lamp ; 

 all that is necessary is that the negative side of the condenser 

 should be in conducting connexion with the circuit of the 

 incandescent carbon. The experiment may be interpreted 

 by considering that this negative charge of the condenser 

 can escape out of the incandescent conductor and discharge 

 across the highly vacuous space to the positively electrified 

 cool middle metal plate; but that a positive charge cannot be 

 discharged out of the hot conductor, or, which amounts to the 

 same thing, a negative charge cannot discharge across from 

 the cool metal plate to the incandescent carbon which is 

 positively charged. We have then a unilateral conductivity 

 exhibited by this highly vacuous space bounded by two 



