96 



Prof. J. A. Fleming on the 



descent simultaneously the condenser in any case was dis- 

 charged, but apparently at an accelerated rate. These 

 experiments show again that if two carbon electrodes are 

 sealed into a high vacuum, negative electricity escapes very 

 freely out of either electrode if it is rendered incandescent, 

 but that the escape or discharge of positive electricity is not 

 in the same way facilitated by heating the positive electrode. 

 Accordingly a highly vacuous space bounded by two carbon 

 electrodes separated by a distance less than the mean free 

 path of the gaseous molecule at that pressure, presents a 

 unilateral conductivity when one of these electrodes is cold 

 and the other highly incandescent. For if the hot electrode 

 is connected to a negatively charged body and the cold 

 electrode to a positively charged body, discharge takes place 

 across the vacuous space, but if the charges are reversed then 

 no discharge takes place. The negative charge can escape 

 from the heated electrode but not from the cold one. 



§ 26. Experiment 24. — The question of the apparent uni- 

 lateral conductivity of the vacuous space bounded by a hot 

 and a cold electrode was then further examined by the aid of 

 the lamp No. 6 formerly used. 



In this lamp an aluminium plate is sealed into the vacuum 

 and placed just outside the carbon horse-shoe. If a sensitive 

 galvanometer (the high resistance Elliott galvanometer) is 

 joined up between the metal plate and the negative electrode 

 of the lamp, then, as in other cases when the lamp is in action, 



Fig-. 21. 



no current of a magnitude much greater than *0001 of a 

 milliampere is detected. If a single Clark standard cell is 

 inserted in the galvanometer circuit (see fig. 21) with its 

 negative pole attached to the middle plate and its positive 



