﻿804 Mr. K. K. Smith on Negative 



could be surrounded with a Dewar flask containing liquid 

 air. The pressure indications were read on the McLeod 

 gauge, G. By raising or lowering the mercury at T, the 

 lamp and gauge could be shut off from the vacuum-pump, or 

 connected to it. The tube C was half filled with coconut 

 charcoal, and could be surrounded with liquid air when it 

 was desired to reduce the pressure as much as possible. A 

 bulb, P, containing phosphorus pentoxide, was attached 

 between C and the Gaede mercury pump, and auxiliary 

 pump. The furnace itself was evacuated by a separate 

 pump. 



The lamp was exhausted and heated at the same lime. 

 The furnace reached a maximum temperature of about 

 600° C, which was maintained for several hours in order to 

 get rid of absorbed gases. At first considerable quantities 

 of gas were given off, but the pressure gradually decreased 

 with prolonged heating, and finally became quite small. 

 The furnace was then allowed to cool slowly, liquid air was 

 applied at U and C, and the filament was heated by an 

 electric current. The cylinder was charged positively, so 

 as to be bombarded by electrons from the filament. At 

 intervals the amount of gas still being driven off was. 

 estimated by closing the trap T for five minutes, and noting 

 the increase of pressure, if any. The details of some of 

 these experiments, which have already been published * r 

 show that the observed thermionic currents were too large 

 to be ascribed to the evolution of gas from the filament, or 

 to an action depending upon impacts between the gas mole- 

 cules and the filament. After the pressure indicated on the 

 gauge had become practically inappreciable, the lamp was 

 either sealed off immediately and removed, or the observa- 

 tions were taken on the unsealed lamp. 



Measurement of Currents. 



The lamp filament was usually made one arm of a Wheat- 

 stone's bridge. The positive terminal of a battery was 

 connected to the receiving electrode, and the other terminal 

 to the positive end of the filament. An electrometer having 

 a sensibility of 790 divisions per volt, and a capacity of 130 

 electrostatic units, was used to measure the smallest currents. 

 A condenser whose capacity could be varied from O'OOl 

 microfarad to 1 microfarad was used in parallel with the 

 quadrants when necessary. With larger currents, resistances- 

 varying from .100 ohms to 1 megohm were put in the 



* Richardson, loc. cit. 



