Electricity through rarefied (rases. 789 



further an J to investigate whether positive ions by striking 

 against the cathoile are really able to liberate electrons from 

 its surface, independently of the part which such electrons 

 may play in the process of the discharge itself. 



5. In such experiments the following method of three 

 electrodes was adopted : — The strip s, coated with aluminium 

 phosphate and strongly heated, was connected with the 

 positive terminal of the battery of small cells, the cathode P 

 being earthed through a galvanometer, and the third elec- 

 trode r raised to a positive potential higher than that of 

 the heated strip. The apparatus was kept well exhausted 

 throughout the experiments, so that the comparatively small 

 potential difference applied to the electrodes (not exceeding 

 2000 v.) could not produce a discharge through the bulb. 

 The positive ions emitted by the strip move towards the 

 cathode P and may produce electrons either hy collision with 

 the gaseous molecules or by impact with the surface of the 

 cathode. These electrons move necessarily towards the third 

 electrode r charged to a higher positive potential than the 

 strip, and the current carried by them is measured by a 

 galvanometer. The ratio of this current to that carried by 

 the po-itive ions to the cathode P, gives the rate of pro- 

 duction of electrons by the positive ions in the bulb. 



It was evidently necessary in the first place to inquire 

 into the validity of the method, since it may seem doubtful 

 whether all the electrons produced in the bulb wail actually 

 reach the electrode r. For this purpose the electrode r was 

 raised to different potentials, and curves were drawn show- 

 ing the current flowing through this electrode as a function 

 of the potential difference between it and the heated strip, 

 other conditions remaining the same. These curves appeared 

 to be similar in shape to the well-known saturation curve in 

 ionized gases, the current increasing at first rapidly with 

 the potential difference and finally remaining constant. A 

 potential difference of 200 v. appeared sufficient to assure 

 the saturation being reached, and in most of the experiments 

 to be described the electrode /' was raised to a potential of 

 4-2<>00 v., the strip being charged to -f 1800 v. Further, 

 it was found that the current received by the third electrode 

 was strictly proportional to the positive emission from the 

 strip, which could be varied within wide limits by changing 

 the temperature of the latter, so that the rate of production 

 of electrons by the positive ions in the bulb was constant at 

 a given pressure. 



The experiments consisted mainly in studying the variation 

 in the rate of production of electrons with the pressure, in 



