790 Mr. S. Ratner on the Nature of Discharge of 



the bulb. If the electrons present in the bulb originate 

 solely in ionization by collision between the positive ions 

 and gaseous molecules, then the rate of production of the 

 electrons should be expected to vary inversely as the mean 

 free path of the positive ions, i. e. it should be strictly pro- 

 portional to the pressure in the bulb and should become 

 negligibly small when the bulb is highly exhausted. If, 

 however, electrons are also produced by impact of the 

 positive ions against the surface of the cathode, then no such 

 proportionality would be expected, and the current through 

 the third electrode should remain considerable even when the 

 highest exhaustion is reached. 



In the estimation of the rate of production of electrons by 

 positive ions an important correction must be introduced, the 

 omission of which led to misleading results in the earlier 

 experiments. When a molecule is ionized by collision a pair 

 of ions is. produced, so that the current through the plate P 

 is carried not only by the positive ions from the heated strip, 

 but also by the positive ions produced by collision. Also 

 when an electron is set free from the surface of the cathode, 

 a positive charge is gained by the cathode. If we denote by 

 C/_) the current through the third electrode, by C (+) the 

 current through the cathode P, and by R the rate of pro- 

 duction of electrons by the positive ions from the strip, then 



c _ 



it is easy to see that R is given not by >~--\ as at first anti- 



Q M+) 



cipated, but by -^ — ( ] n . This correction is small when 



M+)~~ u (-) 

 the bulb is well exhausted and consequently C ( _ } is small 



compared with C (+) . 



As a result of a large number of experiments carried out 

 in this way, it was shown distinctly that the electrons present 

 in the bulb are produced only by collision between the 

 positive ions and the gaseous molecules, and that there is 

 no appreciable electronic emission from the surface of the 

 cathode. TaWe I., in which the results of one set of these 

 experiments are given, shows that the law of proportionality 

 between R and the pressure p holds well within the limits of 

 experimental error. 



It was noticed, however, in some experiments that when 

 the strip was raised to a very high temperature, a dispro- 

 portionately large current flowed through the third electrode, 

 even at the highest obtainable exhaustion of the bulb. This 

 was finally found to be caused by a source of error which 

 can, however, be easily eliminated. It was found that a 

 strongly heated platinum strip coated with aluminium phos- 

 phate or any other salt serving as a source of positive ions, 



