1879.] 



Molecular Pliysics in High Vacua. 



trodes, experiments are described with an apparatus constructed ac- 

 cordingly. The potential of the idle poles (of which there are two) 

 at low exhaustions is very feebly positive ; as the exhaustion gets 

 better the positive potential increases, and at a vacuum so good as to 

 be almost non-conducting, the positive potential of the idle poles is 

 at its greatest. The result is that an idle pole in the direct line of fire 

 between the positive and negative poles, and consequently receiving 

 the full impact of the molecules driven from the negative pole, has a 

 strong positive potential. 



It is found that when the shadow of an idle pole is projected on a 

 phosphorescent screen, the trajectory of the molecules suffers deflection 

 when the idle pole is suddenly uninsulated by connecting it with earth. 

 The same result is produced by connecting the idle pole with the 

 negative wire through a very high resistance, such as a piece of wet 

 string, instead of connecting it with earth. A tube, which has already 

 been described in a paper read before the Royal Society on December 

 5th last, is used to illustrate this deflection. The shadow of an alumi- 

 nium star is projected on a phosphorescent screen. So long as the 

 metal star is insulated the shadow remains sharp, but on uninsulating 

 the star by connecting it with an earth wire the shadow widens out, 

 forming a tolerably well-defined penumbra outside the original shadow, 

 which can still be seen unchanged in size and intensity. On removing 

 the earth connexion the penumbra disappears, the umbra remaining 

 as before. 



It is also found that the shadow of the star is sharply projected 

 when it is made the positive pole, the negative pole remaining un- 

 changed. 



These experiments are explained by the results just mentioned, that 

 the idle pole, the shadow of which is cast by the negative pole, has 

 strong positive potential. The stream of molecules must be assumed 

 to have negative potential ; when they actually strike the idle pole 

 they are arrested, but those which graze the edge are attracted inwards 

 by the positive potential and form the umbra. When the idle pole is 

 connected with earth, its potential would become zero were the dis- 

 charge to cease ; but inasmuch as a constant supply of positive elec- 

 tricity is kept up from the passage of the current, we must assume that 

 the potential of the idle pole is still sufficient to more than neutralize the 

 negative charge which the impinging molecules would give it. The 

 effect, therefore, of alternately uninsulating and insulating the idle 

 pole is to vary its positive potential between considerable limits, and 

 consequently its attractive action on the negative molecules which 

 graze its edge. The result is a wide or a narrow shadow, according 

 to circumstances. 



After a definite shadow is produced, it is found that increasing the 

 exhaustion makes very little change in the umbra, but it causes the 



2 n 2 



