1898.] Fall of Potential at the Electrodes, etc. 15 



The bottom K of the box is made of wood to permit the 

 entrance of the Rontgen rays, which pass from the source F 

 through the aluminium window C in the cover DU of a lead- 

 covered box which contains the induction coil and tube. 



The part of the space, in the box PP', that is exposed to the 

 rays can be regulated as desired by moving the lead strips L and 

 L', while observing the fluorescence on a screen placed above the 

 apparatus. 



The glass-bulb R contains aqua ammonia from which the tube 

 S conducts the ammonia gas into the box PP'. Fastened to the 

 cover of the latter are the two glass tubes T and T', into which 

 drops of hydrochloric acid are introduced. The particles of 

 ammonium chloride formed at the lower ends of the tubes, where 

 the acid is in contact with the ammonia in the air, descend in the 

 box with a slow velocity producing well-defined whitish streams a 

 and b near the plates A and B. 



While the air in the box is at rest the particles move vertically 

 downwards, and this motion is maintained when the plates are 

 kept at the same potential and the air between them is exposed 

 to Rontgen rays, or when the plates are at different potentials 

 and the air is not exposed to the rays. 



If however, while the plates are maintained at different 

 potentials, the whole space between them is exposed to the rays 

 the two streams are deflected or bodily carried each to its 

 neighbouring plate. 



When the rays are turned on, the streams of particles begin to 

 move from their initial position with a velocity that is accelerated 

 for an appreciable length of time, i.e. the particles do not assume 

 a uniform motion at all quickly. 



For a given intensity of the rays, the rate of the motion is 

 dependent upon the potential gradient between the plates. 



When the electric field is not too intense the streams assume 

 a steady state in oblique positions as indicated by a' and b' in 

 fig. 1. In this case the horizontal component of the motion is not 

 large compared to the vertical one. But if the gradient is made 

 much larger, the horizontal velocity may predominate greatly and 

 the streams are carried bodily to the plates. The potential differ- 

 ences used in these experiments ranged from 40 to 320 volts for a 

 distance of two centimetres. 



It becomes necessary to show that the movements indicated 

 by the streams are true indices of the actual motion of the gas, 

 that the gas is set into motion and carries the particles with it, 

 and that these are not themselves the primary cause of the motion. 



If the ammonium chloride particles became charged in some 

 way we might expect them to partake of movements somewhat 

 similar to those observed. 



