582 Professor Sir J. J. Thomson [April 2, 



of the discharge-tube, showing that in this part of the discharge the 

 electric force is negative, while when the path of the cathode rays 

 passed through the bright part of a striation there was a large 

 deflexion of the cathode rays from the cathode of the discharge-tube, 

 showing that in this part of the discharge the electric force was 

 strongly positive. The change from the small negative deflexion to 

 the strong positive one was exceedingly abrupt, so much so that 

 when the anode and cathode were moving downwards, owing to the 

 sinking of the float supporting them, and one striation after another 

 was thus being brought across the path of the cathode rays, the 

 phosphorescent spot moved as abruptly as if it had been struck by a 

 l3low when the bright head of a striation crossed its path. At the 

 low pressures at which these observations are made the potential- 

 difference between the electrodes when the current is large enough 

 to produce striations is exceedingly small, often not exceeding 60 or 

 70 volts. Under these circumstances the negative forces although 

 unmistakable are small : when, however, the current through the 

 tube is reduced until the discharge is no longer striated, the potential- 

 difference between the electrodes is very much increased, and now 

 large negative forces can be observed in the neighl)ourhood of the 

 anode. Sometimes the region in which the force is negative extends 

 a considerable distance from the anode : in one case I observed a 

 negative force for two-thirds of the distance from the anode to the 

 cathode. 



As the corpuscles in the cathode rays have an exceedingly small 

 mass they are able to follow very rapid variations in the electric held ; 

 by means of them we can observe the gradual establishment of the 

 steady state of the discharge and the change in the direction of the 

 electric force at certain places from positive to negative. Thus 

 suppose the steady current through the tube is small and the 

 potential-difference is considerable, and that the pencil of cathode 

 rays is passing through the discharge near the anode, then if we 

 watch the behaviour of the phosphorescent spot in the interval 

 immediately following the application of the potential-difference to 

 the tube, we shall find that when the current first starts through the 

 tube the spot is repelled from the cathode, showing that at this stage 

 the electric force is positive throughout the tube. This repulsion of 

 the cathode rays is however only momentary : the spot jumps back, 

 and after a very short interval the spot is attracted towards the 

 cathode, showing that the force in this region is now negative. 

 Thus during this interval the ions in the gas and those clinging to 

 the walls of" the tube have rearranged themselves in such a way as to 

 reverse the force in the field. This momentary deflexion is much 

 more perceptible near the anode than some distance away from it ; 

 the rearrangement seems to spread from the cathode, and to be 

 established so rapidly close to that electrode that there is no time to 

 observe it, while as we travel away from the cathode the steady state 



