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



yet I felt that the existence of a negative force could never be proved 

 by the nse of wire detectors. 



For let ns consider what the existence of a negative force implies. 

 The electric current is always in the same direction throughout the 

 tube, and therefore the average movement of the ions is in the same 

 direction at all parts of the tube ; thus, whenever the electric force is 

 negative, there must be ions moving against the electric force instead 

 of with it. Now the validity of the method of the wire electrodes 

 depends upon the assumption that the ions in the neighbourhood of 

 the tip of these electrodes follow the lines of force, that if, for exam- 

 ple, the tip were at a higher potential than the gas so that the force 

 on a positive ion were away from the tip, negative ions would follow 

 the direction of the force acting upon them, and run into the tip and 

 lower its potential until it became the same as that of the gas in its 

 neighbourhood. But if the ions do not follow the electric force, and 

 the existence of a negative force implies that some of them at any 

 rate do not, we have no right to assume that the potential of the wire 

 is the same as that of the gas. In some simple cases it is evident 

 that it would not be so. Thus suppose the wire were exposed to a 

 stream of cathode rays, and that there were no positive ions in its 

 neighbourhood, then it is evident that the wire would acquire the 

 potential of the cathode from which the rays started and not that of 

 the gas around the wire. 



For these reasons I felt that the existence of a negative force 

 could not be established by means of wire electrodes, and I adopted 

 an entirely different method of measuring the electric force along 

 the discharge-tube. The principle of this method is as follows : 

 Imagine a very fine pencil of cathode rays, travelling at right angles 

 to the line joining the cathode and anode, to pass through the 

 discharge-tube. As it crosses the discharge at any place it will be 

 acted upon by the electric force at the point of the discharge, and 

 will be deflected by an amount proportional to the electric force. 

 The deflexion will be from the cathode of the discharge-tube if the 

 force is positive, towards it if the force is negative. If very small 

 pencils of cathode rays are used the disturbance of the electric field 

 in the discharge-tube due to the negative charge on the rays is quite 

 insignificant, and there is none of that distortion of the striations 

 which, to a greater or less extent, always occur when exploring 

 metallic electrodes are used. 



The arrano-ement by which this principle is carried out in practice 

 is shown in Fig. 2. The cathode and anode are fastened together 

 by a piece of glass-rod and fastened to a float, floating on the top of 

 a mercury column. By raising or lowering the column the anode 

 and cathode can be moved up and down the discharge-tube. Tliis 

 arrangeiTient is the same as that used with the wire detectors and 

 shown in Fig. 8. The wires a b (Fig. 3.) were replaced by cathode 

 rays generated in the side tube S (Fig. 4) by a small induction-coil : 



