186 Professor Sir J. J, Thomson [April 10, 



between the gauze and the top of the cylinder, the gauze being posi- 

 tive to the cylinder. The ions collected by the upper plates thus 

 entered into a much stronger field which gave to them a velocity 

 much greater than that with which they entered it, so that when they 

 passed through the hole they were all moving with practically the 

 same velocity. 



Beneath the top of the box there was an insulated Faraday cylinder 

 (P) connected with a Wilson electroscope. The distance between 

 the top of the cylinder and the bottom of the plate was 1 mm. in one 

 piece of apparatus, * 5 mm. in anotlier ; the diameter of the hole in 

 the Faraday cylinder was 2 * 3 mm. Beneath this hole there was a 

 metal disk insulated and connected with another Wilson electroscope; 

 the plane of the disk was parallel to X and thus at right angles to 

 the undeviated path of the ions, the axis of the hole in N passed 

 through the centre of the disk. The part of the tube below N was 

 placed between the poles of a powerful electromagnet, the lines of 

 magnetic force being at right angles to- the undeviated path of the 

 ions, and parallel to the direction of the cathode rays. To protect 

 the cathode rays coming from D from the magnetic field, a deep cut- 

 ting was made in one of the poles of the electromagnet and the por- 

 tion from C to D of the tube placed in this and then covered over 

 with layers of soft iron. This tube was sealed on to the tube T of 

 the kind already described, for the determination of ejm for the 

 Canalstrahlen. 



If the ions travelled without deviation parallel to the axis of the 

 tunnel in N, tliey would all strike against the disk Q, and the Faraday 

 cylinder would not receive any charge. If, on tlie other hand, they 

 were very much deflected by the magnetic field, they would all strike 

 against the Faraday cylinder and the disk would not receive any 

 charge. If we measure the charges received by the disk and the 

 Faraday cylinder during any time, the ratio of the charges will be 

 the ratio of the number of ions which strike against the disk to the 

 number striking against the cylinder. The readings of the electro- 

 scopes do not give us directly the charges received by the systems to 

 which they are attached, but the potentials to which these systems 

 are raised. We can, however, if we know the ratio of the potentials 

 easily deduce that of the charges. For let E^Vj be the charge and 

 potential of the Faraday cylinder, E.„ Vo the corresponding quantities 

 for the disk. 



Thus if the ^''s represent coefficients of capacity, we have 



so that 



