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



if V is the potential-difference between the gauze and the top of the 

 box 



thus 



m 



Thus when H increases through the vahie given by the equation 



H2(^^+^'Y- = 2V, (2) 



there ought to be a large increase in the ratio of the charge on the 

 Faraday cylinder to that on the disk. If the pencil of ions coming 

 through the hole were indefinitely thin, and if all the ions travelled 

 with the same velocity in the same direction, the transference of the 

 charge from the disk to the cylinder would be quite abrupt. With a 

 magnetic force less than a certain value, all the charge would be on 

 the disk, while with a force greater than this all the charge would be 

 on the cylinder. In my experiments the diameter of the hole, * 9 

 mm., was a considerable fraction of the length 1 • 6 mm. of the tunnel, 

 so that there was a considerable latitude in the direction of propaga- 

 tion of ions through the hole. This has the effect of making the 

 ratio of the charges on the cylinder and disk change much less ab- 

 ruptly than if they were all projected in the same direction, since 

 those ions which are projected towards the side of the cylinder to 

 which they are bent by the magnetic field, will be carried to the side 

 by a smaller magnetic force than those which are projected at right 

 angles to the disk. When the hole is very small, the charge carried 

 by the ions passing through it in a given time is also very small, and 

 the potentials of the disk and cylinder change very slowly. The pur- 

 pose for which these experiments were made was not so much to get 

 accurate values of e/m for the ions as to find out whether these had 

 masses comparable with the mass of an atom of hydrogen, or of 

 oxygen, etc. The arrangement used was adequate for doing this, 

 and had the advantage of giving a supply of ions which could pro- 

 duce measurable effects in a minute or so, thus avoiding many diffi- 

 culties as to insulation which crop up when the experiments have to 

 be extended over very much longer periods. Experiments with very 

 much smaller holes are, however, in progress. 



The strength of the magnetic field between the poles was deter- 

 mined by comparing the currents induced in a small coil when 

 suddenly withdrawn from the magnetic field with the current obtained 

 by turning an earth inductor through 180° in the earth's magnetic 

 field. When the pole-pieces were 1*15 cm. apart, the magnetic 

 forces H for different currents through the coils of the electromagnet 

 were as follows : — 



