The motion of electrons in gases

1. The methods of investigating the motion of negative ions in gases at low pressure that have been explained in some previous papers may be extended to cases in which larger variations are made in the electric force and pressure. In order to find the kinetic energy of the motion of agitation of the ions, the velocity in the direction of an electric force, and the value of e / m for different forces and pressures, it is necessary to investigate experimentally two properties which are characteristic of the motion of electrons. These are the abnormal lateral diffusion of a stream of ions moving in a uniform electric field, and the deflection of the stream produced by a small transverse magnetic force. In the previous experiments the two phenomena were investigated separately and in each case with apparatus which gave satisfactory results when small electric forces were used and the pressures were limited to a certain range. In order to investigate the motion under larger forces an apparatus of more suitable dimensions was constructed, by means of which both the required sets of experiments may be made. 2. The negative ions were generated by the action of ultra-violet light on the plate A, fig. 1, and after traversing the distance from A to B some of the ions passed through a narrow slit S, 2 mm. wide and 15 mm. long, in the centre of the metal sheet B. The electric force was in the same direction on the two sides of B, so that the ions, after passing through the slit, continue their motion towards the plane electrodes C, which were parallel to the plane of B. The electrodes C were 4 cm. from B, and three flat rings, R1, R2, R3, 7 cm. internal diameter, were fixed at distances of 1, 2, and 3 cm. respectively from the plane of the electrode C. A separate connection for each ring and for the plates A and B was brought out through a large ebonite plug fitted in the brass cover of the apparatus, and was maintained at a potential proportional to the distance of the corresponding ring, or plate, from the electrodes C. The stream of ions that came through the slit moved in a uniform electric field and was received by the three insulated electrodes c 1, c 2, c 3. These were portions of a disc 7 cm. in diameter, the central section c 2 being 4.5 mm. wide and separated from the two equal side plates c 1 and c 3 by air gaps 0.5 mm, wide. The narrow gaps between the electrodes were parallel to the direction of the slit in B. In the calculations it will be supposed that the electrode c 2 is 5 mm. wide, and that the side plates c 1 and c 3 come within 2.5 mm. of the central line.