874 Prof. J. S. Townsend and Mr. V. A. Bailey on 



In the earlier investigations it was found that the motion 

 of electrons in hydrogen and other gases was similar to that 

 obtained in air, but only a few experiments were made with 

 these gases. 



2. One of the principal differences which was observed 

 between the motion of ions and the motion of free electrons* 

 in gases, is that the mean kinetic energy of the motion 

 of agitation of an ion is the same as that of a molecule of the 

 gas, whereas with electrons the energy of agitation increases 

 with the force, and with the smaller pressures and larger 

 forces used in these experiments the mean kinetic energy of 

 agitation of an electron is as great as a hundred times that 

 of a molecule of the gas. The reason for this is that the 

 electron loses only a small part of the energy which it 

 acquires in moving under the electric force when it collides 

 with a molecule, since the mass of a molecule is so large 

 compared with that of an electron. Thus the effect of a 

 collision with a molecule is to alter the direction of motion 

 of the electron without making much reduction in the 

 velocity. The average loss of energy in a collision with a 

 molecule is somewhat greater than the loss which would take 

 place if the electron and the molecule were perfectly elastic 

 spheres, but this loss is so small that the velocity of agitation 

 becomes very large. 



In order to obtain definite information as to the nature of 

 the collisions of electrons with molecules of a gas, it is 

 necessary to determine experimentally both the mean velo- 

 city of agitation u and the mean velocity W in the direction 

 of the electric force. Tue factor k by which the mean 

 energy of agitation mir/2 of an electron exceeds that of a 

 molecule of the gas, and the velocity W in the direction of 

 the electric force Z, were determined by means of the 

 apparatus which was found to be most suitable for deter- 

 mining these quantities for electrons moving in air J. 



3. The principle of the method is shown by the diagram 



fig. i. 



Electrons are set free from a metal plate A by the action 

 of ultra-violet light and travel through the gas to a parallel 

 plate B which is at a distance of I centimetres from A. 

 The middle part of the plate B is of thin brass foil with a 

 slit 2 millimetres wide and 1^- centimetres long in the centre, 

 through which some of the electrons pass in a narrow stream 

 into the lower part of the apparatus and are received on the 



* Proc. Koy. Soc. A, lxxxi. p. 464 (1908). 



t J. S. Townsend and H. T. Tizard, Proc. Eov. Soc. A, lxxxyiii. p. 336 

 (1913). 



