494 Mr Owen, On the Magnetic Deflexion of the Negative 



were driven off the wire. He found the velocity of the ions 

 emitted by the wire to K be smaller when the wire was enclosed in 

 a tube than when open to the air. He explained this as being 

 due to the " loading up " of the ions by heavy particles driven 

 from the wire. On Child's theory these heavy particles " do not 

 aid in the discharge but materially check it." 



The question therefore arises, Is the negative current of 

 electricity from a hot platinum wire at very low pressures carried 

 by corpuscles or by a mixture of corpuscles and heavy particles ? 

 If the mechanism of the discharge is the latter one, How does 

 the relative proportion of corpuscles and heavy particles vary with 

 the temperature of the wire ? It was in the hope of throwing 

 some light on these questions that the following experiments 

 were undertaken. 



The method adopted was to determine the value of ejm for 

 the carriers of the negative electricity at very low pressures. 

 This was done by the method of " Retardation of Discharge by a 

 Magnetic Field " first used by Prof. Thomson in 1899. 



In the number of the Phil. Mag. referred to above, Prof. 

 Thomson proves that the path of an ion under the joint action of 

 uniform electric and magnetic forces (acting at right angles to 

 one another) is a cycloid. If an ion starts from rest at the plane 

 x = 0, then the greatest distance the ion can get from this plane 

 is 2mX/eH Q , where X and H are respectively the electric and 

 magnetic forces, m the mass and e the charge on the ion. 



Now consider a number of ions starting from the plane x = 

 and moving towards the plane x = a. If a is less than 2m X/eH' 2 , 

 all the ions will reach the plane x = a. If on the other hand a is 

 greater than 2mX/eH Q , all the ions will be turned back by the 

 magnetic field before they reach the plane x = a. If a is fixed 

 and H can be varied, then we shall have the following effect. 

 Starting with very small magnetic fields, no diminution is pro- 

 duced in the number of ions reaching the plane. When the 

 magnetic field, however, attains a certain value all the ions will 

 be stopped. Thus if we know X, a and the critical value of H 

 we can find ejm, for e/m = 2X/aH 2 =2V/a 2 H' 2 , when V is the 

 difference of potential between the planes. In his determination 

 of the value of e/m for the corpuscles emitted by a metal plate 

 illuminated by ultra-violet light, and by a glowing carbon 

 filament, Prof. Thomson did not find this change from no effect to 

 a complete disappearance of the current to be quite so abrupt as 

 the theory indicates. He explained this as being due to the 

 ionisation not being wholly confined to the gas immediately in 

 contact with the metal plate or filament but extending into the 

 gas through a layer of finite thickness t say. Thus the ions would 

 start towards the plane at distances ranging from a to a—t. The 



