550 



Prof. J. J. Thomson on the Masses of 



Fij?. 1. 



soon as the distance between the plates was equal to 2X m eH 9 

 the magnetic force would greatly diminish the rate at which 

 CD received a negative charge, and would in fact reduce the 

 rate almost to zero if all the negatively electrified particles 

 came from the surface of AB. Hence, if we measure the 

 distance between the plates when the magnetic force first 

 diminishes the rate at which CD receives a negative charge, 

 we shall determine the value of 2X»?/eH 2 ; and as we can 

 easily determine X and H, we can deduce the value of m/e. 



The way in which this method was carried into practice 

 was as follows, the apparatus being shown in fig. 1. 



AB is a carefully polished 

 zinc plate about 1 centim. in 

 diameter, while CD is a grat- 

 ing composed of very fine 

 wires crossing each other at 

 right angles, the ends being 

 soldered into a ring of metal ; 

 the wires formed a network 

 with a mesh about 1 millim. 

 square. This was placed 

 parallel to AB on the quartz 

 plate EF, which was about 

 4 millim. thick. The orating 

 was very carefully insulated. 

 The system was enclosed in 

 a glass tube which was kept 

 connected with a mercury- 

 pump provided with a McLeod 

 gauge. The ultra-violet light 

 was supplied from an arc 

 about 3 millim. long between 

 zinc terminals. The induc- 

 tion-coil giving the arc was 

 placed in a metal box, and 

 the light passed through a 

 window cut in the top of the 

 box ; over this window the 

 quartz base of the vessel was placed, a piece of wire gauze 

 connected with the earth being placed between the quartz 

 and the window. The plate AB was carried by the handle 

 L which passed through a sealing-wax stopper in the tube K. 

 The magnet used was an electromagnet of the horseshoe 

 type. The magnetic force due to the magnet was determined 

 by observing the deflexion of a ballistic galvanometer when 

 an exploring coil, of approximately the same vertical dimen- 



