566 Prof. J. J. Thomson on 



opening through whieh the rays emerge as the screen on 

 which the phosphoresence had been observed, was placed just 

 behind the wire, and was read by a reading microscope with 

 a micrometer eyepiece. When the magnetic field was put 

 on, the wire was deflected ; and if T is the tension o£ the wire, 

 p the radius of curvature into which it is bent, i the current 

 through the wire, 



T 



■ = m ; 

 p 



or, if yi is the vertical displacement of the wire, 



*&1 - * r 

 dz 2 ~T'' 



Now if ^77 = when z = we have, if y x is the displacement 

 of the wire at the scale, 



»=t£-J>*; V 



Hence, comparing (1) and (2) we have 



e 



: '-J»L (3) 



Hi J_ 



T 



a relation from which the magnetic force is eliminated. To 

 ensure that the tangent to the wire is horizontal when z = 0, 

 the following method is used. P is a chisel-edge carried by 

 a screw and placed about 1 mm. in front of the fixed end of 

 the wire; this is adjusted so that when the magnetic field is 

 not on the wire just touches the edge : this can be ascertained 

 by making the contact with the wire complete an electric 

 circuit in which a bell is placed. When the magnetic field 

 is put on the wire is pulled off from the edge, and the tangent 

 at z=0 is no longer horizontal ; it can, however, be brought 

 horizontal by raising or lowering the pulley D until the wire 

 is again in contact with P, which can be ascertained again 

 by the ringing of the bell. Then y L is the vertical distance 

 between the point where the wire now crosses the edge o£ 

 the scale and the point where it crossed it before the magnetic 

 field was put on. Since y, y l9 ?', and T can easily be mea- 

 sured, equation (3) gives us the value of e/mv, while the 

 deflexion under the electric force gives the value of e/mv 2 . 



