582 Professor J. J. Thomson [March 22, 



Now if i-^i = 

 dz 



the wire at the scale, 



Now if y^i = when 2 = we have, if y-^ is the displacement of 

 a z 



''^-^:-s>'' ^'^ 



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



e 

 ^=^, (3) 



T 



a relation from which the magnetic force is eUminated. 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. TVhen the 

 magnetic field is put on the wire is pulled off from the edge, and the 

 tangent at z = 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^ is the vertical distance between the point where 

 the wire now crosses the edge of the scale and the point where it 

 crossed it before the magnetic field was put on. Since y, yi, i, and T 



can easily be measured, equation (3) gives us the value of — , while 



the deflection under the electric force gives the value of . 



m ^'2 



If y is the vertical displacement of the patch of phosphorescent 

 light on the screen produced by the magnetic field, x the horizontal 

 displacement due to the electrostatic field, we see that 



niv m V 



■M 



1-^ 



where A and B are constants depending on the position of the screen 

 and the magnitudes of the electric and magnetic forces. These 

 quantities can be calculated by means of the equations just given. 



