48 DETERMINATION OF SPEED [CH. v. 



by introducing a pair of supplementary electrodes, one 

 above and one below the course of the rays inside 

 a vacuum tube, and connecting them to the poles 

 of a low-potential battery, a few storage cells for 

 instance, thus obtaining a vertical electrostatic field 

 at right angles to the cathode rays. At the same 

 time a magnetic field, produced by lateral magnet 

 poles or by the lines of force due to an electric 

 current in a circular ring, could be arranged at right 

 angles to both the other directions ; and thus the 

 electrostatic deflection could be compared with, or 

 could be used to neutralise, the magnetic deflection. 

 The electric field, being .fixed in direction and urging 

 the particles along not across the lines of force, 

 will act differently to a magnetic field and will cause 

 the particles to move in a parabola the shape which 

 the earth gives to a horizontal jet of water. 



Fig. 4 shows J. J. Thomson's apparatus for 

 measuring the deflexion of cathode rays caused by 

 an electric field at right angles to them.* The rays 

 starting from the cathode C traverse a couple of 

 earth-connected slits AB and after having passed 

 between the electrified plates D and E, impinge on 

 the glass at P, producing a small but vivid phosphor- 

 escent spot. The position of this spot is read on a 

 scale pasted on the outside of the glass. In ordinary 

 vacua screening prevents any effect from being ob- 

 served, by reason of conducting power in the residual 

 air, developed by ionisation caused by the impact of 

 the flying particles ; consequently Hertz failed to 

 obtain the deflexion which was otherwise to be 

 expected; but by pushing the vacuum to a higher 

 stage J. J. Thomson overcame this difficulty, 

 measured the deflexion PP', and employed this 



* Phil. Mag., vol. 44, p. 293 (1897). 





