the Cathode Rays by Magnetic Force. 479 



too. The dark interval between two bright bands in the 

 " magnetic spectrum " shows the absence of these particles 

 of intermediate velocity. If the velocities are different, then 

 the particles cannot be simultaneously emitted. The poten- 

 tial-difference between the electrodes of a tube through which 

 an induction-coil is discharging may well be imagined to go 

 through several maxima and minima during a discharge. 

 The velocity of the particles is no doubt greater the stronger 

 the electric field in which they move. The greater this velo- 

 city, the less the deflexion by magnetic force. In the neigh- 

 bourhood of a maximum difference of potential the magnetic 

 deflexion will vary only very slowly ; the rays corresponding 

 to this maximum will therefore strike the glass in one parti- 

 cular neighbourhood for a relatively long time. This will 

 account for the concentration of the luminosity in particular 

 places. If this view be correct, the two separate bands cannot 

 really coexist, but are formed successively. The whole pro- 

 cess, of course, occurs far too rapidly for the eye to follow. 



In order to test which of these alternative explanations is 

 the correct one I have experimented on the cathode rays pro- 

 duced by the continuous discharge of a battery of storage- 

 cells. If the magnetic spectrum is not produced in this case, 

 it will show conclusively that its formation is due to some 

 peculiarity of the induction-coil discharge, and that it does 

 not depend on there being more than one kind of particle 

 present in the cathode stream. A tube was constructed as 

 shown in the figure, which is approximately on one-half of the 

 actual scale, a is the flat disk serving as cathode. Facing it is 

 placed the brass tube b. The end of this tube is closed by the 

 brass plate g, in which a slit is cut. This brass tube is con- 

 nected to the anode c. Beyond the slit is the bulb shown in 

 the figure. The tube was connected up to a battery of 800 

 storage-cells, specially constructed for the experiment. It was 

 then exhausted on a mercury pump. When the pressure was 

 sufficiently low, the cathode rays proceeding normally from a 

 passed through the slit in g and fell on the wall of the bulb at d } 

 giving rise to a fairly sharply-defined phosphorescent patch. 



When a magnetic field was produced normally to the plane 

 of the paper the rays were deflected, their course being then 

 somewhat as shown in the figure by the dotted line, and the 

 phosphorescent patch was displaced to /. This patch, so far 

 from being broadened out into a " magnetic spectrum," was 

 now narrower and more sharply -defined than when undefiected. 

 The same tube at the same degree of exhaustion, when 

 used with an induction-coil, showed a very well-marked 

 " magnetic spectrum." It is quite clear therefore that the 

 heterogeneity of the rays is not an essential property, but 

 depends on a peculiarity of the induction-coil discharge. 



