THE PHENOMENA OF ELECTROSTATICS. 229 



X 



The wall of the tube shows a brilliant luminescence everywhere 

 except where it is screened by the obstacle from bombardment 

 by the cathode rays. 



Magnetic deflection of cathode rays and canal rays. A mov- 

 ing charged body is equivalent to an electric current, and when a 

 charged body moves across a magnetic field the magnetic field 

 pushes sidewise upon the charged body and causes the charged 

 body to describe a curved path. The magnetic deflection of the 

 cathode rays is easily shown by placing a horse-shoe magnet 

 with its poles placed on opposite sides of the tube shown in Fig. 

 1 60. The shadow of the cross is thrown up or down according 

 to the arrangement of the magnet. The magnetic deflection of 

 the canal rays is very slight ; a very strong magnetic field is 

 necessary to produce a perceptible deflection. The direction of 

 the magnetic deflection of the cathode rays shows that these rays 

 are negatively charged particles, and the direction of the mag- 

 netic deflection of the canal rays shows that these rays are posi- 

 tively-charged particles. The magnitude of the deflection of the 

 cathode rays shows that the mass of the cathode particles (elec- 

 trons) is very small and that their velocity is very great. The 

 magnitude of the deflection of the canal rays shows that the 

 mass of the canal ray particles is relatively great and that their 

 velocity is less than the velocity of the cathode rays. This mat- 

 ter is explained in detail in Art. 135. 



An object upon which the cathode rays * impinge is heated, it 

 may be, to a very high temperature. Many substances, however, 

 emit light (without being made perceptibly hot) when ^subjected 

 to bombardment by the cathode rays. Such substances\are said 

 to be luminescent. For example, lead sulphate emits a deep 

 violet light, zinc sulphate emits white light, magnesium sulphate, 

 with a slight admixture of manganese sulphate, emits a deep red 

 light under the action of cathode rays. 



* The cathode rays produce effects which are practically important and which can 

 be easily observed. The effects of the canal rays, however, are so slight as to be 

 scarcely perceptible even under the most favorable conditions. Therefore further dis- 

 cussion of the canal rays is not warranted in this brief outline. 



