284 ADVANCED ELECTRICITY AND MAGNETISM. 



142. Magnetic deflection of cathode rays and canal rays. 

 A moving 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 deflec- 

 tion 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. 207. 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 direc- 

 tion of the magnetic deflection of the cathode rays shows that 

 these rays are negatively charged particles, and the direction 

 of the magnetic deflection of the canal rays shows that these 

 rays are positively charged particles. The magnitude of the 

 deflection of the cathode rays shows that the mass of the cathode 

 particles (electrons) 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 matter is explained in detail in Art. 148. 



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

 may be, to a very high temperature. Many substances, how- 

 ever, emit light (without being made perceptibly hot) when 

 subjected to bombardment by the cathode rays. Such sub- 

 stances are said to be luminescent. For example, lead sulphate 

 emits a deep violet light, zinc sulphate emits white light, mag- 

 nesium sulphate, with a slight admixture of manganese sulphate, 

 emits a deep red light under the action of cathode rays. 



The cathode rays pass quite readily through thin metal plates, 

 especially through thin plates of aluminum. By using a Crookes 



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



