460 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



tube of which it is a part, while the current passes freely in the other 

 branch of the U tube. This form of tube rectifies an alternating 

 current. 



The apparent repelling or driving back action of the cathode beam 

 on striae is shown in a suggestive manner in a straight cylindrical tube 

 when a diaphragm is inserted between the anode and the cathode. 

 We will take for illustration one branch of the U-shaped tube (Figure 



Figure 8. 



7), and suppose that the current is led into the tube at A and out at D. 

 A metallic diaphragm with a small hole at its centre is inserted in the 

 tube about one third of the distance beween A and D, measured from 

 the anode A — the latter also having an orifice at its centre. The 

 strise are slowly driven back by the cathode rays as the exhaustion 

 proceeds. At a definite stage of this exhaustion a stria takes refuge 

 behind the diaphragm nearer the anode, where it is protected from 

 the driving back action of the cathode rays ; finally at higher exhaus- 

 tions this stria is driven through the orifice in the anode and shelters 

 itself behind the anode. 



