Radiation by impact of Positive and Slow Cathode Rays. 623 



the part to the left of the opening B. Part of this reflected 

 radiation is corpuscular as it is affected by a magnet ; a part 

 of it is not so affected, and so must, like the incident rays, 

 be a form of Rontgen radiation. 



I should estimate the velocity of the positive rays at about 

 2 x 10 8 cm./sec; the impact of cathode rays possessing the 

 same energy as these would generate a very hard type of 

 Rontgen ray; the t} r pe of Rontgen ray generated by the 

 impact of positive rays more nearly resembles that produced 

 by cathode rays with the same velocity but much less energy 

 than the positive rays. 



I now pass on to consider the production of soft Rontgen 

 radiation by the impact of slow cathode rays. The arrange- 

 ment used is shown in fig. 3. is a Wehnelt cathode — a 



Fie-. 3. 



thin strip of platinum foil with a patch of barium oxide 

 deposited on it by burning away a speck of sealing-w T ax. 

 The anode A is a piece of brass rod with a hole bored through 

 it through which the cathode rays pass on their way to the 

 target B, a copper plate which is at the end of and in metallic 

 communication with a cylinder of wire gauze. The variation 

 in the speed of the cathode rays was produced by putting 

 between the gauze and the anode an electromotive force 

 tending to stop the rays. Thus if Vj is the potential dif- 

 ference between the anode and the cathode, V 2 that between 

 the anode and the gauze, the energy of the cathode rays 

 when they strike the target is proportional to Y 1 — V 2 . This 

 method of varying the energy of the rays was found to work 

 better than altering the potential between the cathode and 

 the anode, as the emission of cathode rays from C was much 

 more regular with a constant potential difference between the 

 anode and cathode. 



To detect the radiation coming from the target, a camera 

 similar to that used in the previous experiment was placed at 

 the end of the side tube T. A slit was placed in front of the 



