682 



Mr. Alan A. Campbell Swinton 



[Fob. 4, 



nally introduced by Crookes, more especially for showing the heating 

 effect of the cathode rays when allowed to impinge upon a piece of 

 platinum foil, and it is to Herbert Jackson that we owe its application 

 to the production of the h'ontgen rays. 



Here is a tube arranged as 

 in Fig. 3 with two concave 

 cathodes opposite one another, 

 both focussing upon a small 

 fragment of quicklime. I em- 

 ploy in this case two cathodes 

 because I am going to use an 

 alternating electric current, such 

 as is supplied from the mains, 

 but transformed up to some 

 *iO,000 volts by being passed 

 through an induction coil. 

 Each aluminium cup serves in 

 turn as cathode and anode, and, 

 as will be observed, when the 

 current is turned on and con- 

 ditions are favourable, a very 

 brilliant aud beautiful light is 

 produced. This, however, only 

 lasts for a short time and then 

 dies out, the strong light re- 

 curring from time to time at 

 unequal intervals. This curious 

 effect, which in result is analo- 

 gous to the hunting of a badly adjusted arc lamp, requires explanation. 

 It appears to be due to absorption of the residual gas by the lime 

 while the latter is white hot, and the giving of it out again at a lower 

 temperature ; this producing a periodic increase and decrease of the 

 vacuum, and a consequent decrease and increase of the energy of the 

 discharge through the tube and of the light. Another curious fact, 

 and one that supports the bombardment theory of the cathode rays, 

 is that the rays after having been allowed to fall upon the block of 

 lime for a little time, are found to bore perlectively straight and very 

 minute holes in the material. This block, which has been used on 

 several occasicms, aud has also been turned round a little, was solid 

 originally, but has now several holes passing right through it, some 

 of these not being more than about half a millimetre in diameter. 

 At the edges the material is somewhat bn ken away, but in the 

 interior the holes have been so accurately eroded by the cathode rays 

 that they look as though they might have been bored with a small 

 drill. This shows the great accuiacy with which the cathode rays 

 cnn be focussed. Again, it is remarkable that though the current is 

 alternating, and the arrangement of the tube and electrodes perfectly 

 symmetrical, so that one would expect the heating and luminous effect 



Fig. 2. — Figure enuraved on the interior 

 of a gla^s bulb by cathode rays. 



