EMISSION AND TRANSMISSION OF RONTGEN RAYS. 



125 



A plane cathode was employed so that there was no focussing of the cathode rays ; the 

 object was to avoid an undue rise of temperature of the anticathode with the consequent 

 liberation of gas. 



TO ELECTROSCO 



EARTH 



CATHODE 



Fig. 1. 



The elements used as anticathodes were mounted in line on ;i ear made of alum- 

 inium which ran along horizontal rails fastened by sealing-wax to the bottom of the 

 tube A. The rails consisted of one-half of a piece of aluminium tube cut along its 

 length from end to end. Underneath each axle of the car was fastened a piece of soft 

 iron, and by means of a small electro-magnet outside, the car could be moved and any 

 metal desired brought under the beam of cathode rays. 



Anticathodes and Screens. The anticathodes were about 2 cms. diameter, 2 mms. 

 thick, and were kept in position by small pins let into the body of the car. Some 

 twenty elements w r ere employed as radiators. The metals platinum, tantalum, tin, 

 cadmium, silver, palladium, '/me, copper, nickel, iron and aluminium had their surfaces 

 cleaned and polished where necessary, first with emery of different grades, and, finally, 

 with jewellers' rouge. The lead, thallium, and calcium surfaces were renewed by 

 planing or filing immediately before sealing up the tube. The elements bismuth, 

 tungsten, antimony, chromium and magnesium were used in the form of powder. 

 For these, small trays were spun in a lathe out of thin aluminium sheet, and into 

 them the powders were packed tightly, their upper surfaces flush with the rims. 

 A disc of gas carbon was used for the carbon anticathode. 



I owe the tantalum to the kindness of Messrs. Siemens and Halske, who were 

 good enough to provide me with a suitable specimen. 



Screens of aluminium, iron, nickel, copper, zinc, silver, tin, platinum, and lead were 

 employed, and the thickness was in every case determined by weighing. Platinum 



