Radiation and Ionization of Helium. 753 



the first time. It has also been found that, with pressures 

 much higher than those at which tests were made in our 

 earlier paper, ionization of helium can be detected before the 

 normal ionization voltage is reached, but this is shown to be 

 mainly due to the ionizing action of the 21*2 volts radiation 

 upon helium atoms which are in an abnormal state owing to 

 the absorption of 20*4 volts energy, rather than to electron 

 impacts upon such atoms, although evidence that this latter 

 effect happens to a small extent has also been obtained. 



Description of the Apparatus. 



A diagrammatic view of the apparatus used is given in 

 fig. 1. It consists of two vertical glass tubes joined together 

 by a short horizontal tube. The wider tube (3*5 cm. in 

 diameter) w r ill be referred to as the main tube, and the 

 narrower one will be referred to as the side tube. At the 

 lower end of the main tube there is a tungsten filament F, 

 which, when heated electrically, serves as a source of elec- 

 trons. This filament (which we shall call the lower filament) 

 is about 2 mm. below the centre of the circular piece of fine 

 platinum gauze D which forms the base of a cylindrical box 

 of platinum gauze which just fits the inside of the glass tube. 

 The cylindrical side of this box completely covers the end of 

 the opening to the side tube as indicated in the figure. C is 

 another circular piece of platinum gauze placed 5 mm. above 

 the top of the sides of the cylindrical box, and 5 mm. above 

 and parallel to it is a third gauze B, which is stretched 

 across the lower end of an aluminium tube E, lined oil the 

 inside with thin platinum foil. Concentrically within the 

 cylinder E a circular platinum plate A is suspended. This 

 acts as the collecting electrode and can be raised or lowered 

 within the cylinder over a range of several centimetres by 

 means of the windlass shown (T). The fine platinum wire 

 supporting the plate A passes through, and makes contact 

 with, a small platinum ring at R, which is connected to the 

 electrode N, and thus to the sensitive electrometer used to 

 measure the currents. The top of the platinum-covered 

 cylinder is partially closed by a circular platinum plate 

 which has a central hole to permit the passage of the 

 supporting wire. 



The side tube contains a tungsten filament II (which will 

 be referred to as the side filament), situated 2 mm. above 

 the circular platinum gauze K, which has cylindrical sides of 

 thin platinum foil welded round its ed^e as indicated in the 

 figure. The platinum gauze L just fits the side tube and is 



