BY THE ACTION OF A RADIO-ACTIVE SUBSTANCE, ETC. 
263 
concentric with the boundary of the disc. When either set of tubes was pushed 
into A, a stream of air down the large tube divides itself ecpially among the twenty- 
four channels provided by the tubes T. The disc a l was soldered to the front of the 
tube B t , and the disc to the front of the tube B,. 
When working with air at atmospheric pressure the stream of air was obtained by 
putting weights on the movable cylinder of a gasometer. For experiments with dry 
air the delivery tube of the gasometer was connected to wide tubes of calcium 
chloride, and a tube tightly packed with glass-wool was put between the drying- 
tubes and the tube leading to A, so that particles of dust should not be carried into 
the diffusion apparatus. When it was desired to make experiments with moist air, 
the calcium chloride tubes were removed, and long tubes half filled with water were 
substituted. The velocity of the air along the tubes T could lie varied by changing 
the weights on the gasometer. 
The radio-active substance was obtained from E. de Ha ex (Chemische Fabrik, 
List vor Hannover), and the preparation labelled “Radio-active Substance A” was 
used. A tube, C, of thin glass, containing some of the radio-active substance, was 
held by means of wire supports inside the tube A as shown in the figure. The 
radiation given out by the active substance was much more intense than uranium rays, 
and after passing through the glass tube was strong enough to ionize the surrounding 
air. The tube C was sealed in order to prevent any moisture from coming into 
contact with the radio-active substance, which was deliquescent. The tube A was 
fixed rigidly by ebonite supports, S, to the top of a heavy box, so that the tube C 
should not get shaken when the tubes B t and B 3 are fixed in position. 
T1 le tube A was connected to one terminal of a battery of forty lead cells, the 
other terminal being to earth. The rod F was connected to one pair of quadrants of 
an electrometer, the other pair of quadrants and the case of the electrometer being 
connected to earth. The rod F and the wire connecting it to the electrometer were 
surrounded by metal screens, so that external electric charges should not give any 
deflection on the electrometer scale. 
Since the tube A is in metallic connection with all the parts of the diffusion 
apparatus except E, there is no electric force acting on a stream of gas until it comes 
out of the tubes T into the space between E and B. The air takes about one second 
to pass the electrode, and a difference of potential of a few volts between E and B 
would, under ordinary conditions, suffice to collect all the ions of one sign on E, but 
owing to the turbulent motion of the gas as it escapes from the tubes T, a much 
greater potential difference (80 volts) was used. The potential of the electrode 
during an experiment never exceeded 1 or 2 volts. It was found under similar 
conditions of ionisation and velocity of air that the electrometer deflection was not 
altered by charging; A to 40 volts instead of 80. We therefore conclude that all 
the ions of one sign are collected on E, so that the electrometer deflection is 
proportional to the number of ions that come through the tubes T. 
