362 Mr. R. K. McClung on the Relative Amount of 
side of this air space is connected to the cathode of the main 
bulb. When the pressure in the bulb becomes too low, due 
to the discharge passing for a time, then a spark will pass 
Fig. 2. 
S 
P 
across the air-gap P and a discharge pass through the side 
tube S. The frequency of the passage of this discharge can 
be regulated by adjusting the length of the. spark-gap P. 
When the discharge passes through S a little gas or vapour 
is liberated from the electrode, and the pressure is thus very 
slightly increased. This arrangement works quite auto- 
matically and maintains a very constant pressure in the bulb, 
and causes it to work very steadily compared with one which 
has no such regulator. | 
In order to test the constancy of the rays during a series 
of observations a standard apparatus was placed between the 
source of the rays and the cylinders B and ©, as shown in 
the diagram. This standard apparatus was similar in con- 
struction to the one used by the author in the experiments 
on the effect of temperature on ionization, and is described 
on page 91 of the Philosophical Magazine for February 1904. 
The constancy of the ionization was tested in the following 
manner :—When the cylinders had been carefully adjusted 
so that a good balance was obtained, the rays were started 
and allowed to run for about fifteen or twenty seconds, so 
that the ionization might become steady. During this time 
the electrodes a, a’, and K, as well as the quadrants of the 
electrometer, were all connected to earth. At the end of 
this time, all these electrodes were insulated simultaneously 
