1433 
method of cooling seemed to us to be preferable to placing the tube 
itself into the liquid gas, which is always a little dangerous. 
The rays that the radium in the tube sends out are partly of a 
penetrating nature. They go through the walls of the aluminium 
tube and those of the vacuum giass, and penetrate through a metal 
wall into the ionisation space. This consists of a cylindrical box D,, 
which is connected to a battery; in the middle of the lid of this 
space a tube is soldered, which is closed at the lower end. The 
insulated electrode £, which is a hollow cylinder, is connected with 
the electrometer. The metal case /, which is connected to earth, 
serves for electrostatic protection. When the apparatus is mounted 
the tube-shaped portion of the vacuum glass is inside the tube D,, 
which is placed centrally in the box D,, while it is closed by a 
thick piece of india-rubber tubing round a piece of amber G which 
is sealed to the vacuum glass. When the tube containing the radium 
is in its place, ions are formed on both sides of the electrode £, 
in the air that fills the box D,. The current that is taken up by 
this electrode is measured by an electrometer and a plate of 
piezoquartz. 
The experiment consists in measuring the ionisation current gene- 
rated by the rays of the radium: 1. when the radium is at the 
temperature of the room, and 2. when the radium is cooled to the 
temperature of liquid hydrogen. The ionisation chamber, which is 
outside the vacuum glass remains at about the temperature of the 
room. The chamber is airtight, and the quantity of gas that it 
contains does not alter during the experiments. 
The accuracy of the measurements is greatly increased if instead 
of measuring the total current, a compensation method is used. This 
consists in compensating the current to be measured by a current 
in the opposite direction, which is generated in a second ionisation 
chamber by a tube containing radium, which is kept at constant 
temperature during the experiments. This current compensator is of 
a type which is greatly made use of in radioactive measurements. 
The insulated electrode G is in the form of a tube which is closed 
at the bottom; it is connected by means of copper wires (electrically 
protected in brass tubes filled with paraffin wax) with the electro- 
meter and with the electrode £,. This tube reaches into a cylindri- 
cal box H, which is connected to a battery and which forms an 
ionisation chamber. The outside case A serves for electrostatic pro- 
tection. The electrodes G and £, are protected in the usual way 
by a protecting ring connected to earth. The tube G contains a 
sealed glass tube with radium salt. The boxes D, and H are kept 
