1435 
increases from 500 to 800 volts, the current increases by 2 to 3 
thousandths. Constant potentials must therefore be used. 
The accuracy is limited by the stability of the apparatus and by 
the oscillations in the radio-active radiation. 
The investigations were made with radium salts in the solid state, 
contained in sealed glass tubes; the salt was finely granular, and 
the tubes were not quite filled. When they are shaken the grains 
can move to a certain extent, which causes a slight change in the 
distribution of the radiation inside the ionisation chamber. The 
danger of this is lessened by giving the grains a definite arrange- 
ment beforehand by tapping the tube. But in spite of this, small 
perturbations of this nature remained in our experiments of not more 
than 1 in 1000. The very greatest care is, therefore, necessary in 
the manipulations which must be made during the experiments. 
The radio-active oscillations of the ionisation current become 
apparent when the sensitivity of the measurements is raised suffi- 
ciently. They cause irregular deviations which can only be eliminated 
by a great number of measurements. They are least to be feared 
when gamma rays are used, as was the case in our experiments. 
In our case they could not do any harm to the determinations. 
It is important that the ionisation chambers should contain an 
unchangeable quantity of air. When working with penetrating rays, 
the current is approximately proportional to the amount of ionised 
air. If one wishes to keep the current constant with great accuracy, 
we must, therefore, take care that the ionisation chambers are 
properly closed. Each chamber is supplied with a tap. By changing 
the amount of air in the compensation chamber, the current in the 
chamber could be so regulated as to ‘get a compensation of the 
amount required. Both the compensation chambers are filled with 
dry air by a tube filled with cotton wool, which can be connected 
to the tap of the chambers by a ground joint, and to an air pump 
and a manometer to regulate the supply. 
We had to take very great precautions to prevent the cryogene 
operations from causing insulation errors in consequence of the 
precipitation of moisture from the surrounding air on the strongly 
cooled parts of the apparatus. The cryogene apparatus used by us 
enabled us to avoid all difficulties of this sort. This instrument, 
which was arranged for working easily and safely with liquid 
hydrogen, had moreover the advantage that the radium tube could 
only come into contact with the gaseous phase of the liquified gas, 
so that when this was hydrogen there was no fear of solid air being 
deposited on the tube. 
