Lonization produced in Gases by Réntgen Rays. 359 
aluminium. The cylinders were made gas-tight so that they 
could be exhausted. The electrodes a and a’ consisted of 
thin aluminium plates 7 cms. by 4 cms., and they were sur- 
rounded by the guard-rings g and g’ made of the same metal 
and connected to earth. The electrodes 6 and bd’ also con- 
sisted of aluminium plates about 23 cms. by 6 cms. All 
these electrodes were supported by stout brass rods which 
passed out through ebonite plugs. In the actual experiments 
the ionization measured was that which was produced in the 
volume of gas contained between the plates a and b and that 
between a’ and 0’, 
Just close to the ends B and C of the cylinders was placed 
a thick lead screen DD’ in which two well-defined rectangular 
holes were cut, so that a definite beam of rays might pass 
into each cylinder. 
As the rays produce secondary radiation at the surface of | 
metals when falling upon or emerging from them, the ends 
B/ and C’ of the cylinders were covered on the inside with 
cardboard, while the inside surface of the ends B and C were 
covered with white paper in order to reduce this secondary 
radiation as much as possible. Besides being thus very con- 
siderably reduced, these secondary rays would have to traverse 
about 10 ems. of gas before they would be able to ionize any 
of the gas on which the measurements were being made. 
Any effect from the secondary rays from the surface of the 
metals would thus be reduced to a very small amount, if not 
practically eliminated altogether. The openings in the lead 
screen DD’ were also made of the proper dimensions, so that 
the rays should pass between the aluminium electrodes without 
impinging upon them, 
Now if the gas in each of the cylinders is ionized and the 
electrodes 6 and 0’ are raised to a high potential while the 
electrodes a and a’ are insulated, then a and a’ will become 
charged. If the potential of bis opposite in sign to that of 
b’, then a and a’ will receive charges of opposite sign. If a 
_and a’ are connected together and the charges received by 
them are equal but of opposite sign, they will of course 
nullify each other, but if not equal there will be a residual | 
effect. In the experiments, the one electrode 6 was con- 
nected to one pole of a battery of accumulators, and the 
other one 6’ was connected to the other pole, while the middle 
of the battery was connected to earth, as shown in the diagram. 
The two electrodes a and a’ were connected together and to 
one pair of quadrants of an electrometer, the other pair of 
quadrants being to earth. 
The method of comparing the amount of ionization in two 
