Prof. Thomson , On Secondary Rontgen Radiation. 109 
On Secondary Rontgen Radiation. By J. J. Thomson, F.R.S., 
Trinity College, Cavendish Professor of Experimental Physics. 
[Read 12 November, 1906.] 
McClelland* has shown that there is a close connection 
between the secondary ionization given out by a substance 
exposed to the /3 and 7 rays from radium, and the atomic weight 
of the body. 
The experiments hitherto made on secondary Rontgen radia- 
tion have not indicated that any such connection exists between 
the secondary Rontgen radiation and the atomic weight. The 
following experiments show however that a very intimate relation 
does exist between these quantities and that this relation varies 
in a very interesting way with the hardness of the primary rays. 
The method used in this investigation is illustrated in Fig. ( 1 ). 
A Rontgen ray bulb contained in a lead box gave off rays, 
a pencil of which passing through the aluminium window ( A ) 
bombarded a horizontal circular plate ( B ) of the substance placed 
directly below it. The plate was of diameter 5 cms. and was 
surrounded by a larger earthed plate of aluminium ( D ) which 
served the purpose of a guard-ring. This guard-ring plate was 
supported from one side by a turn-table having an up and down 
screw adjustment. Thus the guard-ring could be raised or lowered 
so that its upper surface was always in the plane of the upper 
surface of the inner disc. 
Parallel to the metal plate and symmetrically situated above it 
was mounted an iron wire gauze plate ( C ) (2 mms. mesh), braced 
at the edges with a wire ring. The wire gauze was supported, 
like the guard-ring, from one side by a turn-table. By screwing 
the turn-table up or down the wire gauze plate was raised or 
lowered by an amount measured by a scale and vernier. The 
reading was taken when the wire gauze was touching the metal 
plate. Thus in any subsequent position of the wire gauze, its 
distance from the metal plate beneath it was known. 
By rotating the turn-table the wire gauze could be swung 
clear of the apparatus. This was useful when one plate had to be 
substituted for another, for the wire gauze could afterwards be 
swung back exactly to its old position, the height being corrected 
for any difference in thickness of the metal plates. 
The wire gauze was connected to a battery of small storage 
cells and raised to a sufficiently high potential — 200 volts was 
used — to produce the saturation current across the region between 
the wire gauze and metal plate. 
* Scientific Trans. Roy. Soc. Dublin, 1906. 
