Prof. Thomson , On Secondary Rontgen Radiation. Ill 
The metal plate rested on a smaller plate of aluminium, 
connected as shown in Fig. (1) to a Wilson tilted electroscope 
which was well removed and well screened from the Rontgen rays. 
Except in the case of the very soft primary rays it was 
not necessary to have the electroscope extremely sensitive. About 
10 divisions of the eye piece scale per volt was the usual value. 
The electroscope was connected through a mercury key ( K ) 
which could be operated from a distance, and by means of which 
the gold-leaf system could be earthed or insulated. 
The connecting leads were shielded throughout by earthed 
metal tubes to avoid induction effects. 
Whenever possible a metal was used with the surface polished. 
This was so in the case of lead, thallium, mercury, platinum, tin, 
cadmium, silver, zinc, copper, nickel, iron, calcium and aluminium. 
The remainder were used in the form of fine powder packed tightly 
into circular aluminium trays 5 cms. diameter, 7 mms. deep, and 
with very thin rims. The upper surface of the powder was flush 
with the rim. Bismuth, tungsten, tellurium, antimony, molyb- 
denum, selenium, arsenic, cobalt, manganese, chromium, titanium, 
sulphur, phosphorus (red), magnesium, carbon and boron, were 
used in the form of powder. 
For mercury, a similar iron tray was used. 
We owe the pure selenium to the kindness of Mr Threlfall, 
F.R.S. 
In some cases the same metal was used both as a plate and in 
the form of powder. The agreement in the results obtained was 
very close. 
An observation lasted usually from half a minute to a minute, 
and the Rontgen bulb was given 2 minutes rest between successive 
observations. This, of all the methods tried, was found to be the 
most satisfactory for yielding results which were concordant. 
The hammer-break of the induction coil also required repeated 
attention, the surfaces of contact being kept smooth and bright, 
parallel and plane. 
In a comparison of the secondary radiation from two metals, 
readings were taken alternately a great many times, first with one 
metal and then with the other. 
It was convenient to use different thicknesses for the different 
metals. The thickness varied from 7 mms. in the case of aluminium 
to about 1 mm. in the case of platinum and the densest metals. 
In the case of some of the powdered non-metals it was found 
that they became electrified through friction in packing them 
in the tray. However, a few minutes bombardment with the 
rays sufficed to dissipate this charge. 
For non-conductors such as sulphur the surface was made 
conducting by sprinkling over it a very thin deposit of graphite. 
In some cases a sheet of gold-leaf was laid over the surface, 
