ON ENERGY OF RONTGEN AND BECQUEREL RAYS, ETC. 
27 
Two of these grids were constructed as similar as possible, and mounted in the same 
vertical plane on a wooden base. 
Resistance of each grid = 4'2 ohms. 
Ai’ea of platinum surface of grid = 92'2 sq. centims. 
The X rays incident on the grid for the most part passed through two thicknesses 
of platinum, but, on account of the windings not completely overlapping, the rays in 
some portions passed through one thickness only. This was clearly shown in an 
X-ray photograph of the grid, which is sketched in fig. 1 (6), where the shaded 
portions are the areas where the rays only passed through one thickness of the 
platinum. 
The absorption of the rays in the mica frame was very slight, and it was only on 
very careful inspection of the photograph that the outline of the frame could be 
observed. For the X rays .employed, the intensity was cut down to •45 of its value 
after passing through the grid. 
Focus Tube. 
The rays were excited in an automatic focus tube of the pattern shown in fig. 2, 
with a platinum anode and an aluminium cathode. The tube was excited by a large 
coil, using a Wehnelt interrupter on a 110-volt circuit. The alternative spark gap A 
was always kept the same length—about 5 inches. The bulb was a very hard one, 
and there was generally a fairly rapid succession of sjiarks across A during the 
working of the bulb in order to keep the vacuum constant. The constancy of the 
length of the spark A is of great importance in these experiments, in order to obtain 
rays of the same degree of penetration. A diminution of the spark length lowers 
the vacuum of the gas in the tube and produces rays of lower penetrating power. 
E 2 
