34 
PROFESSOR E. RUTHERFORD AND MR. R. K. McCLUNG 
equal in all directions. In the experiment the rays fell normally on the centre of the 
grid, hut on account of the size of the grid, the intensity of the rays could not he 
considered constant over its surface. The intensity of the rays diminishes, and the 
obliquity of the angle of incidence increases from the centre of the grid outwards. 
In consequence of this, a greater proportion of the incident radiation is absorbed at 
the edges than at the centre. 
The intensity of the rays was cut dovm to about '45 of its incident value in passing 
normally through the grid. It can be shovm, by approximate integration over the 
surface of the grid, that for the distance of the grid from the source of the rays, 
namely, 26 centims., and the dimensions of the grid, the actual energy absorbed 
is about 2 per cent, less than if the rays had the same intensity over the surface of 
the grid as at the centre, and had fallen normally at all points of the grid. 
For the special bulb employed, it was shown that the rate of sujDply of heat to the 
grid was equal to 
'00014 gramme calorie per second. 
This corresponded to a maximum rise of temperature of about 1/200° C. 
Distance of the centre of the grid from the source of the rays = 26 centims. 
Area of grid = 92'2 sq. centims. 
Now '55 of the incident radiation was absorbed in the grid. 
Total energy of the rays falling on the grid is aj^proximately 
= '00025 gramme calorie per second. 
Therefore the total heating effect due to all the rays emitted from the front of 
the plate (omitting absorption in the glass, air, and screens) 
27r X (26)2 
92-2 
X '98 X '00025 = '011 gramme calorie j^er second. 
or '046 watt. 
Now the number of discharges per second in the bulb was 57, and Troutox"^ has 
shown that the duration of the rays during each discharge of an induction coil is less 
than 10~® second, and probably about 10“^ second. Assuming the average duration 
of the rays for each discharge is 10”^ second, the rate of emission of energy while it lasts 
= 1'95 gramme calorie per second. 
The heating effect of the sun’s rays falling normally on 1 sq. centim. surface is about 
= '035 gramme calorie per second. 
The maximum rate of emission of energy as X rays from the bulb is thus about 
56 times greater than the amount of energy per sq. centim. due to the sun’s rays. 
* ‘ Brit. Assoc. Report,’ 1896. 
