
68 METHODS OF MEASUREMENT [CH. 
which rapidly lose their activity. Moreover, W. J. Russell has 
shown that the darkening of a photographic plate can be produced 
by many agents which do not give out rays like those of the radio- 
active bodies. This darkening of the plate is produced under very 
many conditions, and very special precautions are necessary when 
long exposures to a weak source of radiation are required. 
The main objection to the photographic method, however, lies 
in the fact that the radiations which produce the strongest electrical 
effect are very weak photographically. For example, Soddy? has 
shown that the photographic action of uranium is due almost 
entirely to the more penetrating rays, and that the easily absorbed 
rays produce in comparison very little effect. Speaking generally 
the penetrating rays are the most active photographically, and the 
action on the plate under ordinary conditions is almost entirely due 
to them. 
Most of the energy radiated from active bodies is in the form 
of easily absorbed rays which are comparatively mactive photo- 
graphically. These rays are difficult to study by the photographic 
method, as the layer of black paper which, in many cases, is 
necessary to absorb the phosphorescent light from active substances, 
cuts off at the same time most of the rays under examination. 
These rays will be shown to play a far more important part in the 
processes occurring in radio-active bodies than the rays which are 
more active photographically. 
The electrical method, on the other hand, offers a rapid and 
accurate method of quantitatively examining the radiations. It can 
be used as a means of measurement of all the types of radiation 
emitted, excluding light waves, and is capable of accurate measure- 
ment over an extremely wide range. With proper precautions 
it can be used to measure effects produced by radiations of 
extremely small intensity. 
49. Electrical Methods. The electrical methods employed 
in studying radio-activity are all based on the property of the 
radiation in question of ionizing the gas, z.e. of producing positively 
and negatively charged carriers throughout the volume of the gas. 
The discussion of the application of the ionization theory of gases to 
1 Trans. Chem. Soc. Vol. 81, p. 860, 1902. 
