CHAPTER III. 
METHODS OF MEASUREMENT. 
48. Methods of Measurement. ‘Three general methods 
have been employed for examination of the radiations from radio- 
active bodies, depending on 
(1) The action of the rays on a photographic plate. 
(2) The ionizing action of the rays on the surrounding gas. 
(3) The fluorescence produced by the rays on a screen of 
platinocyanide of barium, zine sulphide, or similar substance. 
The third method is very restricted in its application, and can 
only be employed for intensely active substances like radium or 
polonium. 
The photographic method has been very widely. used, especially 
in the earlier development of the subject, but has gradually been 
displaced by the electrical method as a quantitative determination 
of the radiations became more and more necessary. In certain 
directions, however, it possesses distinct advantages over the elec- 
trical method. For example, it has proved a very valuable means 
of investigating the curvature of the path of the rays, when 
deflected by a magnetic or electric field, and has allowed us to 
determine the constants of these rays with considerable accuracy. 
On the other hand, the photographic method as a general 
method of study of the radiations is open to many objections. A 
day’s exposure is generally required to produce an appreciable 
darkening of the sensitive film when exposed to a weak source of 
radiation ike uranium and thorium. It cannot, in consequence, be 
employed to investigate the radiations of those active products 
5—2 
