Page:Radio-activity.djvu/102

 CHAPTER III.

METHODS OF MEASUREMENT.

53. 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, zinc 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 used very widely, 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 electrical 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, as a general method of study of the radiations, it 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 like uranium and thorium. It cannot, in consequence, be employed to investigate the radiations of those active products which