220 RADIUM [APR j. 



In a uniform magnetic field the rays will all be seg- 

 ments of circles, and it is easy to estimate the radius of 

 curvature of the ray corresponding to any point in the 

 spectrum, since only one circle can be drawn through 

 three given fixed points (Euclid, IV. 5 or III. 25), such points 

 for instance as a, 6, and d ; so, these three points being 

 known, the circle of which the ray forms a part is known, 

 and its radius of curvature is therefore determined. 



Electric charge carried by Beta-rays. 



The fact that ordinary cathode-rays are negatively 

 electrified was proved most conclusively by Perrin with 

 an apparatus diagrammatically represented in the figure; 

 where a represents the cathode, 



6 an earthed diaphragm with small aperture, and 

 c a Faraday cavity or hollow vessel, arranged to 

 catch the rays in its interior and convey the 

 charge to an electroscope. 



b\ c ^ 

 I i I """"^e. Electroscope 



It is not so easy to prove that the beta rays emitted 

 by radium are electrified, because the air and everything in 

 the neighbourhood is rendered conducting by their impact, 

 but Professor Curie succeeded in establishing the fact by 

 enclosing a piece of metal completely in solid paraffin, 

 and showing that when exposed to the rays this metal 

 became charged. 



Strutt exhibits the converse effect in a simple and 

 interesting manner by hanging up a radium tube in a 

 vacuum and attaching to it a pair of gold leaves. As 

 the electric rays are shot away by the radium the leaves 

 diverge with an opposite charge; they go on diverging 



