Velocities of a, Particles from Radioactive Substances. 555 

 allows us to determine the value of -^-. and the deflexion in 



2 -Hi ' 



an electric field the value of —r=r, where m is the mass of 



XL 



the a particle, E its charge, and v its velocity of escape. 



From the combined observations, both v and E/m are 



determined. 



Deflexion in a Magnetic Field. 



It is of importance to know the value of -^- for the a 



particles emitted from a thin film of a well-known radio- 

 active substance, e. g. radium C. When once the values of 



7YIV 



-~r and E/m have been determined for these homogeneous 



rays of known range, the velocity of expulsion of the a 

 particles from any other product can be deduced with 

 sufficient accuracy from the relation found by Geiger, viz. 

 i ,3 = KR, where R is the range of the a particle and K a 

 constant. 



The arrangement in the magnetic experiments was that 

 usually employed, viz. a line source of homogeneous radiation 

 with a slit parallel to the source and a photographic plate 

 placed parallel to the slit and normal to the line through the 

 source and slit. The slit was equidistant (6*5 cm.) from the 

 source and photographic plate. The magnetic field was 

 parallel to the source and slit and photographic plate 



In our experiments we have used a thin film of radium C 

 as a definite and readily reproducible source of homogeneous 

 u rays. The source generally consisted of a fine platinum 

 wire made active by an exposure of some hours to about 

 100 millicuries of emanation in an electric field, or more 

 frequently by sealing about 1 cm. length of platinum wire 

 into a fine capillary tube, in which partially purified radium 

 emanation was compressed. After exposure for two or three 

 hours, the emanation was withdrawn by means of a pump, 

 and the wire then removed and fixed by clips in a brass 

 holder, which was permanently attached to a brass base plate 

 16 x 2 x 1 cm. This brass plate also supported the slit, which 

 consisted of two strips of thin sheet copper fixed 1/10 mm. 

 apart on a stout brass frame, and a holder for the photo- 

 graphic plate, the whole apparatus being very rigid in 

 construction. The active wire and photographic plate being- 

 fixed in position, the apparatus was then slipped into a 

 rectangular brass box, and the whole exhausted by pumping 

 and finally by charcoal in liquid air, and placed between the 

 poles of a large electromagnet. By the time this had been 



