714 A Positive Ray Spectrograph. 



A form o£ positive ray spectrograph giving a focussed 

 spectrum depending solely on ratio of mass to charge is 

 described. 



Actual photographic results obtained with a preliminary 

 apparatus are submitted showing the great accuracy possible 

 by the method with which it is hoped to compare masses to 

 one tenth per cent. 



Cavendish Laboratory. 

 August 1919. 



Appendix. 



The Construction of the Slit System. 



The very fine slits used in this apparatus were made with 

 comparative ease as follows: — A cylinder of pure aluminium 

 about 10 mm. long by 5 mm. wide is carefully bored with a 

 hole 1 mm. diameter. The resulting thick-walled tube is 

 then cleaned and crushed with a hammer on an an\il until 

 the circular hole becomes a slit about 3 mm. wide. Contin- 

 uation of this treatment would result in a slit as fine as 

 required giving the maximum resistance to the passage of 

 gas, but its great depth would make the lining up of a pair 

 a matter of extreme difficulty. The crushed tuhe is therefore 

 now placed between two V-shaped pieces of steel and further 

 crushed between the points of the V's at ab<>ut its middle 

 point until the required fineness is attained. Practice shows 

 that the best way of doing this is to crush until the walls 

 just touch, and then to open the slit to the required width by 

 judicious tapping at right angles t<> that previously employed. 

 With a little care it is possible to make slits with beautifully 

 parallel sides to almost any degree of fineness, *01 mm. being 

 easily attainable. At this stage the irregularly shaped piece 

 of aluminium is not suited to accurate gas-tight fitting ; it is 

 therefore filled with hard paraffin to protect it from small 

 pMrticles of metal &c, which if entering cannot be dislodged 

 owing to its shape, and turned up taper to fit the standard 

 mountings. These in the present apparatus are taper-holes 

 in the back of the cathode and in a corresponding brass plug 

 at the ends of a wide tube 10 cm. long. When turned, the 

 paraffin is easily removed by heat and solvents. 



