﻿2bC) Mr. Walmsley and Dr. Makower on Magnetic 



oil-pump and then by the use of charcoal cooled in liquid 

 air. Pressures were read on a McLeod gauge. The pressure 

 gradually decreased during the experiment, but was usually 

 of the order of 1/1000 mm. after the first three minutes. 

 Three minutes after introducing the stopper B the field 

 was reversed, and the photographic plate subjected to a 

 further radiation for 9 to 12 minutes. At the end of 

 the exposure the plate was removed and wrapped in black 

 paper for a period of about three hours to allow the active 

 deposit to decay. It was then developed in the usual way. 

 The plate used was a piece of Ilford Process plate 3*3 cm. 

 x 0*8 cm._, which was sufficiently large to prevent the film 

 becoming detached during development. The activity of 

 the wire was also measured. Its 7- ray activity at the 

 maximum usually corresponded to about one milligram of 

 radium C. 



The best arrangement of w r ire, slit and plate is not easy 

 to foresee. From equation (3) it is evident that the accuracy 



obtained for the ratio — depends upon the accuracy with 



which the distances d a and d r can be measured. These 

 quantities increase in the first place with the strength of the 

 magnetic field, which w r as made as intense as practicable 

 without over-heating the magnet. The accuracy with which 



the ratio ^ can be measured depends also on the width of 



Pr 



the images relative to their distance apart. The width of the 

 image can be reduced to a certain extent by employing a 

 narrow slit and a narrow source, but the amount of active 

 matter which reached the plate is rapidly reduced by the 

 use of fine slits, and it is impossible to collect enough 

 radium A on a very fine wire. In practice we obtained verv 

 satisfactory results by using as source a platinum w T ire of 

 diameter 0'4 mm. and a slit 0*1 mm. wide. The distance /j 

 was kept permanently 3*0 cm., the slit carrier being held in 

 position by metal stops. When l 2 was 1*84 cm. we obtained 

 a dense image easily measurable, but the maximum value of 

 d a obtained was only 2 mm. With / 2 = 2*85 cm. the image 

 was just measurable in the case of the recoil lines, and 

 d a increased to 3'7 mm. ; but with / 2 = 3*5 cm. using the 

 same amount of active material, the recoil image though just 

 visible was quite immeasurable. In all cases the a-ray 

 images were remarkably sharp and distinct. This rapul 

 reduction in the intensity of the recoil images is rather 

 striking, and explains many failures in previous experiments. 

 In fig. 1, PI. V., is given a reproduction of one of the 



