294 SECTIONAL TRANSACTIONS.— A. 



Mr. W. SucKSMiTH. — The Gyromagnetic Effect in Paramagnetic Substances. 



An apparatus for the measurement of the gyromagnetic ratio for paramagnetic 

 substances has been designed. The angular momentum produced by a change of 

 magnetic moment is extremely small, and low frequency resonance is utilised for 

 building up the resulting impulse to a measurable magnitude. Results on the Dy'^++ 

 ion give g, the Lande splitting factor, equal to 1 . 28 ± . 07, which indicates that the 

 magnetic moment is due to both orbital and spin contributions. Measurements on 

 other paramagnetic substances are in progress. 



Prof. J. H. VAN Vleck. — Some Problems of Magnetism. 



Afternoon. 



Inspection of the Henry Herbert Wills Physical Laboratory, and of 

 apparatus referred to in the following papers (taken as read). 



(a) Prof. A. M. Tyndall and Dr. C. F. Powell. — The Mobility of Ions in 

 Pure Gases. 



Measurements of the mobility of ions in purified nitrogen and helium have been 

 made in all-glass metal apparatus which can be subjected to rigorous heat treatment 

 and by a method which has a high resolving power for ions of different mobilities. 

 The results disclose the very great importance of small traces of impurity on the 

 mobility of positive ions. In helium, for example, values of mobility for the positive 

 ion as high as 17 and 14 were found instead of the value 5 obtained originally by 

 Franck. Slight contamination of the gas on standing for a week transformed the 

 ions into a mixture of widely differing mobilities with a predominance at about 10. 

 The mere removal of a liquid air trap inserted to prevent any access of vapour from 

 mercury or tap grease to the gas produced a further marked effect. In both gases the 

 negative carriers were electrons and no detectable number of ions were found. Such 

 marked effects are to be expected from the principle of electron capture according to 

 which a positive ion on impact with a molecule of lower ionisation potential can capture 

 an electron from a neutral molecule with a consequent change in the nature of the ion. 



In order to obtain the true mobility of a positive ion moving in its own gas at pres- 

 sures of 100 mms. or more it will be necessary to reduce the impurity to a few parts 

 in one million. As the matter stands at the present time, no significance can be 

 attached to any of the values of positive ion mobility recorded in the literature. 



(b) Mr. W. SucKSMiTH. — Apparatus for determining the Gyromagnetic] 



Effect for Paramagnetic Substances. 



(For abstract, see Morning Session above.) 



(c) Dr. G. F. Brett." — The Syrdhesis of Ammoyiia by controlled Electron 



Streams. 



It is known that a beam of electrons passing through a mixture of nitrogen and ! 

 hydrogen causes these gases to combine, giving ammonia. It is also known that some 

 electron speeds are more efiScient than others in effecting this synthesis. 



The present experiments are concerned with measuring accurately these critical 

 speeds, and with the search for new ones. 



The method consists in freezing out the ammonia as it is formed, and measuring 

 the resulting fall in pressure ; the rate of pressure change measures the rate of reaction. 

 A special Pirani gauge is used for this purpose. 



Five definite electron speeds between 15 and 30 volts are found at which the reaction 

 rate increases sharply ; four of these correspond to ionised or excited states of nitrogen, 

 and the fifth to ionised hydrogen. 



