﻿Secohmmeter Measurement of Resistances and Capacities. 297 



In conclusion I wish to express my thanks to Professor 

 J. J. Thomson for his kindly interest and advice during the 

 research. 



Cavendish Laboratory, June 30, 1906. 



Note. — The fact that lack of saturation in a gas depends 

 on the velocity of the ionizing a particle explains the deviation 

 in certain ratios occurring in the paper on the recombination 

 of ions by Prof. Bragg and the writer. (See Phil. Mag. 

 April 1906, p. 475.) Prof. Bragg was the first who observed 

 that lack of saturation was smaller when the ionization- 

 chamber was brought nearer to the radium ; but he seems not 

 to have been quite sure that it was a true effect, since be did 

 not mention it in the above or in any of the subsequent papers. 



But in a letter to the writer just come to hand he mentions 

 that he has established that there is a distinct change in the 

 lack of saturation with velocity of the a particle from radium. 



XXXVII. The Use of the Secohmmeter for the Measurement 

 of Combined Resistances and Capacities. By S. R. MlLNER, 

 jD.Sc, Lecturer in Physics, University of Sheffield*. 



PROFESSORS Ayrton and Perry's secohmmeter is an 

 instrument which consists essentially of two rotary 

 commutators mounted on the same axle, by the rotation of 

 which rapid reversals of the battery and galvanometer ter- 

 minals of a TTheatstone's-bridge system may be produced. 

 It was originally designed for comparison of inductances, but 

 it may be used equally well for the measurement of any 

 single electrical quantity (e. g. capacity or electrolytic re- 

 sistance) which is best measured by an alternating-current 

 balance on a Wheatstone's bridge. So long as it is a question 

 of measuring single resistances or capacities only, the secohm- 

 meter and galvanometer are indeed much superior to the 

 induction-coil and telephone of the older method. The chief 

 advantage is of course the much greater sensitiveness than 

 the telephone which the galvanometer has as a balance indi- 

 cator; but in addition to this, an important point in practice 

 is the convenience arising from the fact that the galvanometer 

 deflexions themselves indicate the direction in which the 

 adjustable arm requires to be varied. 



Several classes of electrochemical measurements have 

 hitherto had to be carried out by the induction-coil and 

 telephone method, since in them the use of the secohmmeter 



* Communicated by the Author. 



