ON STANDAEDS FOR USE IN ELECTEICAL MEASUREMENTS. 139 



a stroboscopic method to a suitable tuning-fork provided with riders and 

 maintained in vibration electrically. The observer at the fork can shunt 

 more or less current through the electromotor driving the disc, and in 

 this way maintains the rate of rotation as constant as he can. But 

 though the electrically maintained fork is laseful for purposes of control 

 it cannot be relied on to give us the rate of rotation. Its vibration period 

 is not within my experience constant to the degi-ee of accuracy required. 

 If stopped and set going again it may start with a period different 

 by several parts in 10,000. No previous determination of the period 

 of the fork can therefore be relied on to give us the rate of rotation, 

 though once stai'ted the fork goes sufficiently uniformly to give us a 

 means of control. 



Accordingly it is necessary to measure the rate of I'otation during 

 each run while the galvanometer observations are being made. The 

 rotating disc is, by means of an eccentric attached to its axle, made to 

 record its revolutions on the tape of a Bain's electro-chemical telegraph 

 instrument side by side with the record of the standard clock. We have, 

 then, a time record exactly corresponding to the period of observation 

 of the galvanometer deflections. During the run the observer at the 

 galvanometer calls out the galvanometer readings, while the observer at 

 the tuning-fork controls the speed, and the Bain's instrument records it. 



I have made in this way a number of measurements daring the 

 months of July and August of a standard resistance of approximately 

 ■0005 ohm, prepared last year l)y my assistant, Mr. Harrison, and a 

 student in my laboratory, Mr. Parker, with the following results : — 



July 17, morning- 

 „ 17, afternoon 

 ,, 19, morning 



Aug. 2, afternoon 

 „ 3, morning 

 „ 4, „ 

 „ 4, afternoon 

 ,, 5, morning 

 q 



,, 9, afternoon 



Mean 



00050016 

 00050016 

 00050015 

 00050020 

 00050021 

 00050016 

 00050013 

 00050019 

 00050021 

 00050018 



00050017 



The maximum divergence from the mean is -00000004, or about one 

 part in 12,000. Mr. Crompton has recently been issuing standards of 

 low resistance made of manganine sheets, and he was kind enough, at my 

 suggestion, to send me one for measurement towards the end of July. 

 It was prepared in his laboratory as a derivative from the Cambridge 

 ohm by means of his potentiometer. Its value so given was -00050175 

 at 23° C. Its temperature coefficient appears, from the measurements 

 made in Mr. Crompton's laboratoi-y, to be so small that we need hardly 

 consider it for our present purpose. My measurements of this standard 

 were as follows : — 



•00050219 

 ■00050225 

 •00050219 

 -00050226 



Mean . . -00050222 

 ■which differs from Mr. Crompton's value by something less than one part 



