1895.] on the Absolute Measurement of Electrical Hesistance. 613 



Table I. gives a set of measurements of the pitch of a bowed 

 fork ; I bring it before you as indicating the accuracy both of the 

 synchronising and the time measurement. 



Table I. 



Extreme variation from mean is about 1 in 65,000. 



When I first began working at this method I found it extremely 

 difficult when the disc was in rotation to obtain a steady reading at 

 the galvanometer. My galvanometer is a very sensitive Thomson 

 reflecting galvanometer of about 1 ohm resistance, made by Elliott 

 Bros, and fitted by my assistant, Mr. Harrison, with a long suspension 

 for the support of the needle by a quartz fibre. When the disc was 

 in uniform rotation the spot of light roamed continually over 50 or 

 100 divisions of the scale, and frequently there were sudden jerks of 

 a most embarrassing kind. Eeadings with a galvanometer so sensitive 

 were in fact impossible. (The scale divisions are 40 to the inch, and 

 the scale is about 40 inches from the galvanometer.) 



Now the electromotive-force in the circuit of the galvanometer 

 when there is no current through the standard coil is due to three 

 causes : — 



1. The cutting of the earth's flux of induction by the radius of 

 the rotating disc. The electromotive-force due to this cause is small 

 in my apparatus, owing to the fact that the plane of my disc nearly 

 coincides with the magnetic meridian. For a uniform rate of rotation 

 it is constant. 



2. The thermo-electric forces due to the general distribution of 

 temperature in the galvanometer circuit. Changes in this distribution 

 would be gradual, and would not account for the sudden irregular 

 movements of the galvanometer needle. 



3. The thermo-electric force at the contact of the external brush 

 with the circumference of the disc. The sudden movements of the 

 needle were obviously due to variations in this thermo-electric force. 



I made many experiments with a view of diminishing these 

 changes. I first tried an ordinary phosphor-bronze brush made of & 

 Vol. XIV, (No. 89.) 9 * 



