PRACTICAL STANDARDS FOR ELECTRICAL MEASUREMENTS. 



!17 



to render the effect of possible errors in the temperature coeflBcients. 

 negligible to the degree of accuracy aimed at by us. "We have, therefore, 

 used the August 1897 Board of Trade values for these coils as transmit- 

 ting the Board of Trade ohm to the laboratory in Exhibition Road. 



The standard thermometers used in the investigation were sent to 

 Kew and their en-ors were determined at the time by the kindness of 

 Dr. Chree ; also, thanks to Sir J. Norman Lockyer, the clock in the 

 Mechanical Department of the Central Technical College, which trans- 

 mitted seconds to the fast running Bain Chronograph, was frequently 

 timed by reference to the current sent hourly to his room from the 

 General Post OfEce, and at 10 A.M. from Greenwich. 



The results of successive measurements of the absolute resistances 

 became very concordant after, little by little, various possible causes of 

 small errors had been eliminated. Nine sets taken on July 30, 1897,, 

 gave the following results for the value of the Board of Trade ohm in. 

 true ohms, without allowance for the error in the clock rate. 



1-000286 

 1-000256 

 1-000285 

 1-000351 

 1-000295 

 Mean 



1-000277 

 1-000306 

 1-000284 

 1-000307 



1-000294 



or, since the clock was found to lose, during the daytime, at the rate of 

 three seconds per twenty-four hours, it follows that according to this, 

 investigation 



1 Board of Trade ohm= 1-00 026 true ohms. 



It is important to consider in which direction this result will b& 

 affected by sources of error that cannot be removed by careful adjustment,, 

 centering, &c. They may be classified as follows : — 



Source of Error. 



1. Over-estimation of the diameter of the coil arising, 



for example, from the stress on the copper wire 

 having caused it to compress the under side of its 

 silk covering. 



2. Under- estimation of the diameter of the phosphor 



bronze disc from a neglect of the tips of the cir- 

 cumferential brush tubes being possibly pushed 

 away from the disc by the stream of mercury 

 issuing, &c. 



3. Presence of iron pipes, girders, &c. in the neighbour- 



hood of the apparatus. 



4. Traces of iron in the phosphor bronze disc. 



support of the 

 of the circum- 



5. Defective insulation between the 



central brush and the supports 

 ferential brushes. 



6. Defective insulation between the convolutions on the 



coil. 



7. Traces of iron in the marble ring. 



Effect Produced. 

 Result would be toe- 

 small. 



Result would be too 

 small. 



Result would be too 



small. 

 Result would be very 



slightly too small. 

 Result would be tocv 



large. 



Result would be toc> 



large. 

 Kesult would be too 



large. 



