ON STANDARDS FOR USE IN ELECTRICAL 3IEASDREMENTS. 



65 



correct. In terms of this unit we find, Table IX., tlie present values at 

 the temperatures of 1867. We also give in the last colnmn but one the 

 temperatures at which these coils have the value I B.A. Unit, and in the 

 final column the temperature coefficients per 1° Centigi-ade also in B.A. 

 Units. 



There remains now for consideration the result of comparisons which 

 we have made on various other standard coils originally issued by the 

 Committee, and which have most kindly been put at our disposal by their 

 owners for the purposes of the report. 



Messrs. Elliott Bros, have three coils. One, No. 41 of the original 

 set, was made by Matthiessen in 1864. A second, No. 56, was first 

 examined by Lord Rayleigh in 1882 : these two are B. A. Units, while the 

 third, Elliott, No. 117, is a legal ohm, first tested by R.T.G in 1884. 

 These coils are all of platinum silver, with a temperature coefficient of 

 •00028. Table X. gives the temperatures at which they were found 

 correct at different dates. 



Table X. 



The observations made in 1887 are separated from the others by a 

 double line because daring 1886 it was observed that the paraffin used 

 in the insulation was becoming green, and it was therefore removed and 

 replaced by pure ozoherit. In consequence of this some change may 

 easily have taken place in the coils, and the record after 1884 must be 

 treated as a fresh one. 



In the first coil the most noticeable point is the drop of 2° between 

 1864 and 1879 ; but since this drop is followed by a rise of 1° in the next 

 twelve months one may feel uncertain as to whether it is real or due to 

 some error in 1879. 



In the nest five years there appears to be a gradual rise in tempera- 

 ture corresponding to a fall in resistance ; the total amount would cor- 

 respond to a change in resistance of about '0004 B.A. Unit. The 

 removal of the paraffin has seriously affected No. 41. 



The next coil also of platinum silver, is one belongiiig to Professor 

 Carey Foster. He writes as follows : — ' It professed to be equal to 



1 B.A. Unit at I4°-2 C. 



I had it direct from Matthiessen, who, I believe, adjusted it specially for 

 me from his standards.' On comparing it with F in May 1887 we find 

 that it has a resistance of "99983 Mean B.A. Unit at 16°'2. It would 

 therefore be right at 16"8. 



This, of course, shows a considerable change, corresponding apparently 

 to a fall in its resistance of about -00073 B.A. Unit. It will be noticed 



1888. F 



