42 REPORT—1883. 
originally said to be correct are slightly different. The greatest difference 
is that between the coils marked C and G, and amounts to ‘0011 mean 
B. A. Unit. 
The mean of all these resistances at the respective temperatures is 
taken as the mean B. A. Unit, and is that to which the resistance coils sent 
for testing are referred. 
The coils examined are those marked as below in previous reports. 
| A | B | Cc | D E | F G | 
36 | (29 43 | Flat | 1867 
Or 
bea)? | ale 
In comparing the single unit coils the form of resistance bridge devised 
by Mr. Fleming and described by him (‘ Proceedings of the Physical 
Society,’ vol. iii.) is employed. 
The bridge, with battery, keys and a suitable galvanometer, is per- 
manently fitted up in a ground-floor room with a north aspect. The stan- 
dard coils are kept in a case in the same room, and the baths in which the 
coils are to be immersed are always ready filled with water, which is thus 
at the temperature of the room. 
When a coil is to be tested, a suitable standard is chosen, and the two 
are placed in the water baths and left at least three or four hours—more 
usually over night. The comparison is then made in the ordinary manner 
by Professor Carey Foster’s method,! and the coils again left for some time 
without being removed from the water. After this second interval another 
comparison is made. he temperatures of the water baths are taken at 
each comparison, and as a rule differ very slightly. 
We thus have two values of the resistance of the coil to be tested at 
two slightly different temperatures. 
The mean of these will be the resistance of the coil in question at the 
mean of the two temperatures. 
We are thus able to issue a certificate in the following form :— 
‘ This is to certify that the coil No. X has been compared with the British 
Association Standards, and that its value at a temperature of A° C. is 
P B.A. Units or P’ R. ohms; 1 B. A. Unit being ‘9867 R. ohms.’ We 
further propose to stamp all coils in the future with this monogram iy 
and a reference number. 
One single unit coil by Messrs. Latimer Clark, Muirhead, & Co., three 
by Messrs. Elliott Brothers, for Professor Mascart, and one by Messrs. 
Simmons & Co., have been tested. 
It will be noticed that nothing is said about the temperature coeffi- 
cient of the coil or the temperature at which the coil is accurately 1 B. A. 
Unit. To determine this exactly is a somewhat long and troublesome 
operation, but at the same time it is one which every electrician, if he 
knows the value of the coil at one given temperature, can perform for 
himself with ordinary testing apparatus. It does not require the use of 
the standards. For many purposes the approximate value of the tem- 
perature coefficient obtained from a knowledge of the material of the 
coil will suffice; we may feel certain that anyone requiring greater 
accuracy would be quite able, and would prefer, to make the measure- 
1 Journal of Soc. of Telegraph Engineers, 1874. 
ea a i a cad hah es | 
