612 PROF. J. J. THOMSON AND MR. G. F. C. SEARLE ON THE RATIO OF THE 
The Galvanometer. 
This was one made in the Laboratory. It has two coils, each with about 16,000 
turns of wire. The resistance of the cobs when in series is 17,380 legal ohms. Great 
care was taken with the insulation, which, when tested by means of a gold leaf 
electroscope, v/as found to be practically perfect. 
The Battery. 
We used 36 very small storage cells, two sets of 18 being placed in parallel. 
The battery had thus an electromotive force of about 36 volts. The small size of the 
battery enabled us to insulate it with ease. The insulation was tested by a gold leaf 
electroscope, but no leak could be detected. 
The Resistances. 
The resistances used were contained in three boxes. 
I. A Wheatstone’s bridge box. No. 1256, Elliott. Legal ohms, with coils 
ranging from 1 to 5000 ohms, and proportional arms, each containing 10 , 
100 , 1000 ohms. 
11. A box by Elliott, containing 4 coils, 10,000, 20,000, 30,000, 40,000 B.A. 
units. 
III. A box by Muiriiead, containing originally 10 coils, each 100,000 B.A. 
units. 
The box I. was provided with an aperture for a thermometer. The other boxes 
had no such provision, but as they were always kept permanently on the same table 
as box I. we may hope that their temperature did not differ much from that of I. 
The resistances of the coils in II. and III. were always ascertained by means of I. 
It was, therefore, necessary to obtain some definite knowledge of I. with reference to 
some coils whose values are accurately known. 
The standard coils used were 
No. 141, C.L.C. No. 102 . . . 10’00103 legal ohms at 16°'7 C. 
No. 143, C.L.C. No. 104 ... 99-9977 „ „ 16°-05 C. 
No. 145, C.L.C. No. 106 ... 1000-306 „ „ 17°-4 C. 
The Legal ohm being assumed to be 1/-9889 B.A. unit. 
See ‘ British Association Ileport,’ 1885. 
Mr. Glazebuook informs us that the temperature coefficient of these coils may be 
taken as -0003 per 1° C. We shall take the same temperature coefiicient for the 
coils in the boxes. 
Calling the coils in the “proportional arm” which joins the rest of the box 10„, 
100a, 1000a, those in the other arm 10/„ 100^, IOOO 4 -we finind 
