VALUE OF THE BRITISH ASSOCIATION UNIT OF RESISTANCE. 
233 
If we remember that when the coils are parallel the value of M is a maximum, so 
that the error due to the small angle between them depends on the square of the 
angle, it is clear that in our case this error is vanishingly small. 
The galvanometer also was designed by Professor Maxwell, and wound by Professor 
Chrystal. It is also referred to in the article “Galvanometer,” in the ‘ Encyclopaedia 
Brittanica,’ 9th edition. The description of it is taken from his account in the 
j laboratory book, dated July, 1876. 
There are two channels of rectangular section, and the following approximate 
dimensions :— 
indies 
Depth of channel ...... 
• • h-V 
External diameter of bobbin 
. . 4 
Breadth of channel ..... 
0 1 
8 
Distance between channels . 
02 3 
Each channel contains 20 layers of thin copper wire and 16 layers of thick, making 
about 465 and 202 double turns respectively, so that there are 667 double turns in 
each channel, and about 2668 single turns on the galvanometer. 
inches. 
[Diameter of copper in thin wire. '014 
\ Silk and all 82 thicknesses lie in.1| 
[Diameter of copper in thick wire.*029 
l Silk and all 34 thicknesses lie in ..... 1^- 
The two thicknesses of wire were employed in order to till the channels, and at the 
same time permit the resistance of the galvanometer to be reduced to the requisite 
amount. The ends of the wires are connected to binding screws on the bobbin marked 
A, B, &c., a, b, &c. A to a is one wire, B to b another. In our experiments the coils 
were connected up in series, the total resistance being about 60 ohms at a temperature 
of 13°-2 C. 
The needle of the galvanometer was suspended from the Weber suspension by 
three single cocoon fibres of 60 centims. in length. 
The magnet was a small bar of hardened steel 1‘5 centim. long, '6 centim. broad, 
and T2 centim. thick ; its weight was ’708 grin. The magnet was attached by two 
! small screws to a brass stirrup to which the mirror was fixed. A piece of brass wire 
5'6 centims. long, with a screw thread cut on it, was fixed to this stirrup at right 
angles to the plane of the mirror, projecting equally on either side of the mirror. 
Two small brass cylinders could be screwed along this brass wire, and by means of 
them the moment of inertia and time of swing of the needle could be adjusted as 
required. The stirrup and mirror weighed 6'6 grms. 
The galvanometer rested on a solid wooden base of about 18 centims. diameter, and 
this base was supported on three levelling screws. 
MDCCCLXXXIIL 2 II 
