MR. LATIMER CLARK ON A STANDARD VOLTAIC BATTERY. 
9 
centre of the telescope reflected in the silvered glass also coincided with the cross wires, 
the telescope was of course directed normally to the centre of the plane of the coils, 
which were adjusted in the magnetic meridian ; excessive care was taken with all the 
adjustments and readings, which were repeated with reversed currents and positions. 
In order to maintain a current through the coils of the dynamometer and to ensure 
that the difference of potential between its poles should be precisely equal to that of the 
standard cell, an auxiliary battery was used in the manner before described. This con- 
sisted of five large Daniell’s cells working through a circuit consisting of the dynamo- 
meter and a rheostat, r (fig. 4) ; a and b are the two terminals of the dynamometer, and 
the poles of the auxiliary battery, C, Z, are connected with them ; the similar poles of the 
standard elements c, z, and the galvanometer, g, are also connected to the same*termi- 
nals ; and the rheostat r is adjusted so that no current passes through the galvanometer. 
In this case it is. evident that the poles a and b are maintained at a difference of potential 
precisely equal to that of the standard elements. In this arrangement not the slightest 
difficulty is experienced in maintaining a perfectly steady and uniform current through 
the coils of the dynamometer. The poles of the dynamometer were so arranged that 
they could be connected immediately with a Wheatstone balance in order that its 
resistance could be measured promptly after each observation. 
Eig. 4. 
The winding of the large coils of the instrument was superintended by Professor 
Clerk Maxwell, who kindly supplied me with the measurements as follows : — 
millimetres. 
Mean circumference of first coil 1558*48 
Mean circumference of second coil 1559*16 
The depth of each coil is 12*90 
The breadth of each coil is 15*00 
The distance apart of the planes of the coils . 250 
mdccclxxiv. c 
