ON STANDARDS OF ELECTRICAL RESISTANCE. 165 
vite, and by a hollow bearing K, working in brass: this bearing worked in 
a kind of stuffing-box & (fig. 3), which by three screws and a flat spring 
washer between it and the frame at 7, could be adjusted to fit the collar e 
with great nicety, preventing all tendency to bind or shake. Supported in 
this way the coil revolved with the utmost freedom and steadiness. 
The coil of copper wire was necessarily divided into two parts on the two 
rings II,, to permit the suspension of the magnet 8. The two brass rings 
were each formed of two distinct halves, insulated from one another by vul- 
canite at the flanges ff,. This insulation was necessary to prevent the in- 
duction of currents in the brass rings. These rings, after being bolted 
together, were turned with great accuracy by Messrs. Elliott Brothers. The 
insulated copper wire was wound in one direction on both rings; the inner 
end of the second was soldered to the outer end of the first; the two extreme 
ends of the conductor thus formed were soldered to two copper terminals hh’, 
insulated by the vulcanite piece x bolted to the brass rings. Each terminal 
was provided with a strong copper binding screw, and had a mercury-cup 
drilled into its upper surface. The two coils could be joined, so as to form a” 
closed cireuit, by a short copper bar between the binding screws. The bar, 
binding screws, and nuts were amalgamated to ensure perfect contact. When 
the copper coils were to be connected with the electric balance, the short 
copper bar was removed and the required connexions were made by short 
copper rods, one quarter of an inch in diameter, dipping at one end into the 
mereury-cups on the terminals hh’, and at the other end into the mercury- 
cups of the electric balance. The absence of all induced currents influencing 
the suspended magnet when the circuit was broken at hh’ was repeatedly 
proved by experiment. 
Rotation was communicated to the coils by a catgut band simply making 
half a turn round the small V-pulley 7. The band could be tightened as 
required by the jockey pulley z and weight w (fig. 4). 
A short screw of large diameter, n, gearing into a spur-wheel of one hun- 
dred teeth, 0, formed the counter from which the speed of rotation was 
obtained, as follows. A pin p on the wheel o lifted the spring g as it 
passed ; this spring in its rebound struck the gong M. The blow was of 
course repeated at every hundred revolutions, and the time of each blow was 
observed on a chronometer. The arrangement was equally adapted for rota- 
tion in either direction. 
A second V-pulley r served for the band ¢¢, communicating motion to the 
governor by which the speed was controlled. 
The manner in which the suspended magnet was introduced to the centre 
of the coil is best seen in fig. 3. A brass tripod N, bolted to the main 
frame, supported the long brass tube O, which passed freely through the 
hollow bearing at K. A cylindrical wooden box P slipped on to the end of 
the tube O. The magnet hung inside this box, the lower part of which 
could be removed to allow the exact position of the magnet to be verified. 
The support N also carried a short brass tube R, on which the glass case 
T could be secured by a little sliding tube. The mirror t, attached to the 
magnet S by a rigid brass wire, hung inside this glass case by a single 
cocoon fibre about eight feet long. This fibre was protected against currents 
of air by a wooden case (not shown in the Plate), extending from the point 
of support down to the glass case. A little sliding paper prolongation of the 
Wooden case made it nearly wind-proof by fitting at the bottom against the 
main brass frame. An opening in the case allowed the mirror to be seen.. 
The fibre at the top was suspended from a torsion head, by which it could 
