166 REPORT— 18638, 
be turned ; it could also be raised and lowered by a small barrel, and was 
adjustible in a horizontal plane by three set screws. The care taken in sus- 
pending the magnet and in protecting it both against currents of air and 
vibration was repaid by success, for the image of the scale reflected in the 
magnet was as clear and steady when the coil was making 400 revolutions 
per minute as when it was at rest. 
The governor used was lent by one of the Committee and will not be 
described in detail, as an improved governor on the same principle will be 
adopted in future experiments, in describing which an account of its construc- 
tion will be given. It may be said, however, that the little instrument actually 
employed generally controlled the speed to such uniformity as allowed the 
deflections to be observed with as much accuracy as the zero-point. 
The scale and telescope hardly require special description; they were 
arranged in the usual manner for this kind of experiment, at about three 
metres from the mirror. The scale was an engine-divided paper scale nailed 
to a wooden bar. This plan will in future experiments be abandoned, as 
variations in the weather had a very perceptible influence on the scale. 
The annexed diagram shows the electric balance by which the copper coil 
B 
ce “7 
4. S 8 y 4 Dw & 2 
| 4 ‘ oy ny SRS > ¢ a, 
wo) B ° x 3 
K f Ss / a, * 
, we s 
TM a 
K 
Vv 
C was compared with an arbitrary German-silver standard § before and 
after each induction experiment. The arrangement is that of the ordinary 
Wheatstone’s balance, as described in Appendix H of the Report of your 
Committee for 1862. A and C represent the arms of the balance as there 
described, 8 the German-silver standard, and R the copper coil to be mea- 
sured. JJ,, HH,, MM,, and LL, are four stout copper bars with mercury- 
cups at aa, d,.., 6b, b,..,¢¢,, anddd,. Twoshort copper rods F and F, can 
be used to connect a with 6 and ¢ with d. When this is done the arrange- 
ment is exactly that of the simple Wheatstone balance with the keys at K 
and K,, as deseribed in Appendix H of the last Report. A and C were coils 
formed of about 300 inches of No. 31* German-silver wire, and were adjusted 
to equality with extreme nicety, and each assumed equal to 100 arbitrary 
units. If Ron any occasion had been exactly equal to S, the galvanometer G 
would have been unaffected on depressing the keys K K,, when a was joined 
* Diameter =0'01 inch, 
