164 REPORT—1863. 
The coil of wire is made to revolve about a vertical diameter with constant 
velocity. The motion of the coil among the lines of force due to the earth’s 
magnetism produces induced currents in the coil which are alternately in 
opposite directions with respect to the coil itself, the direction changing as 
the plane of the coil passes through the east and west direction. If we con- 
sider the direction of the current with respect to a fixed line in the east and_ 
west direction, we shall find that the changes in the current are accompanied 
with changes in the face of the coil presented to the east, so that the absolute 
direction of the current, as seen from the east, remains always the same. If 
a magnet be suspended in the centre of the coil, it will be deflected from the 
north and south line by the action of these currents, and will be turned in 
the same direction as the coil revolves. The force producing this deflection 
is continually varying in magnitude and direction, but as the periodic time 
is small, the oscillations of the magnet may be rendered insensible by in- 
creasing the mass of the apparatus along with which it is suspended. The 
resistance of the coil may be found when we know the dimensions of the 
coil, the velocity of rotation, and the deflection of the magnet. The intensity 
of terrestrial magnetism enters into the measurement of the electromotive 
force, and also into the measurement of the current; but the measure of the 
resistance, which is the ratio of these two quantities, is quite independent of 
the value of the magnetic intensity. 
Part II. Description of the Apparatus—For convenience of description, 
the apparatus with which the experiments were made may be divided into 
five parts:—1°, the driving gear; 2°, the revolving coil; 3°, the governor ; 
4°, the scale, with its telescope, by which the deflections of the magnet were 
observed; 5°; the electric balance, by which the resistance of the copper coil 
was compared with a German-silver arbitrary standard. 
The general arrangement of the first four parts is shown in the diagram, 
fig. 4, Plate VI. 
~ The driving gear consisted of a leaden flywheel X on a shaft A, turned 
by hand, and communicating its motion by a band, 66, 6,...., arranged in 
a way equivalent to Huyghens’s gearing, to a shaft B, a pulley on which drove 
the revolving coil by a simple bandaa,a,..... The arrangement of the band 
bb, b,.... communicating the motion of shaft A to shaft B may be easily 
understood from the diagram. CC are two guide-pulleys running loose on 
pins attached to the main framing. DD are two loose pulleys maintained at 
a constant distance by the strut E, to which the weight W is hung. 
When the rotation of shaft B.is opposed by a sufficient resistance, the 
effect of turning the flywheel in the direction shown by the arrow is to lift 
the weight W from the ground, tending to turn the shaft B with a definite 
force, which will be sensibly constant so long as the weight is kept off the 
ground and the band 46, 6,.... remains unaltered in length. Wherever, as 
in the present experiments, the resistance increases with the speed of rota- 
tion, the speed of the driving-wheel can easily be regulated by hand, so as to 
keep the weight from falling so low as to touch the ground, or rising so high 
as to foul the gear, and thus, with a little care, a constant driving force can 
be applied to the shaft B, and to the machinery connected with it. 
The revolving coil formed the most important part of the apparatus. It is 
shown one-fifth full size in figs. 1 and 2, Plate VI. 
A strong brass frame H H was bolted down by three brass bolts F F F, 
dowelled into a heavy stone. It could be accurately levelled by three stout 
screws GGG, The brass rings II,, on which the insulated copper wire 
was coiled, were supported on the frame by a pivot J, working in lignum 
