OF ELECTROSTATIC UNITS IN THE ELECTROMAGNETIC UNIT. 
425 
Determination of the Area of the Movable Coil. 
Before unwinding the coils for the purpose of measuring the length of the wire, and 
counting the number of turns in each coil, a comparison of the large coils and the sus- 
pended coil was made to determine the magnetic moment of the latter. This quantity, 
which appears in the calculations as inc 2 , could not be obtained very accurately by mea- 
surement of the outer and inner radii of the coil, seeing that the difference between these 
radii was very considerable in comparison with either. 
The method of comparison followed was suggested by Professor Clerk Maxwell. 
The simultaneous action of the large coils and the smaller coil upon a magnet placed 
in their neighbourhood was observed. The distances between the coils and the magnet 
being so arranged that the simultaneous action of the opposing coils upon the magnet 
was zero, and these distances being known, the ratio between the areas of the coils was 
easily obtained. The chief difficulty being to measure accurately these distances, two 
symmetrical observations were made with the smaller coil on opposite sides of the magnet. 
The distance between the positions of the same part of the coil in these two observations 
was measured, and half this distance taken as the distance of the centre of the coil from 
the magnet in each position. 
The coils (see Plate XXXII. fig. 5) rested upon a narrow table ( t ) which had been pre- 
pared for this comparison. A V-groove (v) ran from end to end of it parallel to its edge, 
near one side, to guide the adjustment of the magnet and smaller coil. The stands (s, s') 
to which the large coils had been attached were placed at opposite ends of the table, and 
were so adjusted that the line joining the centres of the coils passed through the centre 
of the magnet. 
The magnet (m), which had been used as a declinometer, was suspended by a long fibre 
in an enclosed space. It carried a light galvanometer-mirror, whose motions were visible 
through a small lens, which formed part of the glass front of the instrument. The 
declinometer, two of its feet resting in the V-groove, was placed between the coils, so 
that the centre of the mirror or magnet was equidistant from the centres of the large 
coils. The “ suspended” coil, supported on a wooden stand (p), was placed between the 
magnet and one of the large coils, its motions to and from the magnet being guided 
parallel to itself by the groove in which two of the feet of the stand rested : it was 
adjusted so that its centre also should be in the line joining the centres of the large 
coils. The three coils were arranged so as to have their planes parallel, and so that 
the action of the two large coils should conspire to oppose the action of the smaller coil, 
which was placed much nearer the magnet. The motions of the magnet were observed 
in the usual manner, by reflection from the mirror. A current from a battery of 120 
cells was made to pass through the three coils in series. The smaller coil was moved 
to and fro parallel to itself until no deflection of the magnet could be detected. During 
this observation it was found that the magnet was sensitive to the smallest motion of 
the coil that was appreciable. 
mdccclxxiii. 3 L 
