1852.) OF THE ROYAL INSTITUTION. 107 
A compound bar magnet was so fitted up that it could revolve on 
its axis, and a broad circular copper ring was fixed on it at the middle 
distance or equator, so as to give a cylindrical exterior at that place. 
A copper wire being made fast to this ring within, then proceeded to 
the middle of the magnet, and afterwards along its axis and out at 
one end. A second wire, touched, bya spring contact, the outside of 
the copper ring, and was then continued outwards six inches, after 
which it rose and finally turned over the upper pole towards the first 
wire, and was attached to a cylinder insulated from but moving round 
it. This cylinder and the wire passing through it were con- 
nected with the galvanometer, so that the circuit was complete ; but 
that circuit had its course down the middle of the magnet, then 
outwards at the equator and back again on the outside, and whilst 
always perfect, allowed the magnet to be rotated without the external 
part of the circuit, or the latter without the magnet, or both together. 
When the magnet and external wire were revolved together, as one 
arrangement fixed in its parts, there was no effect at the galvan- 
ometer, however long the rotation wascontinued. When the magnet 
with the internal wire made four revolutions, as the hand of a watch, 
the outer conductor being still, the galvanometer needle was deflected 
35° or 40° in one direction : when the magnet was still, and the outer 
wire made four revolutions as the hands of a watch, the galvan- 
ometer needle was deflected as much as before in the cont#ary direc- 
tion: and in the more careful experiments the amount of deflection 
for four revolutions was precisely the same, whatever the course of 
the external wire, either close to or far from the pole of the magnet. 
Thus it was shewn, that when the magnet and the wire revolved in 
the same direction, contrary currents of electricity, exactly equal to 
each other, tended to be produced ; that those outside resulted from 
the intersection by the outer wire of the lines of magnetic force 
external to the magnet ; that wherever this intersection was made 
the result was the same; and that there were corresponding lines 
of force within the magnet, exactly equal in force or amount to those 
without, but in the contrary direction. That in fact every line of 
magnetic force is a closed curve, which in some part of its course, 
passes through the magnet to which it belongs. 
In the foregoing cases the lines of force, belonging as they 
did to small systems, rapidly varied in intensity according to their 
distance from the magnet, by what may be called their divergence. 
The earth, on the contrary, presents us, within the limits of one 
action at any one time, a field of equal force. The dipping needle 
indicates the direction or polarity of this force; and if we work in 
a plane perpendicular to the dip, then the number or amount of 
the lines of force experimented with will be in proportion to the 
area which our apparatus may include. Wires were therefore 
formed into parallelograms, inclosing areas of various extent, as 
one square foot, or nine square feet, or any other proportion, and 
being fixed upon axes equidistant from two of the sides could 
