ON THE LINES OF MAGNETIC FORCE. 
31 
secting parts during the time of the motion: the direction of which current will be 
determined (with lines having a given direction of polarity) by the direction of the 
intersection, combined with the relative amount of the intersection in the two or 
more efficient and determining (or intersecting) parts of the circuit. 
3088. Thus, if fig. 3 represent a magnetic pole N, and over it a circuit, formed of 
metal, which may be of any shape, and which is at first in the 
position c ; then if that circuit be moved in one direction into 
the position 1 ; or in the contrary direction into position 2 ; or 
by a double direction of motion into position 3 ; or by transla- 
tion into position 4 ; or into position 5 ; or any position between 
the first and these or any resembling them ; or, if the first posi- 
tion c being retained, the pole move to, or towards, the posi- 
tion n ; then, an electric current will be produced in the circuit, 
having in every case the same direction, being that which is 
marked in the figure by arrows. Reverse motions will give currents in the reverse 
direction (256. &c.). 
3089. The general principles of the production of electrical currents by magnetic 
induction have been formerly given (27- &c.)=^, and the law of the direction of the 
current in relation to the lines of force, stated (114, 3079 note). But the full mean- 
ing of the "above description can only be appreciated hereafter, when the experimental 
results, which supply a larger knowledge of the relations of the current to the lines 
of force, have been described. 
3090. When lines of force are spoken of as crossing a conducting circuit (3087.), 
it must be considered as effected by the translation of a magnet. No mere rotation 
of a bar magnet on its axis, produces any induction effect on circuits exterior to it; 
for then, the conditions above described (3088.) are not fulfilled. The system of 
power about the magnet must not be considered as revolving with the magnet, any 
more than the rays of light which emanate from the sun are supposed to revolve 
with the sun. The magnet may even, in certain cases (3097.), he considered as 
revolving amongst its own forces, and producing a full electric effect, sensible at the 
galvanometer. 
Fig. 3. 
»/'■' 
> 'A 
3091. In the first instance the wire was carried down the axis of the magnet to 
Fig. 4. 
the middle distance, then led out at the equatorial part, and returned on the out- 
side ; fig. 4 will represent such a disposition. Supposing the 
magnet and wire to revolve once, it is evident that the wire 
a may be considered as passing in at the axis of the magnet, 
and returning from h across the lines of force external to the 
magnet, to the axis again at c ; and that in one revolution. 
* Philosophical Transactions, 1832, page 131, &c. 
