ON THE LINES OF MAGNETIC FORCE. 
,‘35 
curves equal to half those of a great plane, however small or great the length of the 
wire may be ; and though by its tortuous course it may pass out of one plane into 
another on its way to the equator. 
3101. Further, if such a wire as that last described be revolved once round the 
end of the magnet to which it is related, a slipping contact at the equator being 
permitted for the purpose, it will intersect all the lines of force during the revolu- 
tion ; and that, whether the polar contact is absolutely coincident with the magnetic- 
axis, or is anywhere else at the end of the bar, provided it remain for the time un- 
changed. All this is true, though the magnet maybe subject, by induction at a distance, 
to other magnets or bodies, and may be exerting part of its force on them, so as to 
make the distribution of its power very irregular as compared to the case of the in- 
dependent bar (3084.), or may have an irregular or contorted shape, even up to the 
horseshoe form. It is evident, indeed, that if a wire have one of its ends applied to 
any point on the surface of a magnet, and the other end to a point in the magnetic 
equator, and the latter be slipped once round the magnetic equator, and the loop of 
wire be made to pass over either pole, so as at last to resume its first position, it 
will in the course of its journey have intersected once every line of force belonging 
to the magnet. 
3102. A wire from pole to pole which passes close to the equator, of course inter- 
sects half the external lines of force in a great plane, twice, in opposite directions as 
regards the polarity ; and, therefore, when revolved round the magnet, has no electric 
current induced in it. If it do not touch at the equator, still, whatever lines it inter- 
sects, are twice intersected, and so the same equilibrium is preserved. If the magnet 
rotate under the wire, it acts the part of the central rotating wire already referred to 
(3095.) ; or if any course for the electric current other than a right line is assumed 
in it, that course is subject to the law of neutrality above stated, as will be seen by 
reference to the internal condition of the magnet itself (3117.). Hence the reason 
why no currents are produced, under any circumstances of motion, by the application 
of such conducting circuits to the magnet. I may further observe, in reference to 
the intersection of the lines of force, that if a wire ring, a little larger in diameter 
than the magnet, be held edgeways at one of the poles, so that the lines of force 
there shall be in its plane, and be then turned 90° and carried over the pole to 
the equator (3088.), it will intersect once all the lines of the magnet, except the very 
few’ which will remain unintersected at the equator. 
3103. Whilst endeavouring to establish experimentally the definite amount of the 
power represented by the lines offeree, it is necessary to take certain precautions, or 
the results will be in error. For instance, ten revolutions of the wire about the 
magnet, or of the magnet within the fixed wire (3097-), ought to give a constant de- 
flection at the galvanometer, and yet without any change in the position of the wire 
the results may at different times differ very much from each other ; being at one 
time 9°, and at another only 4° or 5°. I found this to be due to difference of velocity 
within certain limits, and to be explained and guarded against as follows. 
F 2 
