LINES OF MAGNETIC FORCE — MAGNETIC POLARITY, 
51 
intended to receive and carry one or more discs of metal that might be screwed on to 
it. This end projected so far beyond the support, that such discs could be partly 
introduced between the poles of a horseshoe magnet, so as when revolving, to move 
across the lines of force at their most intense place of action ; and, whilst the mag- 
net and the apparatus continued fixed, to revolve continuously across the same lines 
of force. One of the galvanometer wires was pointed, and so held as to bear into and 
against the surface of a cup-shaped cavity at the end of the axial screw; and the 
other was applied by the hand, or so fixed as to bear by a rounded part against the 
rim of the disc, at that point which was furthest within the poles of the magnet. 
3160. Discs of metal were prepared for this apparatus, each 2*5 inches in diameter, 
and of different thicknesses and material. When a disc of copper was fixed on the 
axis, and adjusted in association with the large horseshoe magnet (3159.), as de- 
scribed above, three, or even two revolutions of it, would deflect the needle of the 
thick wire galvanometer through a swing of 30°. In this apparatus, the most effec- 
tual part of the portion of the disc which is at any moment passing across the mag- 
netic axis, is that which is near the circumference ; for it has the greatest velocity, 
consequently moves through more space, and that in a part where the lines of force 
are most concentrated. 
3161. The contact at the end of the axle should always be carefully watched and 
made good. The degree of pressure on the edge of the disc should not be too slight ; 
otherwise the contact, under the circumstances of the motion, is not sufficient to 
carry forward the same constant proportion of current generated. Neither should it 
be made at the angles of the disc edge ; if a grating or cutting friction occur, an 
electric current is generated by it. With a smooth hard friction of copper wire 
against the copper disc there is very little evolution of current. When the copper 
wire presses against the edge of an iron disc there is far more. In either case, 
however, the effect may be eliminated or compensated ; for, in whichever direction 
the disc is revolved without the magnet, the deviation of the needle, if any be pro- 
duced, remains the same; whereas, when the magnet is in place, the deviations pro- 
duced by it are in the reversed direction for reversed revolutions. Hence, if an 
equal number of revolutions be made in the two directions, and the unequal deflec- 
tions in opposite directions be noted, the half of their sum will give nearly the amount 
of deflection which would have occurred if no current had been exerted by friction 
at the edge, i. e. provided the deflections have not been through large arcs. These 
effeets of friction are no doubt objections to the principle in this form ; still the 
results are, as it appears to me, valuable in relation to copper and iron, and are as 
follows. 
3162. A eopper disc, 0*05 of an inch in thickness, gave a swing deflection for two 
revolutions, which, being the average of several experiments, =20°*8. A second 
copper disc, of 0*1 of an inch in thickness, gave an average deflection of 27°*8. 
A third copper disc, of 0*2 in thickness, gave a deflection of 26°*5. Here, therefore, 
H 2 
