36 DR. FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XXVIII.) 
3104. If a wire move across lines of force slowly, a feeble electric current is pro- 
duced in it, continuing for the time of the motion; if it move across the same lines 
quickly, a stronger current is produced for a shorter time. The effect of the current 
which deflects a galvanometer needle, is opposed by the action of the earth, which tends 
to return the needle to zero. A continuous weak current, therefore, cannot deflect it so 
far as a continuous stronger current. If the currents he limited in duration, the same 
effect will occur unless the time of the swing of the needle to one side he not con- 
siderably more than the time of either of the currents. If the time of the needle-swing 
be ten, and the time of ten quick rotations he six, then all the effect of the induced 
current is exerted in swinging the needle; but if the time of ten slow rotations be 
twelve or fifteen, then part of the current produced is not recognized by the extent of 
the vibration, but only by its holding the needle out awhile, at the extremity of a 
smaller arc of declination. Therefore, when quick and slow velocity was compared, 
and, indeed, in every case of comparative rotations of the wire and magnet, only that 
number of rotations was taken which could be well included within the time of the 
needle’s journey to one side; when the needle, therefore, was seen to travel on to its ex- 
treme distance after the rotation and the inducing current had ceased. If the needle 
began to return the instant the motion was over, such an experiment was rejected 
for purposes of comparison. When these precautions were attended to, and velocities 
of revolution taken, which occupied times from one-third to three-fourths of that 
required for the swing of the needle, then the same number of revolutions (ten) gave 
the same amount of deflection, namely 9°'5, with my apparatus, though the time of 
revolution varied as 1 : 2, or even in a higher degree. 
3105. Another cause of difference produced by varying velocity, is the diminution 
of the action of the current on the needle, as the angle which the latter forms with 
the convolutions of the coil increases. Hence a constant current produces more 
effect on the deflection of the needle for the first moments of time than afterwards. 
This effect, however, was scarcely sensible for swinging deflections of 9° or 10°, pro- 
duced by currents which were over before the needle had moved through 4° or 5°. 
3106. It has already been shown, that it is a matter of indifference whether the 
wire revolve in one direction or the magnet in the other (3091.) ; and this is still 
further proved by the cases where the magnet and the wire revolve together (3092.) ; 
for then the currents which tend to form are exactly equal and opposed to each other, 
whatever the position of the wire may be. As the immobility of the needle is a point 
more easily ascertained than the extent of an arc, indicated only for a moment, and 
as the rotations of the magnet and wire conjointly can be made rapid and continuous, 
the proof in such cases is very satisfactory. 
3107. Proceeding to experiment upon the effect of the Fig- H- 
distance of the wire c, fig. 11, from the magnet, the wire was 
made to vary, so that sometimes it was not more than 8 
inches long (being of copper and 0*04 of an inch in dia- 
meter), and only half an inch from the magnet, whilst at 
