178 DR. FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XXIII.) 
2668. Now the action of the iron is, by experiment, of this nature. If an iron wire 
be simply introduced or taken out of the experimental helix with different conditions 
of the commutator, the results are exactly those which have been stated. If the ma- 
chine be worked with an iron wire core, the commutator changing at the stops (2665.), 
then the current gathered up and sent on to the galvanometer is a maximum ; if the 
commutator change at the moments of maximum velocity, or at any other pair of 
moments equidistant from the one stop or the other, then the current at the com- 
mutator is a minimum, or 0. 
2669. There are two or three precautions which are necessary to the production of 
a pure result of this kind. In the first place, the iron ought to be soft and not pre- 
viously in a magnetic state. In the next, an effect of the following kind has to be 
guarded against. If the iron core be away from the dominant magnet at the begin- 
ning of an experiment, then, on working the machine, the galvanometer will be seen 
to move in one direction for a few moments, and afterwards, notwithstanding the con- 
tinued action of the machine, will return and gradually take up its place at 0°. If the 
iron core be at its shortest distance from the dominant magnet at the beginning of the 
experiment, then the galvanometer needle will move in the contrary direction to that 
which it took before, but will again settle at 0°. These effects are due to the circum- 
stance, that, when the iron is away from the dominant magnet, it is not in so strong 
a magnetic state, and when at the nearest to it is in a stronger state, than the mean 
or average state, which it acquires during the continuance of an experiment ; and 
that in rising or falling to this average state, it produces two currents in contrary 
directions, which are made manifest in the experiments described. These existing 
only for the first moments, do, in their effects at the galvanometer, then appear, pro- 
ducing a vibration which gradually passes away. 
2670. One other precaution I ought to specify. Unless the commutator changes 
accurately at the given points of the journey, a little effect is gathered up at each 
change, and may give a permanent deflection of the needle in one direction or the 
other. The tongues of my commutator, being at right angles to the direction of mo- 
tion and somewhat flexible, dragged a little in the to and/rowz parts of the journey : 
in doing this they approximated, though only in a small degree, to that which is the 
best condition of the commutator for gathering up (and not opposing) the currents; 
and a deflection to the right or left appeared (2677-)- Upon discovering the cause 
and stiffening the tongues so as to prevent their flexure, the effect disappeared, and 
the iron was perfectly inactive. 
2671. Such therefore are the results with an iron core, and such would be the 
effects with a copper or bismuth core if they acted by a diamagnetic polarity. Let 
us now consider what the consequences would be if a copper or bismuth core were to 
act by currents, induced for the time, in its moving mass, and of the nature of those 
suspected (2642.). If the copper cylinder moved with uniform velocity (2665.), then 
currents would exist in it, parallel to its circumference, during the whole time of its 
