the galvanometer in circuit with that spiral. Now midway between the two
spirals will be placed a plate of iron, as shown in Plate 2, and at once
you observe the deflection of the galvanometer is reduced by less than
one half, showing clearly that the presence of the iron plate is in some
way influencing the previous effects. The iron will now be removed, but
the spirals left in the same position as before, and by increasing the
speed of the reversals you see a higher deflection is given on the
galvanometer. Now, on again interposing the iron plate the deflection
falls to a little less than one-half, as before. I wish this fact to be
carefully noted.

The experiment will be repeated with a plate of copper of precisely the
same dimensions as the iron plate, and you observe that, although the
conditions are exactly alike in both cases, the interposition of the
copper plate has apparently no effect at the present speed of the
reversals, although the interposition of the iron plate under the same
conditions reduced the deflection about fifty per cent. We will now
remove the copper plate, as we did the iron one, and increase the speed
of the reversals to the same as in the experiment with the iron, and you
observe the deflection on the galvanometer is about the same as it was
on that occasion. Now, by replacing the copper plate to its former
position you will note how rapidly the deflection falls. We will now
repeat the experiment with a plate of lead; you will see that, like the
copper, it is unaffected at the low speed, but there the resemblance
ceases; for at the high speed it has but very slight effect. Thus these
metals, iron, copper, and lead, appear to differ as widely in their
electrical as they do in their mechanical properties. Of course it would
be impossible to obtain accurate measurements on an occasion like the
present, but careful and reliable measurements have been made, the
results of which are shown on the sheet before you, marked 3.