superpose, one on the other, a hundred flat bobbins of a centimeter
in thickness in such a way as to form a single solenoid one meter in
height, and that the incoming and outgoing wires of each of them be
connected with the contiguous bobbins exactly in the same way as they
are in the consecutive sections or a dynamo-electric machine ring.
Finally, let us complete the resemblance by causing each junction of the
wire of one of the bobbins with the wire of its neighbor to end in a
metallic plate set into an insulating piece containing as many plates as
there are bobbins, plus one. Over this species of collector, which maybe
rectilinear or wound around a cylinder, let us pass two brushes fixed to
an insulating piece that may be moved by hand. Now, if we place these
two brushes at a distance such that the number of the plates of the
collector included between them be, for example, equal to ten, and we
give them any degree of displacement whatever, after rendering them
interdependent, the current entering through one of these brushes and
making its exit through the other will always traverse 10 bobbins.
Everything will occur, then, as if we caused the ten-bobbin solenoid to
move instead of the brushes. This granted, and the brushes being in any
position whatever, let us send a current into the apparatus, and place
therein a soft iron cylinder. By virtue of a well known law, such
cylinder will remain suspended in the interior of the solenoid, and its
longitudinal center will place itself at so much the greater distance
from that of the solenoid the more the current increases in intensity.
It would even fall entirely if the current had not an intensity above a
minimum value dependent upon many elements concerning which we have not
now to occupy ourselves. We will suppose the current intense enough to
keep the distance of the two centers much below that which would bring
about a fall of the cylinder. When such a condition is fulfilled, it is
found that if we try to remove the iron cylinder from the equilibrium
that it is in, we must apply a pressure that increases with the amount