motion--work being the product of mass and distance.

Large sums of money have virtually been thrown away in the endeavor
to produce energy, and there are intelligent persons who to this day
imagine that, by indefinitely increasing the strength of a magnet, more
power may be got out of it than is put in.

Large field-magnets are advantageous, and the tendency in the
manufacture of dynamo machines has been to increase the mass of iron,
because with long and heavy cores and pole pieces there is a steady
magnetism insured, and therefore a steady current, since large masses
of iron take a long time to magnetize and demagnetize; thus very slight
irregularites in the speed of an armature are not so easily perceived.
In the case of electro-motors these conditions are changed. In the first
place, we assume that the current put through the coils of the magnets
is continuous; and secondly, we can count upon the momentum of the
armature, as well as the momentum of the driven object, to assist us
over slight irregularities. With electric launches we are bound
to employ a battery current, and battery currents are perfectly
continuous--there are no sudden changes; it is consequently a question
as to how small a mass of iron we may employ in our dynamo as a motor
without sacrificing efficiency. The intensity of the magnetic field
must be got by saturating the iron, and the energy being fixed, this
saturation determines the limit of the weight of the iron. Soft wrought
iron, divided into the largest possible number of pieces, will serve
our purpose best. The question of strength of materials plays also an
important part. We cannot reduce the quantity and division to such a
point that the rigidity and equilibrium of the whole structure is in any
way endangered.