through a pair of concentric solenoids, and in the annular space
between these is hung a solenoid, the upper half of which is wound in
the opposite direction to the lower half. By the use of what Mr. Boys
calls "induction traps" of iron, the magnetic force is confined to a
small portion of the suspended solenoid, and by this means the force
is independent of the position. The solenoid is hung to one end of a
beam, and its motion is resisted by a pendulum weight, by which the
energy meters may be regulated like clocks to give standard measure.
The beam carries the tangent wheels, and the rotation of the cylinder
gives the energy expanded in foot-pounds or other measures. The use of
an equal number of turns in opposite directions on the movable
solenoid causes the instrument to be uninfluenced by external magnetic
forces. Mr. Boys showed on the screen an image of an electric arc, and
by its side was a spot of light, whose position indicated the energy,
and showed every flicker of the light and fluctuation of current in
the arc. He showed on the screen that if the poles are brought too
near the energy expended is less, though the current is stronger, and
that if the poles are too far apart, though the electromotive force is
greater the energy is less; so that the apparatus may be made to find
the distance at which the greatest energy, and so the greatest heat
and light, may be produced.

At the conclusion of the paper, Prof. W.G. Adams and Prof. G.C. Foster
could not refrain from expressing their high admiration of the
ingenious and able manner in which Mr. Boys had developed the subject.

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