an instrument so constructed would only be limited. But it is very easy
to eliminate the magnetic effect of the coils of the electro magnet on
the needle, by introducing an opposite magnetic effect, so that only
that part of the force remains which belongs to the soft iron core
proper. One way of doing this is by surrounding the needle with a coil,
the plane of which is at right angles to the line, S N, and coupling
this coil in series with the deflecting coil, D D. If the proportions
of this transverse coil and the direction of the current through it
be properly chosen, its magnetic effect can be made to exactly
counterbalance that of the exciting coils, _e e_, without perceptibly
weakening the magnetism of the iron core. But instead of employing two
coils, one parallel and the other transversely to the zero position of
the needle, we can obtain the same result in a more simple manner with
one coil only, if this be placed at such an angle that its magnetic
effect can be substituted for the combined effects of the two coils. In
other words, we set the deflecting coil, D D, at a certain angle to the
zero position of the needle.

A similar arrangement, though not precisely for the same purpose, has
already been suggested and tried by Messrs. Deprez, Carpentier, Ayrton,
and Perry, in galvanometers with permanent steel magnets. If the coil,
D D, be so placed, the deflecting force which now acts obliquely can be
considered as the resultant of two forces, one acting at right angles to
the line, S N, as in an ordinary galvanometer, and the other parallel to
this line, but in a sense opposed to the action of the electro magnet
and its exciting coils. If the angle of obliquity be so chosen that this
latter component exactly equals the magnetic effect of the exciting
coils _per se_, an equality which holds good for all currents, then we
shall have an almost perfect imitation of a tangent galvanometer with
permanent magnets. But we can go a step further than this; we can