circuit. The length of the arc can be determined by the adjustment of
the screw, G, by which the amount of movement of the armature is
limited.

Fig. 2 represents a modified form of Mr. Hedges' lamp designed for
installation when it is desirable to burn a number of lamps in series.
In this arrangement the carbons are separated by the attractive
influence of a solenoid upon an iron plunger, to which is attached (by
a non-magnetic connection) the armature of an electro-magnet, the
coils (which are of fine wire) forming a shunt circuit between the two
terminals of the lamp, and so disposed with respect to the armature as
to influence it in an opposite direction to that of the solenoid. When
the circuit of the lamp is completed with the electric generator the
carbons are drawn apart by the action of the solenoid on the plunger,
and the distance to which they are separated is determined by the
difference of attractive force exercised upon the armature by the
solenoid and the magnet; but as the latter forms a short circuit to
that of the arc, it follows that should the resistance of the arc
circuit increase either through the arc becoming too long or through
imperfection in the carbons or contacts, a greater percentage of
current will flow through the magnet coils, and the arc will be
shortened, thereby reducing its resistance and regulating it to the
strength of the current. In other words, the distance between the
carbons, that is to say, the length of the arc, is determined by the
position of the armature of the electro-magnet between its magnets and
the solenoid, which position is in its turn determined by the
difference between the strength of current passing through the coil of
the solenoid and that of the magnet.

Mr. Killingworth Hedges exhibits also a third form of his lamp, in