(a) A mechanism for automatically shunting an arc or other lamp when it
ceases to work properly. It is generally worked by an electro-magnet of
high resistance placed in parallel with the arc. If the arc grows too
long the magnet attracts its armature, thereby completing a shunt of
approximately the resistance of the arc, and which replaces it until the
carbons approach again to within a proper distance. Sometimes a strip or
wire of fusible metal is arranged in shunt with the arc. When the arc
lengthens the current through the wire increases, melts it and a spring
is released which acts to complete or close a shunt circuit of
approximately arc-resistance.

(b) See Safety Device–Safety Fuse.

(c) See below.