The current
entering by the terminal, B', passes through the coils of a bobbin, S,
before reaching the points of attachment, a and b, of the lamp, L, the
normally working one. Thence the circuit runs to B. Within the coil, S,
is a small hollow cylinder, T, of thin sheet iron, which is raised
parallel with the axis of the bobbin during the passage of the current
through the latter. At its base the cylinder is prolonged into two little
rods, h and h', which plunge into two mercury cups, G and G'. The cut
shows that one of the cups, G', is connected to the terminal, B', and the
other, G, to the terminal, a', of the other lamp, L'. An inspection of
the cut shows just what ensues when an accident happens to the first lamp
while burning. The first circuit being broken at ab, the magnetizing
action of the current in the bobbin ceases, the cylinder, T, descends,
and the rods, h and h', dip into the mercury. It follows that the
current, always starting from the terminal, B', will by means of the
cups, G and G', pass through the lamp, L', to go by the original return
wire to B.
[Illustration]
The substitution of the lamp, L, for L' is almost instantaneous. It can
scarcely be perceived. It goes without saying that such an arrangement of
automatic commutation is applicable to lamps with two or more filaments
of which only one is to be lighted at a time.
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