The inner end of the carbon E is supported by a piece of No. 25
German-silver wire, C, which is about 6 in. long. This wire runs
through the
[Illustration: Arc in a Large Can]
porcelain tube to the binding post D. The binding post is fastened
to a wood plug in the end of the tube. The tube B is adjusted so
that the end of the carbon E is pressing against the carbon F. The
electric wires are connected to the carbon F and the binding post
D. A resistance, R, should be in the line.
The current, in passing through the lamp, heats the strip of
German-silver wire, causing it to expand. This expansion lowers
the end of the carbon E, separating the points of the two carbons
and thus providing a space between them for the formation of an
arc. When the current is turned off, the German-silver wire
contracts and draws the two carbon ends together ready for
lighting again. The feed can be adjusted by sliding the carbon F
through its insulation.
A resistance for the arc may be made by running the current
through a water rheostat or through 15 ft. of No. 25 gauge
German-silver wire.
--Contributed by R.
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