the back end, and in the positions shown in the sketch. Two of the
holes are cut large enough to hold a short section of a garden
hose tightly, as shown at AA. A piece of porcelain tube, B, used
for insulation, is fitted tightly in the third hole. The hose
insulation A should hold the carbon F rigidly, while the carbon E
should rest loosely in its insulation.

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