The zinc strip should not be so long that it will not clear the solution when raised up if the jar is about half filled.
[Illustration: FIG. 188.—A Handy Light.]
The positive electrode is a battery carbon mounted on the lower end of a brass rod. The brass rod in this case is not movable because the carbon can remain in the solution indefinitely without any waste.
The joint between the brass rods and the electrode should be coated with paraffine so that the chemicals in the solution will not cause any corrosion. The rods should be connected to the lamp socket. When the zinc is lowered into the solution, the lamp will be lighted by the current generated.
The electrolyte should be that described as the "bichromate" solution on page 34 of Vol. I of this book.
AN EXPERIMENTAL ARC LAMP.
An arc lamp is operated by causing an electric current to pass between two carbon rods. If the voltage is high enough, carbon vapor will form between the rods when they are separated a short distance. It requires an E. M. F. of approximately fifty volts to produce and maintain an arc.
A six volt storage battery will heat a pair of sharp pointed carbons to incandescence and produce a light somewhat similar to the electric arc.
[Illustration: FIG. 189.—Experimental Arc Lamp.]
Ordinary carbon rods such as those used in flashlight batteries will serve very nicely for a "battery arc lamp." The ends should be filed to sharp points. Figure 189 shows a simple method of mounting the carbons. The lower one is soldered to a piece of brass strip by taking advantage of the brass cap pressed over the upper end of flashlight battery carbons.