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. The upper carbon is mounted on the forward end of a brass lever which is pivoted near the back end so that an adjustable counterweight may be used to regulate the pressure on the point of contact between the two carbons. If the carbons are touching lightly and are connected to a strong battery of six volts or more, the point of contact will be heated to incandescence and will produce a dazzling white light. A real arc lamp for operation on the 110 volt current, in series with a bank of lamps, may be constructed on this same scheme by using larger carbons and eliminating the counterweight. The upper arm should be pivoted in the same manner but should move stiffly so that it will remain in any position in which it is set. If the 110 volt current is used, the carbons should be brought together and then separated a short distance. The current will continue to flow across and will form an arc producing a very powerful purplish white light. The arc will consume the carbons quite rapidly and also generate a great deal of heat. Therefore if it is intended to use the light for any length of time it is advisable to fasten the carbons by means of a clamp so that they may be easily replaced. A MAGNETIC DIVER. This is a mystifying piece of apparatus to those who do not know its secret. The little diver apparently travels up and down from the bottom of a jar full of water to the surface at a word of command.