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.
The diver consists of a small glass test tube about two inches long having the figure of a man painted upon it. The lower end is closed by a very small cork through which is pushed a large iron tack with the head on the outside. The test tube contains just enough water to float head up even with the surface of the water. The exact amount of water required to just permit enough buoyancy for the diver to rise again to the surface after he has been to the top will have to be determined by careful experiment.
The jar containing the diver should be set on a table having a drawer in which a pair of powerful electromagnets can be placed directly under the jar. The electro-magnets are connected to a strong battery and a switch located under the rug or carpet and so arranged that it may be closed by stepping on it lightly and without attracting attention. If the diver does not have too much bouyancy the attraction between the tack and the electro-magnets when the switch is closed should be sufficient to cause him to descend to the bottom of the jar. When the current is turned off the diver should rise again.
[Illustration: FIG. 190.—The Magnetic Diver.]