Page:Popular Science Monthly Volume 89.djvu/158

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��Popular Science Monthly

��in. deep, to receive the coil flush with its top. The leads of the coil are run through the disk. The surface of the coil is then plastered evenly with retort cement. The legs are fastened to a second piece of insulating material with round-headed brass machine-screws, )4 in. long, with nuts. See Fig. 6.

���Cartridge shells make neat electrical contacts for a rheostat

Using Cartridge Shells for Electrical Contacts

A NOVEL use for cartridge shells of the old center-fire kind, certain to interest the electrical experimenter, is in making rheostats, small switch- boards and important contacts on wire- less apparatus, where efficiency is considered. A hole a trifle smaller than the diameter of the shell is made in the base and the cartridge shell forced into the hole made, as shown in the diagram. The proper wires are then soldered to the metal on the inside, or the wire ma>' be placed inside of the shell and held securely in position by driving a wooden plug into the empty shell, as depicted. A complete rheostat may be so made. The heads of the shells ofi^er efficient contacts.

The Best Crystal Detectors

IN spite of the fact that crystal detect- ors play so important a part in the experimentation of electrical amateurs, their use is not understood as well as it should be. There arc various combina- tions in use. It will be found, in general, that the more .sensitive a crystal is, the more readily will it lose its adjustment or "knock out" from loud signals (jr static. The a\'erage amateur will get more satis- faction from using a single crystal than from a combin.ition. For exam|)le, al- though perikon has many desirable char- acteristics, there is apt to be trouble from particles of one of the crystals rubbing

��ofif, and adhering to the surface of the other. This is constantly occurring.

Chalcopyrite and zincite, arsenic and silicon, and antimony and silicon are all used in combination, and are remark- ably sensitive. An occasional wash with carbon disulphide helps to remove grease and dirt from the surfaces, and often restores them to sensitiveness.

Carborundum is proof against all manner of knockouts, but is unfortu- nately not very sensitive. A stifif wire or needle makes the best contact with this substance, and should be pressed down into it with considerable force. A battery must be used with this detector.

Silicon is more sensitive than carbo- rundum, and is correspondingly more easily knocked out by static. A light contact is required, and the efficiency is often improved by applying an exceed- ingly small potential.

Galena is probably the most sensitive of the crj'stal detectors, but is hard to keep in adjustment. It is especially sensitive to static. The wire used should be as fine as possible. Certain violin and mandolin strings are wound with very fine silver wire; this wrapping is most suitable for use with galena.

A minute fraction may be cut off the end of the wire from time to time, thus always insuring a clean point. Ceruslte requires about the same treatment as galena. Iron pyrite works best with a firm contact, and is almost as sensitive as silicon.

Testing Electric Lamps Quickly

WHERE many incandes- cent lamps are tested, much time is lost in screwing them in and out of the testing-socket. This can be reme- died by means of an ordinary porce- lain lamp-socket. Remo\e the inner shell and hammer its threads down on a ^g-'m. [lipe. The tliamotor should then be great enough to admit the llnvailed base of the lamp. After re[)laciiig the shell, lamps can be readily tested by merely sliding them into this socket.

���A test-socket

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