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Page:Popular Science Monthly Volume 90.djvu/496

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��Popular Science Monthly

��High Frequency Tesla Coil for Electrical Experimenters

ANY one who owns a wireless equipment L has the apparatus necessary to operate this coil. It should give at least a 6- in. spark under favorable conditions. The drawing gives the necessary dimensions and shows parts cut away, in order to make the construction clearer. The only apparatus necessary is a spark-gap, an induction coil or transformer and plate-condenser.

���INO. COIL

��Detail of the high frequency coil and the dia- gram showing how to connect the apparatus

The primary winding consists of 22 turns of No. 14 rubber-covered copper wire wound on a fiber tube, 6 in. outside diameter, and of a length that will hold the 22 turns of wire. The secondary coil is made up of one layer of No. 34 double silk-covered wire, wound on a tube 3 in. outside diame- ter and 12 in. long. The tall posts carrying the high potential spark-gap should be at least 18 in. high. Fasten the parts to-

��gether with glue, and do not use any nails or screws except for fastening parts to the base. Finish all over with two or three good coats of thin shellac, drying well between coats.

It is well known that a spark drawn from a coil of this type is not dangerous and only a slight burning sensation is experienced when it passes to the body. Sparks may be drawn from any part of the body with- out discomfort to the operator, but to the astonishment of the onlookers. This coil may be used quite satisfactorily for enter- tainments or for various high-frequency experiments. The diagram shows how to connect the apparatus.

��Testing Wires and Battery Terminals for Polarity with Litmus Paper

POLARITY indicators are expensive instruments and the testing of a few wires would not warrant the purchase of one. One simple test method utilizes litmus paper. Procure from a local drug store some red litmus paper and thoroughly soak it in a solution made up of one table- spoonful of salt to a glass of water. When it is thoroughly soaked remove the paper carefully so as not to tear it, and hang up to dry where nothing will touch it except the hangers. Never dry it between blotters.

These test papers are used as follows: Take a piece of the paper about ij^ in. long by }/2 ifL. wide and slightly moisten it and place the ends of the wire to be tested on the paper so that they will be about ^ in. apart. A deep red spot will appear on the paper at the end of one of the wires and a blue spot at the other. The red designates the positive and the blue the negative. Sometimes the red spot will not show owing to the low potential difference between the wires, but the blue will always appear, designating the negative wire.

There are two kinds of litmus paper, red and blue. If the red cannot be obtained get the blue, but the potential difference between the wires will make the red show up more in this instance. After using this paper it can be dried as when taken from the salt solution, and used again.

��The Editor of the Popular Science Monthly is always glad to hear from readers who have made simple and useful things for the home and the shop with their own hands and who would like to tell others of their own success. Articles from amateur mechanics, electricians and wireless operators are paid for on acceptance, promptly and liberally. But contributors must understand that only unpublished contributions, offered exclusively to the POPULAR SCIENCE MONTHLY, are desired.

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