Page:Popular Science Monthly Volume 89.djvu/969

 Popular Science Monthly

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��changed by the short-circuit. The current is increased, because of the fact that the primary resistance is lowered by the short-circuit, but the number of turns is decreased. Usually the number of ampere-turns (i. e., the number of effective turns times the number of am- peres) is lowered by the defect ; and since the energy received by the secondary circuit depends on the number of am- pere-turns in the primary of the trans- former, it is evident that the efficiency is greatly reduced.

Therefore, an increase in the amperage of the primary circuit with a decrease in the reading of the voltmeter, and a large decrease in the energy of the secondary circuit are good indications of a short- circuit in the primary of the transformer. The amount of the variations mentioned will depend on the number of turns cut out by the short-circuit. Often the shorted section will become very hot.

Trouble of this kind is caused by poor insulation, or excessive voltage being ap- plied. It depends upon the extent of the puncture of the insulation as to what is best to be done for repairs. Occasionally the entire primary must be rewound. If only slightly punctured, it is easily fixed by wrapping the wire in the damaged portion with insulating tape. Much care must be exercised in replacing the wire in its original position.

��Practicing the Code Without Using a Buzzer

FOR the amateur who wants to prac- tice the code and has no buzzer, a good substitute can be made from an old 75-ohm telephone receiver. The re- ceiver is hooked up in series with a i6-c.p. light and a Morse key on a iio-volt

��REC.

��LAMP — 1

��i

��o

��KEY.

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��The connections for a key and tele- phone receiver with an incandescent light

alternating circuit, as shown in the drawing. If the buzz is too loud a smaller c.p. lamp may be used to soften the tone. This method should not be tried on good receivers. — Malcolm Macurda.

��How To Take Up the Slack

in Your Aerial Wires

^HE difficult job of getting aerial

wires stretched evenly can be

easily accomplished by utilizing the

arrangement shown in the illustration.

��T

���Porcelain tube in cross-bar insulating the bolt for taking up slack in wires

It consists of a porcelain tube — an ordinary unglazed tube about 3 in. long will do — run through a hole in the arm and the holding bolt run through the tube. If bolts having long threads are used, considerable slack can be taken up by merely turning the nut with a wrench. In putting up the wires place the nut in the center of the threaded portion ; then it will be easy to shorten or lengthen as desired. — Lee Schertz.

��PHI ^

��Canada to Protect Her Parks with Radio Service IILIP E. EDELMAN of St. Paul, Minn., has been engaged by the Canadian Government as electrical engi- neer to prepare plans for wireless tele- phony and telegraphy installations over the 7,000 square miles embraced by the Dominion Parks of Western Canada. The installation will be of a new design specially adapted to the difficult moun- tain service.

The object of the installation is to prevent game trespassing and to afford a means of instantaneous communica- tion for reporting forest fires and calling for aid in territory where ordinary means of communication are out of the ques- tion, as is often the case in Canada.

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