Page:Popular Science Monthly Volume 90.djvu/976

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��Popular Science Monthly

��An excellent oil-tight case can be made at small cost from an aluminum cup about ^^2 ill- i" diameter. Glass cases can be made from large paste pots, oil-cup glasses, etc., or if desired complete composition cases can be purchased ready made. When mounted in any cabinet set the appearance of the instruments in question can be made to equal that of any condensers made. The maximum capacity of the condensers described is approximately .008 microfarad.

If these instructions are carefully fol- lowed, the results will be very gratifying. There are many minor points brought out in the description which represent several months* experimentation on the part of the author. Although slight variations of con- struction may appear feasible, it should be borne in mind that such deviations should be attempted only with the idea of experi- mentation. In order to do anything right the first time, it must be done according to rule. Experimentation often means de- struction instead of construction.

The total cost for materials used in making two condensers by the above method should not exceed $1.50 exclusive of cases. The time required may vary from 6 to 20 or more hours. Tests should be made for short circuits with head-phones and a battery.

��An Improved Method of Aerial Construction

THE construction of an aerial is greatly simplified if the wires are cut equal to the length of the aerial proper, plus the length of the lead-in, and connected as in

���AERIAL

the drawing. It is understood that this represents but one wire of the aerial. As many may be used as required. The chief advantages of this form of con- struction are that it requires no soldered joints, since the wire is in one piece from the lightning switch to the top of the aerial ; that it has no sharp corners to start brush discharges, and that the lead-in has very nearly the same current carrying capacity as the aerial, since it is made up of exactly the same number of ,wires. — Edward L. Norton.

��LEAD-W

��Simplified connec- tion for an aerial wire and the lead-in

��Some Useful Hints for the Radio Operator

IF your tuner is wound with bare wire on a threaded core, and the wire has v/orked loose, put it in the refrigerator over night so that the chill will contract the tube. Immediately upon removing it, work the wire tight with the hands and fasten. As the tube warms to the temperature of the atmosphere it will expand, thus drawing the wire tighter than could be done with a machine.

Wipe off the tuner switch contacts with a piece of silk and rub the blade back and forth over some 0000 French emery paper about once a week. This will keep the blade from wearing rough, and the switches will always work without scraping.

Silk is one of the best materials for keep- ing the surfaces of rubber and nickel parts bright.

Rub old and discolored hard rubber panels briskly with some cheesecloth sat- urated in household ammonia, and finish with a piece of silk dipped in crude oil, to restore to the surfaces their former black color and polish.

Use a battery-switch instead of a button on your buzzer test circuit. This will enable you to use both hands in adjusting the detector.

If your cabinets are all finished with wax or oil instead of varnish, they will not show dust, finger marks, smears, etc. Dull nickel plate holds the same advantage over the polished kind.

A relay can be employed to advantage by connecting the magnets with an arm on the antenna switch and using the contacts for short circuiting the detector while sending. With the relay in circuit, throwing over the aerial switch may perform five operations, namely: It may disconnect the receiving set from the aerial, short circuit the detec- tor, connect aerial to oscillation transform- er, connect power to transformer primary, and start the rotary gap motor.

A pair of amber, smoked or blue glasses will protect the operator's eyes from the strain and burning sometimes caused by the spark.

If you desire to get efficiency on a 200- meter wave, which means that the closed circuit connections must be as short as possible, mount your condenser and trans- former together in the same case, and arrange your gap on top of them. Fill the case with oil. — Edwin L. Powell.

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