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Page:Popular Science Monthly Volume 89.djvu/647

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Popular Science Monthly


��Making a Crystal Detector from Cheap Materials

THE base is a piece of oak about 2 by 3 in., having the top edge beveled to improve its appearance. A iiole is drilled jj in. from one end and in ihe center of block, to admit a sliori length of 3 8-'"- threaded brass rod, to be held in place by two nuts taken from an old battery. Next a small piece of thin spring brass A, 2 in. long by i/i6 in. at one end and tapering to yi in. wide at the other is made. At the larger end a hole is drilled to allow this spring to be mounted on ihe upright brass rod by two thumb nuts, also taken from an old battery.

At the smaller end a hole is drilled, large enough to allow a piece of brass hatpin to slide freely through. Make a small tapered coil spring by winding fine springy wire on a match which has been whittled to a point and solder this spring at the smaller end to the piece of hatpin, which should be about lyi in. long, so that the free end of the spring will be K in. from the point of the hat- pin. Now place the pin, point down- ward, in the hole in the smaller end of brass strip. Solder the larger end of the fine coil spring to the brass strip so that

The pointer may be moved about Over the crystal to find the most sensitive spot

���the hatpin floats freely within the hole. Now you are ready to fasten your detector cup with crystal mounted in it, to the base so that the pointed end of the hatpin will make a light contact with the surface of the cr>'stal. If the point of the pin docs not touch the crystal its height should be adjusted by means of the thumb nuts B.

��The hue coil spring C allows the con- tact point to be moved about over the crystal to locate the most sensitive spot, and also acts as a shock absorber to take up all ordinary vibration. The holder can be improved by mounting the crystal cup movably, according to any of the well-known methods. — R.\Y Maxwell.

��Saving the Picture Show with a New Rheostat

RECENTLY a new moving picture theatre found itself short a rheostat on its opening night, with no time to get



An emergency rheostat constructed for theatre use when one could not be obtained

one if the show was to start prompth". The man from the power-house was equal to the emergency, however, and made one, shown in the drawing, from the following materials:

One baseboard, l8 in. by 4 ft.; six porcelain knob insulators; six brass screws and three common coiled steel gate springs. These springs are lyi in. by 16 in. coils. The baseboard is covered with heavy asbestos paper. The springs are connected in series with each other and the lamp carbon circuit. They are mounted with sufificient tension to open the spring far enough to prevent the coils from touching.

As sliown in the drawing, they will pass about 30-35 amperes, without heating very much. This insures a nearly constant current, as there is not much change in resistance. More springs in series will cut down the current, and a reduction of the number will the amperage at the carbons.

��Wireless Telegraph Stations in the West Indies

THE Cuban government has nine wireless telegraph stations in Cuba and on the Isle of Pines. There are two radio stations in Haiti. The respec- tive governments have also equipped stations in Bermuda, Na.ssau, Curacao, Bonaire, Trinidad and Tobago.

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