Popular Science Monthly 



G33 



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 



IIOVDC 



TO CARBONS- 



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 increa.se 

 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. 



