4S8 



Popular Science Monthly 



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i 



Fig. 2. Carbon But 



t on in Center of 



Each Diaphragm 



(liapliragni willi 

 s li e 1 1 a c . T li e y 

 should be placed 

 in a hot oven to 

 insure the com- 

 ])k'te evaporation 

 of alcohol in the 

 sliellac, and thus 

 hold the carbon 

 firmly to the dia- 

 |)hragni. 



The receivers are mounted so their 

 faces will be about 2 ins. apart and con- 

 nected in scries to two liinding posts at 

 the left, of the base. The thin flexible 

 wires from the carbon buttons are also 

 connected to two binding posts. A neat 

 method of running the connection from 

 these wires through the base makes use 

 of cap nuts as detailed in Fig. 3. 



A hard pencil 

 should now be 

 boiled in water till 

 the wood comes off 

 and the lead is left 

 whole. Cut off a 

 piece a triffe longer 

 than the distance 

 between the receiv- 

 ers when the one 

 at the right is ad- 

 justed about to the 

 middle of the slot. 

 This lead is care- 

 fully sharpened to 

 a poin : at each ;nd. 

 The point should 

 not be too sharp, 

 for if so it will 

 break off when a 

 little pressure is 

 applied. 



The f)encil lead 

 is now to be slipped 

 into position by 

 backing out the 



right hand receiver and |)lacing the ends 

 of the rod in the cavities in tlie carbon 

 l)uttons. This completes the microphone. 

 A good arrangement for controlling the 

 apparatus is shown. A w<jo<1 box is con- 

 structed large enough to contain three 

 standard dry cells. Smaller batteries may 

 be used but they will not last as long. 

 ( )n one side of the box is moimtcd a four- 

 point switch (I* ig. i)an(l a small (inely ad- 

 justable rheostat is mounted on the frt)nt. 



The batteries 

 and switches are 

 wired as clearly 

 shown in Fig. 

 5 , the binding 

 posts marked 

 A^ — A- being 

 on the left side of 

 the box and those 



B'—B' 



the 



Fig. 3. Cap Nuts for 



Connecting Wires 



Through Base 



Fig. 4. Detail of Support on Right Hand 

 Side with Thumbscrew Regulator 



Fig. 5. Method of Connecting Posts 

 Microphone to Posts of Control 



right side. This 

 completes the am- 

 plifier, with the 

 excei)tion of the loud talking receiver 

 which is connected to B', E'. 



The loud sjieaker can ho made from 

 an ordinary 75-ohm telephone recei\er 

 of the kind that sells for about 40 cents. 

 The fine wire is remo\x'd by cutting or un- 

 winding and heavier wire put in its place. 

 The receiver siiould 

 then be rewound 

 with about No. 42 

 wire, to a resistance 

 of approximately 

 14 ohms. 



.■\fter rewinding 

 the receiver a short 

 piece of brass tub- 

 ing I in. in diameter 

 and 2 inches long 

 is fastened to the 

 cap of the recei\-er. 

 Obtain two pieces 

 of brass tubing that 

 will just fit over 

 this tube with a 

 good sliding fit. 

 Tiiese pieces are to 

 be 2 ins. and 4 ins. 

 long respectively, 

 and form a variable 

 acoustic resonator. 

 The binding 

 l^osts of the receiv- 

 ers on the micro- 

 phone .ire connected to the receiving set. 

 The posts A — A of tiie microphone are 

 connected to the posts B — B o{ the con- 

 trol box while tlie loud talker is con- 

 nected across posts Hi— 82 (see Fig 5.) 



An extra 1,000-oiim receiwr should be 

 connected in series with the telephones 

 actuating the microi)hone, to test the 

 adjustment of the detector. A switch may 

 bearrangcd toshort circuit thisadililional 

 receix'er when it is not re(]uired. 



of 



