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��Popular Science Monthly
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��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 the 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. These 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 the microphone are connected to the posts B — B o{ the con- trol box while the 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.
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