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Page:Popular Science Monthly Volume 90.djvu/327

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Popular Science Monthly


��D, D^. It is best to make the switch buttons marked zero the lowest; then as the switch-arms are turned toward the upper positions, more and more turns of the coil are cut into the circuit between




���FIG. 5

��The transformer coil wiring diagram show- ing only eight taps taken from the tiims

the two binding posts. As is obvious from the diagram, the "tens" switch cuts in ten turns at a time, while the "units" switch gives steps of a single turn. By various combinations of both switches, any number of turns from i to 149 can be had.

The Transformer Coil

Another coil very much like the first is now to be built. The mechanical features are exactly the same; but only 8 tsvisted taps are taken out from the 150 turns of No. 20-gage wire. The end of the wire is connected to a binding post F, Fig. 5, and taps at the tenth, thirtieth, fiftieth and each twentieth turn thereafter are led to the corresponding points of two nine-point switches mounted on the panel. These are marked "Coupling" and "Secondary" re- spectively, and take the places of the 10- point and 15-point switches on the loading coil just described. Their buttons are marked o, 10, 30, 50, and so on to 150, in steps of 20 turns, and the soldered connec- tions are made as shown in Fig. 5. The binding post F may take the place of one of the screws E in Fig. 3, and the posts G and // are located as D and D^ in Fig. i.

It is to be noted that in the loading coil the adjustment of inductance may be had in single turn steps by the use of two switches, but that there are only two points of connection. In this last described coil there are three points of connection, two of which are variable in steps of 10 and 20 turns from o to 150. Thus the two coils, while superficially alike, may be used for very different purposes.

��It is possible to buy lever-switches from the various supply houses, but usually the type with the knob at the outer end of the switch-arm is furnished. This is not nearly so convenient for tuning as the kind with central-knob shown in Fig. i, 3 and 6, since considerably greater movement of the hand must be made in order to accom- plish a given adjustment. Central-knob switches are not difiF»cult to make; a simple and eflFective design is shown in Fig. 6. The switch-points / may be of any sort whatever; most supply houses will sell these for one or two cents apiece. The switch-arm K is cut out of thin spring- brass, and is formed of two pieces, each having one end punched to fit over the machine-screw and the other end bent down to make smooth contact with the buttons /. The arms are fastened to the turned hard rubber knob L by means of the brad or escutcheon pin Af, and are further clamped by the two nuts N, iV^ The central machine-screw passes through the switch-panel Q within a short sleeve P and through two washers shown but not lettered. Two additional nuts N^^ and N^^^ are clamped on the^ inside, just enough play being allowed for free turning when the arm K bears upon the contact points. Connection to the arm is made through the spiral R, the free end of which is soldered to the screw 0. Soldered con- nection is made to the switch-points at /.

Setting Up the Receiver The loading coil and the transformer coil as described above are to be combined with a detector, telephone set and stopping-


��Diagram of the central knob used on switches. It is very effective and not difficult to make

condenser as shown in Fig. 7. The antenna circuit passes through the loading coil from D to D^ and thence to binding post G of the transformer coil. This leads to the "coupling" switch, and the circuit runs from there through whatever part of the coil is cut in, and out to ground through

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