Popular Science Monthly 



311 



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 



Switches 



U-io 



COUPLING 



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- 



F16.6 



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 



