310 



Popular Science Monthly 



through the wire, and there is always 

 difficulty in maintaining good contact. 

 Further the slider usually short circuits 

 several turns at its point of contact, and 

 thus causes mistuning and loss of signal 

 strength. These disadvantages of the slid- 



The rotary switches mounted on the panel, 

 one having ten and the other fifteen points 



ing contact, taken together with the poor 

 circuit design often employed, have brought 

 the direct-coupled circuit of the sort 

 described into disrepute; there is really 

 little choice between the direct and in- 

 ductively coupled receivers, however, pro- 

 vided that both are properly built. 



By making the loading and transformer 

 coils in separate units, and fitting each 

 with switch contacts instead of sliders, it 

 is entirely practicable to produce a receiving 

 installation which has all the selectivity of 

 the inductively coupled type and still 

 avoid some of its disadvantages. Such 

 coils are described below, and their mode 

 of connection is shown in Fig. 7, which cor- 

 responds to Fig. 5 of the December article. 



The Loading Coil 

 This instrument is shown in Fig. i. The 

 basic piece is a paper tube of about 4 in. 

 diameter and 8 in. length. Beginning 

 about I in. from one end. No. 20 gage 

 double silk-covered magnet-wire is wound 

 on evenly for about 6 in., which will take 

 149 turns. A tap must be taken out for 

 each of the first ten turns, and one for 

 each tenth turn thereafter, as shown in 

 Fig. 4. The best plan is to fasten the end 

 of the wire, before beginning winding, 

 through two small holes punched in the 

 paper tube, leaving about 2 in. of wire free 

 for the tap marked "9" in Fig. 4. Then a 

 single turn is wound on, and a small loop 

 twisted in as shown in Fig. 2. This twist 

 and loop stand up from the surface of the 

 coil, and the wire leading to contact "8," 

 Fig. 4, is later soldered to the loop. The 

 second turn of wire is then wound on the 



tube, and another twisted loop for the 

 contact "7" is made. Thus a twist is put 

 in for each of the contacts, at the end of 

 each turn, till that marked "O" is reached. 

 Then ten turns are wound without a tap, 

 the twist for "10" being taken out at the 

 19th turn of the whole coil. Similarly 

 taps for "20", "30", etc., are made at each 

 tenth turn thereafter. 



The completed coil is to be mounted 

 between two end blocks of wood or hard 

 rubber. A, in Fig. i. Three small pieces 

 are fastened to the inside faces of each of 

 the end blocks, as shown in dotted lines, 

 to keep the coil from slipping sidewise; and 

 the whole is held together by a piece i in. 

 square passing through from end to end 

 in the center of the coil. Screws B, with 

 washers under their heads, pass through 

 holes bored in the end blocks and clamp 

 the tube by threading into the ends of this 

 central stick. 



As further shown in Fig. i, a switch- 

 panel is mounted on top of the end blocks. 

 This should be made of hard rubber or 

 fiber, about 3^ in. thick, though hardwood 

 /^ or 3^ in. thick will do. It is fastened to 

 the end blocks by means of two wood-screw 

 binding posts D, D^, Fig. i and 3, and 

 the screws E, Fig. 3. On it are mounted 

 two rotary switches, one having 10 and the 

 other 15 points, as shown in Fig. 3. Any 

 type of switch-arm will do for these, but 

 the easiest manipulation will be obtained 

 if a center-knob type is used. 



The ID-point switch should be markec 

 "Units" and the 15-point "Tens"; the 



The wiring diagram for the loading coil 

 showing the turns and where they are tapped 



buttons are to be numbered from zero to 9 

 for the former and from zero to 140 in tens 

 for the latter. The taps from the coil itself 

 are to be connected with these switch- 

 points by means of short soldered leads, 

 as shown in Fig. 4. Great care must be 

 taken to see that no short circuits are made 

 as this wiring is put in place. The central 

 points of the switches, i. e., their arms, 

 have wires leading to the binding posts 



