The Tuning Coil.--The transmitting inductance, or sending tuning coil, consists of 20 to 30 turns of No. 8 or 9 hard drawn copper wire wound on a slotted insulated form and mounted on a wooden base. It is provided with clips so that you can cut in and cut out as many turns of wire as you wish and so tune the sending circuits to send out whatever wave length you desire. It is shown at d, and costs about $5.00. See also Oscillation Transformer, page 63 [Chapter IV].
The High Tension Condenser.--High tension condensers, that is, condensers which will stand up under high potentials, or electric pressures, can be bought in units or sections. These condensers are made up of thin brass plates insulated with a special compound and pressed into a compact form. The capacitance [Footnote: This is the capacity of the condenser.] of one section is enough for a transmitting set using a spark coil that gives a 2 inch spark or less and two sections connected together should be used for coils giving from 2 to 4 inch sparks. It is shown at e.
Connecting Up the Apparatus.--Your sending set should be mounted on a table, or a bench, where it need not be moved. Place the key in about the middle of the table and down in front, and the spark coil to the left and well to the back but so that the vibrator end will be to the right, as this will enable you to adjust it easily. Place the battery back of the spark coil and the tuning coil (oscillation transformer) to the right of the spark coil and back of the key, all of which is shown in the layout at A in Fig. 20.
For the low voltage circuit, that is the battery circuit, use No. 12 or 14 insulated copper wire. Connect all of the dry cells together in series, that is, connect the zinc of one cell with the carbon of the next and so on until all of them are connected up. Then connect the carbon of the end cell with one of the posts of the key, the zinc of the other end cell with one of the primary posts of the spark coil and the other primary post of the spark coil with the other post of the key, when the primary circuit will be complete.
For the high tension circuits, that is, the oscillation circuits, you may use either bare or insulated copper wire but you must be careful that they do not touch the table, each other, or any part of the apparatus, except, of course, the posts they are connected with. Connect one of the posts of the secondary coil of the spark coil with one of the posts of the spark gap, and the other post with one of the posts of the condenser; then connect the other post of the condenser with the lower spring clip of the tuning coil and also connect this clip with the ground. This done, connect the middle spring clip with one of the posts of the spark gap, and, finally, connect the top clip with the aerial wire and your transmitting set is ready to be tuned. A wiring diagram of the connections is shown at B. As this set is tuned in the same way as Set No. 2 which follows, you are referred to the end of this chapter.
A Better Transmitting Set (No. 2).--The apparatus for this set includes: (1) an alternating current transformer, (2) a wireless telegraph key, (3) a fixed, a rotary, or a quenched spark gap, (4) a condenser, and (5) an oscillation transformer. If you have a 110 volt direct lighting current in your home instead of 110 volt alternating current, then you will also need (6) an electrolytic interrupter, for in this case the primary circuit of the transformer must be made and broken rapidly in order to set up alternating currents in the secondary coil.
The Alternating Current Transformer.--An alternating current, or power, transformer is made on the same principle as a spark coil, that is, it has a soft iron core, a primary coil formed of a couple of layers of heavy wire, and a secondary coil wound up of a large number of turns of very fine wire. Unlike the spark coil, however, which has an open magnetic core and whose secondary coil is wound on the primary coil, the transformer has a closed magnetic core, with the primary coil wound on one of the legs of the core and the secondary wound on the other leg. It has neither a vibrator nor a condenser. A plain transformer is shown at A in Fig. 21.
