A transformer of this kind can be bought either (a) unmounted, that is, just the bare transformer, or (b) fully mounted, that is, fitted with an iron stand, mounted on an insulating base on which are a pair of primary binding posts, while the secondary is provided with a safety spark gap. There are three sizes of transformers of this kind made and they are rated at 1/4, 1/2 and 1 kilowatt, respectively, they deliver a secondary current of 9,000, 11,000 and 25,000 volts, according to size, and cost $16.00, $22.00 and $33.00 when fully mounted; a reduction of $3.00, $4.00 and $5.00 is made when they are unmounted. All of these transformers operate on 110 volt, 60 cycle current and can be connected directly to the source of alternating current.
The Wireless Key.--For this transmitting set a standard wireless key should be used as shown at B. It is made about the same as a regular telegraph key but it is much heavier, the contact points are larger and instead of the current being led through the bearings as in an ordinary key, it is carried by heavy conductors directly to the contact points. This key is made in three sizes and the first will carry a current of 5 amperes [Footnote: See [Appendix] for definition.] and costs $4.00, the second will carry a current of 10 amperes and costs $6.50, while the third will carry a current of 20 amperes and costs $7.50.
The Spark Gap.--Either a fixed, a rotary, or a quenched spark gap can be used with this set, but the former is seldom used except with spark-coil sets, as it is very hard to keep the sparks from arcing when large currents are used. A rotary spark gap comprises a wheel, driven by a small electric motor, with projecting plugs, or electrodes, on it and a pair of stationary plugs on each side of the wheel as shown at C. The number of sparks per second can be varied by changing the speed of the wheel and when it is rotated rapidly it sends out signals of a high pitch which are easy to read at the receiving end. A rotary gap with a 110-volt motor costs about $25.00.
A quenched spark gap not only eliminates the noise of the ordinary gap but, when properly designed, it increases the range of an induction coil set some 200 per cent. A 1/4 kilowatt quenched gap costs $10.00. [Footnote: See [Appendix] for definition.]
The High Tension Condenser.--Since, if you are an amateur, you can only send out waves that are 200 meters in length, you can only use a condenser that has a capacitance of .007 microfarad. [Footnote: See [Appendix] for definition.] A sectional high tension condenser like the one described in connection with Set No. 1 can be used with this set but it must have a capacitance of not more than .007 microfarad. A condenser of this value for a 1/4-kilowatt transformer costs $7.00; for a 1/2-kilowatt transformer $14.00, and for a 1-kilowatt transformer $21.00. See E, Fig. 19.
The Oscillation Transformer.--With an oscillation transformer you can tune much more sharply than with a single inductance coil tuner. The primary coil is formed of 6 turns of copper strip, or No. 9 copper wire, and the secondary is formed of 9 turns of strip, or wire. The primary coil, which is the outside coil, is hinged to the base and can be raised or lowered like the lid of a box. When it is lowered the primary and secondary coils are in the same plane and when it is raised the coils set at an angle to each other. It is shown at D and costs $5.00.
Connecting Up the Apparatus. For Alternating Current.--Screw the key to the table about the middle of it and near the front edge; place the high tension condenser back of it and the oscillation transformer back of the latter; set the alternating current transformer to the left of the oscillation transformer and place the rotary or quenched spark gap in front of it.
Now bring a pair of No. 12 or 14 insulated wires from the 110 volt lighting leads and connect them with a single-throw, double-pole switch; connect one pole of the switch with one of the posts of the primary coil of the alternating power transformer and connect the other post of the latter with one of the posts of your key, and the other post of this with the other pole of the switch. Now connect the motor of the rotary spark gap to the power circuit and put a single-pole, single-throw switch in the motor circuit, all of which is shown at A in Fig. 22.
