A 100 Mile C. W. Telegraph Transmitting Set.--The principal pieces of apparatus for this transmitter are the same as those used for the 100 Mile Continuous Wave Telegraph Transmitting Set described and pictured in the preceding chapter which used direct current, except that an alternating current power transformer is employed instead of the more costly motor-generator.
The Apparatus Required.--The various pieces of apparatus you will need for this transmitting set are: (1) one hot-wire ammeter for the aerial as shown at E in Fig. 75, but which reads to 5 amperes instead of to 2.5 amperes; (2) one tuning coil as shown at A in Fig. 77; (3) one aerial condenser as shown at B in Fig. 77; (4) one grid leak as shown at C in Fig. 77; (5) one telegraph key as shown at G in Fig. 75; (6) one grid condenser, made like the aerial condenser but having only two terminals; (7) one 5 watt oscillator tube as shown at E in Fig. 77; (8) one .002 mfd. 3,000 volt by-pass condenser, made like the aerial and grid condensers; (9) one pair of choke coils for the high voltage secondary circuit; (10) one milli-ammeter; (11) one A. C. power transformer; (12) one rheostat as shown at I in Fig. 75, and (13) one panel cut-out as shown at K in Fig. 75.
The Choke Coils.--Each of these is made by winding about 100 turns of No. 28, Brown and Sharpe gauge, cotton covered magnet wire on a spool 2 inches in diameter and 2-1/2 inches long, when it will have an inductance of about 0.5 millihenry [Footnote: A millihenry is 1/1000th part of a henry.] at 1,000 cycles.
The Milli-ammeter.--This is an alternating current ammeter and reads from 0 to 250 milliamperes; [Footnote: A milliampere is the 1/1000th part of an ampere.] and is used for measuring the secondary current that energizes the plate of the oscillator tube. It looks like the aerial ammeter and costs about $7.50.
The A. C. Power Transformer.--Differing from the motor generator set the power transformer has no moving parts. For this transmitting set you need a transformer that has an input of 325 volts. It is made to work on a 50 to 60 cycle current at 102.5 to 115 volts, which is the range of voltage of the ordinary alternating lighting current. This adjustment for voltage is made by means of taps brought out from the primary coil to a rotary switch.
The high voltage secondary coil which energizes the plate has an output of 175 watts and develops a potential of from 350 to 1,100 volts. The low voltage secondary coil which heats the filament has an output of 175 watts and develops 7.5 volts. This transformer, which is shown in Fig. 81, is large enough to take care of from one to four 5 watt oscillator tubes. It weighs about 15 pounds and sells for $25.00.
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| The Transformer and Tuner of the World's Largest Radio Station. Owned by the Radio Corporation of America at Rocky Point near Port Jefferson L.I. |
Connecting Up the Apparatus.--The wiring diagram Fig. 82 shows clearly how all of the connections are made. It will be observed that a storage battery is not needed as the secondary coil of the transformer supplies the current to heat the filament of the oscillator. The filament voltmeter is connected across the filament secondary coil terminals, while the plate milli-ammeter is connected to the mid-taps of the plate secondary coil and the filament secondary coil.
