The step up transformer takes the battery voltage and boosts it to many thousands of volts. Since transformers do not work on direct current some form of interrupter is required. It takes the form of a simple clapper on the end of the coil. It opens the battery current flowing through the primary side when ever the magnetic field created by the current flow reaches a certain strength. In practice the interruptions occur, for explanation purposes, about 200 times a second. The collapsing magnetic field caused by the interruption to primary current flow cuts through the secondary winding turns inducing a voltage. The voltage induced is directly proportional to the "turns ratio" between the two windings.
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In the case
of the apparatus shown in the image, the core is a bundle of soft
iron 22 SWG wires with a diameter of 2" and a length of 18".
Primary winding is 300-400 turns of #12 SWG
copper wire wrapped on the core. It is wound bobbin style
around the soft iron core. The spark coil secondary is not wound smoothly back and forth (bobbin style) across the primary windings, but is done using many insulated disk shaped coils, joined together as illustrated on the left. This prevented high voltage breakdown within the transformer as the high voltages generated were now distributed along the width of the secondary coil, and not through the thickness. |
Hook a couple of capacitors and a coil of wire as shown in the top diagrams. The capacitors shown here are glass jars, lined with tin foil inside and out and can handle the high voltages. The value of the capacitor would typically be in the order of 0.1 mfd. The RF coil will eventually be used to couple energy to the antenna system.
Applying battery to the primary circuit causes current to flow and, due to the interrupter, be cycled off and on about 200 times a second. The voltage generated, due to transformer's step up action, on the secondary side is several times higher when the interrupter opens the circuit since the rate of change of the magnetic field is much faster collapsing. When the interrupter closes the circuit the magnetic field's rate of change is much slower due to the inductive effect of the primary coil. The resulting high voltage on the secondary alternates at the same frequency as the interrupter buzzes.
If no coil and capacitors were across the gap, the spark could be several inches long--hence references to a 10" spark coil transmitter in the old books. In practice the capacitors initially act as a short circuit across the coil's secondary, the voltage rising as the value of the capacitors and winding resistance of the spark coil's secondary allow. The applied high voltage isn't available for the length of time required, due to the quick action of the interrupter, and as a result the capacitors do not fully charge. Thus high voltage is quite a bit less than the peak supplied by the transformer and as a result the spark gap must be decreased to the order of 1/4 to 1/2 inch before a spark is obtained.
The antenna system has distributed capacity to earth and with the inductance of the coupling coil creates a resonant circuit. The amount of inductance is adjusted, by means of taps, to resonate at a particular frequency. In those days the wavelength would be somewhere in the region of 600 to 2000 meters (500 kHz to 150 kHz).
On the rise of the high voltage the capacitors charge up via the antenna coil until the flash over voltage of the spark gap is realized. The spark appears, for all intents and purposes, as short circuit. The capacitors are now able to discharge their energy into the resonant antenna via the coupling coil, then the coil back into the capacitors via the still arcing gap. This back and forth transfer of the energy will continue until the arc finishes, or the resonant circuit's energy is dissipated in overcoming the resistance losses. In practice the resonant circuit's energy will diminish before the arc is extinguished due to the fall of secondary voltage.
Thus the resonant antenna produces a burst of radio frequency energy at each arc of the spark gap. (A crude analogy would be a mallet [the energy pulse] hitting a gong [creating a diminishing tone].) The power induced into the antenna circuit is proportional to the voltage charge on the capacitors which, in practice, is governed by the spark gap spacing. A larger spacing allows the voltage on the capacitors to reach a higher value before the spark gap is overcome.
A simple amplitude modulation receiver, such as a crystal set, tuned to the frequency of the transmitter, would hear a tone, the frequency of which is equal to that of the arc (400 Hz not 200 Hz), which in turn is governed by the frequency of the interrupter.
This unit would be quite similar to the transmitters carried on the first wireless equipped vessels. The only changes would be the insertion of a telegraph key in series with the battery which the operator would manipulate to form the Morse characters, and suitable coupling of the RF coil to an antenna.
This transmitter is inefficient, even by turn of the century standards. Go to the coast station transmitter write up to see a discussion of the required changes.
Just as an aside, if the RF coil and capacitor are removed, put a spark plug in place of the spark gap, we will have a car ignition system. That explains why a passing car can often be heard on a radio--its ignition system is a wireless transmitter. Note that the missing coil and capacitor are still there--the ignition wiring has its own distributed inductance and capacitance.
This link lets you hear what a spark transmitter sounded like. The whole recording explains itself.
The professional images are from "An Elementary Manual of Radio-Telegraphy and Radio-Telephony, 3rd edition by J.A.Fleming/Longmans, Green & Co. London 1916).
