In Marconi's early experiments there was little or no attempt at tuning the instruments for waves of certain lengths, but this art has been developed to a high state in modern wireless telegraphy and we shall see how the operator tunes his instruments to talk to any one special station.
The distinguishing feature of the modern wireless transmitter, now familiar to every boy who has ever taken a trip aboard a large ship, or attended an electrical show, as it was in the old days, is the "crack, crack, cr-r-r-ack, crack" of the spark as it flickers between the brass knobs of the instrument, as the operator pounds away at his key. In some of the great high-power land stations, where long distance work is done the crash of the spark is like that of thunder, the flame is as big around as a man's wrist and of such intensity that it could not be looked at with unshaded eyes. On ships where the crash is too loud it has become necessary to cover the spark gap with a wooden muffler so as to deaden the noise.
While the simple spark gap of the early Marconi instruments was enough to send out the Hertzian waves, the modern transmitter is a marvel of electrical construction utilizing as it does the latest discoveries in electrical apparatus.
The most noticeable difference in the sending apparatus is in the arrangement of the two wires between which the spark flies. In the early instruments the wires were set in a horizontal line, and connected to an induction coil, but in the later ones the oscillator was turned up lengthwise with the spark gap between the vertical wings.
The different position of the spark gap is a change only in form, and not in principle. In the Marconi apparatus used nowadays the current comes from a dynamo of more than 110 volts, direct current. The two terminals of the circuit are connected with an induction coil, and from there to the two ends of the wires, making the terminals of the spark gap. The upper wire runs from the spark gap to the aerial, and the lower runs through a battery of Leyden jars, through a high tension transformer (as does the other side of the circuit), and thence to the ground. Aboard ship the ground connection is simply made by attaching a wire to the hull of the ship, which is in connection with the water, the best possible earth connection.
MARCONI TRANSMITTER LAYOUT
A—Key. B—Induction coil. C—Spark gap. D—Dynamo. E—Rheostat. F—Interrupter magnet. G—Aerial. H—High tension transformer. I —Ground wire. K—Battery of Leyden jars.
There are, of course, a great many different kinds of transmitters, but they are all worked out on the same general principle—a spark gap which creates electrical oscillations that are sent into the ether from the aerials.