Wireless Telegraph Construction For Amateurs

by Alfred Powell Morgan

The key serves to break the current into periods corresponding to the dots and dashes of the telegraph code. When the high voltage of the induction coil is impressed upon the plates they become charged, and being of opposite polarity, when at a maximum the energy rushes across the gap and produces a disruptive spark. Each discharge, although appearing like a single spark passing in one direction, is in reality made up of a large number of rapid oscillations or surgings. The first passage of current serves to more than discharge the plates and they become charged in the opposite direction. A reverse discharge then occurs which also oversteps itself, and thus the oscillations go on, but gradually become weaker and weaker until they die completely or are damped out. The heated air of the spark gap becomes a conductor during the passage of the spark, and the oscillations are enabled to surge back and forth at the rate of 15,000 to 1,000,000 per second, although the actual discharge may take only a fraction of a second.

Fig. 2. Hydraulic Oscillator.

The generation of electrical oscillations may perhaps be made more clear by reference to the hydraulic apparatus illustrated in Fig. 2. T and T' are communicating tubes divided by an elastic membrane M. The tubes may be likened to the metal plates t and t' or the arms of the oscillator. The membrane may be likened to the layer of air between the knobs which separates the opposite arms of the oscillator. P is a pump connected to the two tubes T and T', and the broken lines in the apparatus represent water. The pump corresponds to the induction coil in Fig. 1, and the water to the secondary currents of the induction coil. When the pump is set in operation, the water is drawn from the tube T and injected into T'. The pump valves prevent it from flowing back. When the level becomes very high in T', the great pressure distends the membrane in the direction shown by the dotted line until finally it bursts and the water is allowed to flow with a rush into the tube T. But the inertia of the water causes it to rise higher in the tube than its final position of equilibrium, while in returning and endeavoring to seek its level its inertia carries it below this position. Thus the water oscillates back and forth until finally it comes to rest.

Similarly the difference of potential of the oscillator arms is not immediately equalized upon the breaking down of the air gap, and the apparatus becomes the seat of extremely rapid electrical oscillations, as explained above.

All space is supposed to be filled with a highly attenuated, invisible and weightless medium called ether. When the electrical oscillations surge back and forth through the arms of the oscillator, portions of the energy are thrown off from the apparatus and travel in enlarging circles like the ripples on a pond. These consist of lines of dielectric stress or electrostatic flux which pass through the ether and constitute electromagnetic waves.

The receptor or resonator R, Fig. 1, consists of a circle of wire having in it a small spark gap capable of minute adjustment. Two metal plates r and r' are sometimes attached to the opposite sides of the spark gap. When the key is pressed at the transmitting station and waves are sent out through the ether, they strike the resonator and set up therein electrical oscillations which pass across the gap in the shape of sparks.

Fig. 3. "Hydraulic" Transmitter and Receptor.

To make the explanation clearer, let us consider Fig. 3 in which two floats or blocks of wood are represented as resting on the surface of a tank or pool of water. One float, A, is connected by a rope and pulley so that by jerking the rope the float may be made to oscillate and cause little ripples or waves to pass outwards in a gradually enlarging circle. When the waves reach the float, B, they cause it to rise and fall with each wave or to oscillate and reproduce the movements of the float, A. Likewise the oscillations set up by a wireless transmitter are sent out into space to be caught and duplicated at the receiving station. Of course this analogy to the propagation and reception of electric waves is not the same as the true electrical actions, but is merely a graphical, representation.