ELECTRIC WAVE -METHOD. The last and most practical method employs electric waves. The apparatus for the emission and reception of elec tric waves in its simplest form consists of a sending instrument similar to the one just de scribed in the electro-static method, but having the additional factor of the disruptive discharge included in the transmitter, together with a coherer in the receiver.
These devices are illustrated in Fig. 6, and all workable wireless telegraph apparatus are con structed upon fundamental principles involved in the electric wave method and are modifications of that shown in the diagram.
The sending apparatus. A. consists of an induc tion coil, a, the primary winding of the coil, 1, ber of turns of wire on the coils, on the size of the lather and their distance apart, etc. Etbeimo-STATte NIErnon. In the electrostatic method of -ignaling through space the primary being included in a circuit with a battery, 2, an interrupter, 3. and a Morse key. 4 ; a condenser. 5, is shunted around the interrupter. 3. The ter minals of time secondary coil. 6. lead to the spark gap. 7. one side of which leads to the aerial wire, S, and the opposite side to the earth at 9.
The receiving device. II. consists of similarly elevated and earthed wires. 10 :mil 11, connected to a wave-detector. 12. usually of the coherer type. The local battery circuit includes the coherer, a battery, 13, and a telephone receiver. 1 I. When in action the function of the transmitter is to radiate electromagnetic waves (see ELECTRICITY ; WAVES) ; this is accomplished by a series of of the primary electric current generated by the battery. This low voltage 'Ii net enrrent is rendered intermittent by the in terrupter. 3, and for every impulse through the primary winding of the induction coil an alter mating current is set up in the secondary; since the secondary coil has many turns of wire in it compared to the primary coil, the low-voltage current is transformed into one of high potential (see INDUCTION COIL) having a frequency equal to the rate at which the primary circuit is made and broken by the interrupter. This low-fre
quency, high-potential current charges the aerial wire and the wire leading to the earth with posi tive and negative electricity, establishing a dif ference of potential of several thousand volts. When this potential difference is maximum the resistance of the air film of the spark-gap is broken down, and a spark passes between the brass balls forming the gap, and the difference of potential is equalized.
The process of restoring the electrical equi librium requires a very small fraction of a sec ond, since the high-frequency currents oscillate from the top of the aerial wire, 8, to the earthed plate, 9, sometimes at the rate of a million times per second.
Now electric oscillations transform their en ergy into electric waves, and these electric waves are propagated through space with the rapidity of light to the receiver. When the electric waves impinge upon the aerial wire, 10, of the receiving system they are retransformed into electric oscillations, identical in frequency with those in the sending circuit, but their po tential is very much reduced; these enfeebled oscillations surging in the receiving circuit affect the coherer, breaking down its resistance, per mitting the local current from the battery, 13, to Clow through the wave detector and telephone receiver or relay, 14, and indicating the imping ing wave in the form of a dot or a dash, depend ing upon its length; it ceases, however, the instant the oscillations stop.