Fortunately, it is very easy to add these properties to an oscillatory circuit in a very simple manner. A certain little instrument called a "tuner" is con
nected up in the circuit and by the simple movement of a few handles the desired result can be obtained quickly even by an operator with but a moderate experience. He has certain graduated scales to guide him, and he is only called upon to work according to a prearranged rule in order to obtain any of the regulation wave-lengths.
As a matter of fact, the instruments are not directly inserted in the antenna circuit, the circuit that is which is formed by the aerial wires, the earth and the inter-connecting wires. Instead, the two sides of the spark-gap are connected together so as to form a separate circuit of their own, the local circuit as we might call it, and then the two circuits, the antenna circuit and the local circuit, are connected together by "induction."
A coil of wire is formed in each, and these two coils are wound together so that currents in one winding induce similar currents in the other winding, and by that means the oscillations set up by the coil in the local circuit are transformed into similar oscillations in the antenna circuit. This transformation involves certain losses, but it is found in practice to be by far the most effective arrangement. Both the circuits have to be tuned to the desired wave length, but that is done quite easily by the operation of the handles in the tuner already referred to.
It is to this coupling together of tuned circuits that Marconi's most famous patent relates. It is registered in the British Patent Office under the number 7777, and hence is known as the "four sevens" patent. It has been the subject of much litigation, which
proves its exceptional importance, and it is to the fact that the Marconi Company have been able to sustain their rights under it that they owe their commanding position to-day in the realm of wireless telegraphy.
The Receiving Apparatus also consists of a separate local circuit which can be coupled when desired to the antenna circuit through a transformer. The same simple tuning arrangement is made to affect this circuit also, so that the "multiple tuner," as the instrument is called, controls all the circuits both for sending and for receiving. The oscillations caused in the antenna circuit by the action upon it of the etherial waves flowing from the distant transmitting station pass through one winding of the transformer and thereby induce similar oscillations in the local receiving circuit which are made perceptible by the receiving instrument.
Reference has already been made to the original form of receiving apparatus called the Coherer. This, however, has been very largely superseded by the Magnetic Detector of Marconi and the Crystal Detector, both of which make the signals perceivable as buzzing sounds in the telephone.
The magnetic detector owes its existence to the fact that oscillations tend to destroy magnetism in iron. It is believed that every molecule of iron is itself a tiny magnet. If that be so one would expect every piece of iron to be a magnet, which we know it is not. We can always make a piece of iron into a magnet by putting another magnet near it, but when we take the other magnet away the iron loses its power,
or to be precise it almost loses it. A piece of even the best and softest iron having once been magnetized retains a little magnetic power which we call "residual" magnetism.