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 This, however, has been very largely superseded by the Magnetic Detector of Marconi and the Crystal Detector, both of which make the signals perceivable as buzzing sounds in the telephone.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. All this is easily explained if we remember first that a heap of tiny magnets lying higgledy-piggledy would in fact exhibit no magnetic power outside the heap. If, however, we brought a powerful magnet near them it would have the effect of pulling a lot of them into the same position, of arranging them in fact so that instead of all more or less neutralizing each other they could act together and help each other. Then the heap would become magnetic. On removing the powerful magnet, however, a lot of the little ones would be sure to fall down again into their old places and so the heap would at once lose a large part of its power, yet some would remain and so it would retain a certain amount of "residual" magnetism. If, then, you were to give the table on which the little magnets rest a good shake, the "higgledy-piggledyness" would be restored and even the "residual" magnetism would vanish.