Now the telephone, by which term is always meant the receiver (the sending part of the telephone apparatus being a "microphone"), is one of the most remarkable pieces of electrical apparatus which the mind of man has ever conceived. It is astonishingly robust. With ordinary care you cannot damage it. There is no need whatever to keep it wrapped in cotton wool or even to keep it in a case. Without harm you can put it loose in your pocket. Within reason you may even drop it a few times without harm. Its cost is only a few shillings. Yet its sensitiveness is simply astounding. It will detect the existence of currents so small that any other type of instrument to deal with them has to be extremely delicate and costly. It consists of a magnet fitted into a little brass case with a little piece of soft iron fixed on each pole, while each of these "pole-pieces" is surrounded by a tiny coil of wire. The lid of the box is a disc of thin sheet-iron, and things are so proportioned that the pole pieces nearly but not quite touch this sheet-iron "diaphragm." An outer cover, generally of ebonite, serves to catch the sound-waves caused by any movement of the diaphragm and convey them to the ear. The action of the permanent magnet tends to pull the diaphragm inwards--to bulge it in slightly--so that it is in a state of very unstable equilibrium. Because of this instability a very tiny current flowing through the coils and either adding to or subtracting from the strength of the magnet is sufficient either to draw it still closer or to let it recede a little. Whether it approaches or recedes depends upon the direction of the current through the coils and makes no difference to the sound. The movement of the diaphragm is great or small according as the current is strong or weak: any variation in the current causes a perfectly corresponding movement in the diaphragm. Even those very small and very complex changes in air-pressure which give us the sensation of sound are very faithfully followed by this simple bit of sheet iron, so that the sounds are faithfully reproduced for our benefit. At the moment, however, we are not dealing with speech but with buzzing sounds, which are very simple, being merely a rapid succession of "ticks." The telephone, it must be remembered, takes no notice of a steady current, except when it starts and stops. But each time that occurs it gives a tick. Hence, if we start and stop a current very rapidly, or to use another term, make it rapidly intermittent, we get a rapid succession of ticks, and if rapid enough they form a humming, buzzing, or singing sound. If very fast you can get a positive shriek. The precise character of the sound depends entirely upon the rapidity of the intermittency.