It might reasonably be asked why a telephone receiver could not be directly connected to the aerial and ground so that it would respond directly to the high frequency currents generated by the incoming waves without the medium of a detector. There are two very good reasons why such a method would not be possible, the first being that the little magnet coils contained in the telephone receivers exert a choking action upon alternating currents of high frequency which effectually blocks their passage. Low frequency alternating currents, intermittent direct currents and continuous direct currents will readily pass, producing a sound-each time there is any change in their value. The purpose and action of most types of detectors is to act as a valve allowing the current to pass through in one direction but not permitting it to pass in an opposite one. The high frequency oscillating currents may be represented by a curved line crossing and recrossing a zero line and gradually decreasing in amplitude as shown by A in Fig. 66.
FIG. 66.—Illustrating the valve action of a rectifying detector.
FIG. 67.—A new type of silicon detector in which a crystal of arsenic may be brought to bear against the surface of one of several silicon crystals.
The detector, acting as a valve, eliminates one half of the alternating current so that the result may be represented by B, in reality a pulsating direct current which rises and falls but is able to flow through the telephone receiver and produce a motion of the diaphragm with consequent sound waves audible to the ear.
FIG. 68.—Diagram drawing analogy between rectifying action of a detector and a pump.
The accompanying sketches and the following analogy drawn between the electric currents and the flow of a stream of water may serve to render a better conception of how it is possible for the valve action of a detector to rectify an alternating flow, continuously reversing its direction to an intermittent current passing in one direction only. The illustration shows two pumps A and B. Each pump is immersed in a pool of water and consists of a cylindrical tube T and T' having a small opening, O and O', at the lower end to admit the water and a piston, P and P', operating up and down inside the tube. Every time that the piston P is raised in the pump A it will draw in water through the small hole O. As soon as it descends, however, the water will reverse its direction and pass out. The action of the water represents that of an alternating current because it passes in first one direction and then in the other. The pump B is fitted with a valve whose action is to permit the water to flow in one direction only. The valve is fitted to the piston P'. It is a little flap which opens a hole in the piston when the latter is descending and closes when it is rising. Suppose that the piston is raised. Water will be drawn in through the little hole O'. As soon as the piston reaches the limit of the stroke it commences to descend. In falling it exerts a slight pressure on the valve which opens and allows the water to pass through. The hole in the piston is larger than the hole in the pump and so there is almost none of the water forced back into the pool. The next up stroke of the piston draws more water in, that which is on top flowing out through the overflow. The nature of the stream passing through the hole O' is intermittent, passing principally in one direction. It may be likened to the intermittent direct current produced by the detector.
