Radio Broadcast (May-Oct 1922)

Care and Operation of a Crystal Receiving Set By EDGAR H. FELIX, A. I. R. E. A RADIO receiving set is an instrument to which alternating currents from an antenna system are suppHed, and which converts these alternating currents into sound waves. The chief advantage of a crystal receiving set over other types is the small investment involved and the fact that there is no up-keep cost after the initial investment has been made. The chief disadvantages are the comparatively short range, lack of selectivity and instability of crystal detectors. A crystal receiving set, however, if it receives at all, receives perfect music without any distortion of sound quality. In this respect it is superior to a phonograph. The output of a crystal detector, however, cannot be amplified by vacuum tube amplifier, and, for this reason, head receivers are always necessary when using a crystal set. To operate any mechanical device successfully we must not only know what the controls are, but also what they do, how they do it and why. For instance, you could start an automobile if you were told to depress a certain foot pedal and pull a certain lever toward you, and then release the depressed foot control. But you would certainly be a failure as a driver if you did not know the function of the clutch and the purpose of the transmission. For this reason, in describing to you the best way to operate a crystal receiving set, 1 shall tell you the function as well as the effect of each control. A crystal receiving set performs three functions: first, it tunes the antenna system to resonance with incoming ether waves; second, it rectifies the incoming oscillating energy so that it can be converted to sound; third, it converts the rectified oscillations into sound waves by means of head telephone receivers. Each of these functions will be taken up in succession. Resonance is a familiar term to you, but it is hard to understand its application in radio without the recourse to analogies. If a tuning fork of a certain period of vibration is set into vibration it starts a series of sound waves. If a second tuning fork of the same period is held near the first, it is set into vibration by the sound waves sent out by the first tuning fork. When two vibratory systems of any type have the same period of vibration they are said to be in resonance. In the case of the tuning forks the period of vibration is determined by the material and dimensions of the tuning forks. The period of vibration or frequency of an electrical circuit is determined by its inductance and its capacity. Whenever there is a change in the quantity or direction of current flowing through a wire, a magnetic field is set up. On the other hand, a change in the strength of a magnetic field always causes an electric current to flow through any electrical circuit within its influence. If current is passed through a length of wire, the magnetic field built up induces a current in the opposite direction from that which caused the magnetic field. In other words a magnetic field builds up a current opposing that which caused it. In this way magnetism in electricity corresponds exactly to inertia in mechanics. If, instead of a straight piece of wire, a given length of wire is wound in the form of a cylindrical coil, the magnetic field of one turn unites with that of the next and so on, greatly intensifying the magnetic effect. The coil in this form is called a tuning coil. The inductance of any electrical circuit or instrument is a measure of the facility with which magnetic fields are built up. A cylindrical coil, for instance, possesses considerably greater inductance than a straight piece of wire of the same length as that used upon the coil. The greater the inductance of a circuit the longer the time required for the magnetic fields to build up and to get up the opposing