Radio Broadcast (May-Oct 1922)

How to Begin to Enjoy Radio of these bands, thus increasing the number of wavelengths available for simultaneous communication. In the receiving set the band can be greatly narrowed by the use of two selective circuits and this is a common arrangement. The first circuit selects a band of frequencies (wavelengths) from all the radio waves present in the ether. The second circuit selects a very narrow band of frequencies from those present in the first selective circuit. This double selection is very effective as the effect is cumulative. A TYPICAL RECEIVING CIRCUIT ATYPICAL receiving circuit is shown in Fig. 2. All the symbols used are those customarily employed. The reader should fix them firmly in mind, for they are not usually labeled in diagrams. Wherever an arrow appears it signifies that the quantity represented by the symbol through which it passes or to which it points is variable. Thus a coil represents an inductance; the arrow, A, pointing to the coil means that it is variable. Two straight lines of equal length near and parallel to each other represent a condenser, which is the name of the instrument that furnishes a capacity. An arrow through the lines means that the capacity is variable. The long arrow through the two inductances, one in either circuit, means that the strength with which the Antenna Inductance Condenser — ^ (Capacity) Ground To Detector PRIMARY ' SECONDARY (First Selective (Second Selective Circuit) Circuit) Fig. 2 primary circuit affects the secondary circuit mav be varied at will. The method of doing this will be explained in a later article. The dashed lines are not a part of the circuit but simply indicate its division into two selective circuits, each of which, you must notice contains an inductance and a capacity. It is only a tuner that is shown; the detector and phones not being included in the diagram. Note that there are five variable quantities represented in this tuner; that means that there are five knobs or handles to adjust. Your tuner may not be like this one, it may have fewer variables, but it will certainly have some of the features of this typical tuner. For instance, your tuner may have only one selective circuit and it may have only one variable quantity in this circuit. Again your tuner may have two selective circuits with only one variable in each and no means of adjusting the effect of the one circuit or the other. Other combinations are possible also. INDUCTANCE AND CAPACITY LET' us conclude this article by describing ^ how an inductance and capacity are made and how they are made variable. We will first consider capacity. Capacity may be likened to a tire. Compare a bicycle tire and an automobile tire. Suppose we take 4 cubic feet of the air in a room and pump it into an automobile tire and also pump another 4 cubic feet into a bicycle tire. The same amount of air has been put into each tire but the results are different. The pressure in the bicycle tire is, say, 100 pounds per square inch. This means that the air inside the tire is trying to escape with a force of 100 pounds. It also means that, in order to blow up the tire, a force just greater than 100 pounds per square inch must be applied. But the same amount of air in the automobile tire causes a pressure of only, say, 20 pounds per square inch. The air inside is trying to escape with a force of only 20 pounds, and it required a force of only a little more than 20 pounds to put it in the tire. So in electrical capacity, the amount of capacity determines the forc-e (electromotive) with which a certain quantity of electricity will try to escape and also the force needed to put that amount of electricity into the capacity. Using the samp amount of electricity, the larger capacity requires less force to be charged and exerts less force trying to discharge itself. In Fig. 3-A there are shown two conducting plates placed close together but separated by a nonconductor. The plates may be of any metal and for the sake of compactness they are usually a number of small plates all joined together as shown in