Radio Broadcast (Nov 1923-Apr 1924)

Various Circuits and What They Mean from the antenna to the tube circuit, and from one circuit to another. But it is quite apparent that if a set consisted of inductance alone, it would not work, because due to the reactance, the current would lag — be altogether useless — and no signals would be heard. There must, therefore, be some way of overcoming the difficulty of neutralizing the reactance. There is, and in order to name this something, the reactance of inductance has been qualified by the word "positive." That is, instead of saying a coil of wire merely has reactance, it is said that it possesses positive reactance, while the thing that overcomes it is known as negative reactance : the reactance of a capacity or condenser. It has been shown that positive reactance causes the alternating current to lag or slip behind the voltage, but a circuit having a great deal of capacity has just the opposite effect, and causes the current to "lead" or jump ahead of the voltage. Thus, by carefully balancing the lag and lead, with coils and condensers, it is possible to bring the current into step, or electrically, into "phase" with the voltage. When this condition is realized, efficiency will be the greatest and the most work accomplished. This current lead which characterizes a circuit containing a predominance of capacity, is due to a "displacement current" which anticipates the direction of the current which is to follow. This displacement current varies with the capacity of the condenser and the frequency of the E. M. F. in a manner that is best illustrated by the formula for negative reactance: XC = z^fc XC, the reactance, varies inversely with the frequency and capacity. (It will be remembered that positive reactance varies directly with the change in the qualities which are responsible for it.) If the condenser or frequency is increased (the denominator of the fraction), the reactance drops, and vice versa. We now arrive at the point where this information throws light upon the tuning of a radio circuit. WHAT REALLY HAPPENS WHEN YOU TUNE BOTH kinds of reactance, positive and negative, change with the wavelength or frequency, and to receive energy at any particular frequency, the two kinds of re actance must be equal to each other at that frequency so that the current will be in phase with the voltage. This point of equilibrium or balance is called the "resonance point." All circuits have some resonance point, no matter what the values of capacity and inductance may be. In other words there is always some wavelength at which the negative reactance will balance out the positive reactance. This may be shown by examining the two formulas simultaneously: X=2xfL and XC=^. Now, follow this carefully: If we start at an extremely high frequency (a high value of f), we shall find (regardless of the values of L and C) that XL is much greater than XC. As the frequency is lowered (the wavelength raised) XL of course becomes smaller, while XC (the denominator decreasing) becomes greater. Hence, at some point or another (the resonance point), XL will be exactly equal to XC, and the wave (frequency) at which this occurs will be that to which the circuit is tuned. All of this may seem slightly confused, but we believe it will be clarified by a more concrete example, for which we shall specify a regular three-coil honeycomb, three-circuit receiver, the circuit for which is shown in Fig. i. During our hypothetical tests, the honeycomb coils will not be changed — in other words, L, or inductance, will be constant. There is a condenser in series with the primary coil, and another shunting the secondary. The capacity between antenna and ground acts as an addiditional capacity, which, in conjunction with the primary variable, is virtually a shunt across the primary coil. Thus each coil, primary and secondary, have condensers across them, the capacities of which are decreased by turning down the scales of the variable condensers. Hence what proves true of one circuit (in so far as varying the condenser is concerned) will be true of the other, and so, only one circuit, the primary, will be considered. We shall assume, at the start of our experiment, that the primary is tuned to four hundred meters, and the primary condenser is set at fifty degrees, about one half of its scale. We now move the condenser down to twenty-five degrees. What has happened? We have increased the negative reactance (by decreasing C, or the denominator of the fraction). If the negative reactance has been increased, then the new resonance point will be one at which the