Radio Broadcast (Nov 1923-Apr 1924)

314 Radio Broadcast one need boast, in order to understand the theoretical significance of tuning, is a knowledge of fractions and equations which is herewith slightly reviewed. A common fraction is designated by two numbers, the numerator and denominator, placed respectively above and below a bar indicating a fraction, — §, i\, J etc. There are very few of us who are unfamiliar with these relics of elementary schooling. If the denominator of a fraction is made larger, the value of the fraction is quite obviously made smaller. Thus tV is less than \. And so, if we have two quantities called X and Y, X being'equal to tV (X = iV) and Y being equal to \ (Y = J), Y is a larger quantity than X. THE EQUATION AN EQUATION, as its name de. clares, is a statement of equality: For instance, 10 = 2 x 5 or 8 = -V6-. We can see by observation and from the definition of an equation, that if one side of an equation is multiplied or increased, the remaining side increases automatically (in order to maintain an equation) and in proportion. Thus in the equation 10 = 2 x 5, if the 2 on the right-hand side of the equal mark is changed to 4, the 10 must be changed to 20, and the equation read 20 = 4 x 5 . In the equations X L = 2^ f .2 5 , and XL = 27rf .5 in the latter equation represents twice the amount of XL in the former equation. Likewise, if one side of an equation is reduced, the other side decreases in proportion. REACTANCE AND RADIO CURRENTS THOUGH it has been many times stated and explained, it will do no harm to reiterate that radio currents in the antenna and in most of the circuits of receiving and transmitting apparatus are ' 'alternating," that is, they travel first in one direction and then in another, changing polarity from positive to negative. As was shown in the preceding article on "inductance," a coil of wire carrying an alternating current will have induced in it an E. M. F. (electro motive force which results in a current) in a direction opposite to the inducing current. This is called, for obvious reasons, a "counter E. M. F." Therefore, we have, in an inductance carrying radio electricity, perhaps a tuning coil, variometer, etc., two currents which are opposing each other, the counter E. M. F. tending to hold back the original current; and, if the counter E. M.F. is sufficiently powerful (the inductance of the coil sufficiently high), it will retard the current until the voltage has changed polarity. In other words, the current may be on the plus side of an alternation when the voltage is on the negative side; and as work (such as making a sound in the receiver) can be accomplished by electricity only when voltage and amperes come together, power is lost by this inductive effect. It is obvious that the slightest loss of weak radio currents must be fatal to reception. This ability with which a coil builds up a counter E. M. F. is known as reactance, i, e., "acting back on." Now, as the counter E. M. F. is caused by the rise and fall of the magnetic flux generated by the original current, the strength of the counter E. M. F. varies with the rapidity with which this field rises and falls, in other words with the frequency of the current. Indeed, all of this may be summed up in the following formula for reactance: XL = 2 7T f 1 XL is the reactance which is computed in ohms, for its effect on a circuit is very similar to that of resistance. L is the inductance in the circuit (for the greater part, the coils of wire, etc.), and as L increases, for instance, by adding more turns of wire, the reactance, XL, will necessarily become greater. (Increasing one side of an equation automatically increases the other side). F is the frequency of the current, and, likewise, as that becomes higher, so does the reactance. (The frequency of a radio current, of course, changes with the wavelength. However, the proportion is an inverse one. That is, when the wave is decreased, the frequency increases; when the wavelength increases, the frequency is lessened. The reader must bear this in mind, so that when we have occasion to speak of a wave change, he may instantly interpret our words in terms of frequency, and vice versa.) In brief: in a coil of wire carrying a radio current, there exists a tendency to nullify or make useless that current — a tendency that varies directly with the siie of the coil and the wavelength of the current. Coils of wire, or inductances, are necessary in every radio set, for the transference of energy