EQUIPPING A COACH FOR WIRELESS RECEPTION
Page 29
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What Happens inside an Oscillatory Circuit when it is brought into Resonance is told in this Article.
IN THE last two instalments of this series we have dealt somewhat fully with the mechanical analogies to resonance in oscillatory circuits. We have attempted to make clear the exact nature of the phenomenon of resonance, because it is the basis of all our present methods of tuning wireless receivers and transmitters, and, therefore, a clear idea on this important part of the subject will greatly help all those who operate and construct wireless receiving sets. In this present article we will attempt to complete our survey of the question of resonance by considering what actually happens in a tamed electrical circuit. ,
We know that the difference between a direct and an alternating current is that, whereas the former is steady as regards its direction—i.e., it is said to flow always in the same direction, from positive to nega. tive, the latter changes its direction more or less AMPS.
4-4 Graph illustrating the growth 4 3 and reversal of an alternating + 2 current. .4. 1
rapidly. On an alternating current supply there is, therefore, neither. positive nor negative side, which, incidentally, is the reason why accumulators cannot be charged from such a source, since the chemical change to be brought about in the battery needs a current always flowing in one definite direction. Now, the change from one direction to the other on the part of the current is not absolutely sudden ; the current does not flow at a certain rate for a certain time and then suddenly reverse and flow in the other direction, but, on the contrary, it starts quite small, builds up to a certain maximum quantity, then begins to die away, and, finally, when it has reached zero, it again commences to build up, flowing, however, in the other direction.
We can illustrate this action if we make a diagram in which the strength of the current is shown in height and the time over which the changes take place is represented by horizontal distances. Such a diagram, which is technically 'mown as a graph, is shown in Fig. 29. Here we draw two lines at right angles and divide the vertical one to represent amperes and the horizontal one to represent time (in this case divided into centi-seconds, or hundredths of a second). To represent the behaviour of our alternating current we must plot its value at every division of time, either above or below the horizontal line. If we do this, and join all the points thus obtained, we shall get the wavy curve shown in the figure. We see that, starting from zero at the point 0, the current reaches a maximum value half-way between points 3 and 4. It then starts to die away again until, at the point 7, it has again reached zero. It now commences to build up in the other direction, until at midway between points 10 and 11 it has again reached a maximum negative value, after, which it again dies away until zero is again reached at point 14.
This represents a complete cycle, which is thereafter repeated so long as the current flows. The curvethus generated is known as a "sine"" curve, from the trigonometrical function of that name, whose varying values it also represents. It is the curve of all alternating motion which follows the simple harmonic law, such as a swinging swing, an oscillating pendulum, a vibrating string, a tuning fork, and also approximately the pistons of all reciprocating engines. The number of complete cycles performed in one second is known as the frequency ; in this case it amounts to 100 divided by 14, or, roughly, 7.
Now, the curve shown in Fig. 29 deals only with the current or quantity of electricity passing through the circuit. If we considered the' voltage or pressure from moment to moment, we should find that it also varied in the same way and could be represented by a similar " sine " curve. Moreover, we should find that, provided we have a circuit without either capacity or inductance, these two curves would keep in time with one another ; that is, the current and the voltage would reach a maximum on the same side of the horizontal line at the same _moment, and, likewise, they would also reach the zero point together. The effect of capacity and inductance is to cause the current curve and the voltage Ourve to get out of step. And the more we increase the value of the capacity and inductance in a circuit the further the current and voltage get out of step, until a point is reached when these two differ from one another by 90 degrees or a quarter of a cycle. When, this happens the circuit is in resonance, for that particular frequency.
The state of affairs is shown in Fig. 30. At points A, C, and E the current is at a maximum and the voltage is zero ; at points 0, B, and D the voltage is at a maximum and the current is zero. The net result of this is that, so far as a useful result is concerned, when a circuit is resonant to any particular frequency it practically blocks that frequency from passing through it, since current without voltage, or voltage without current, can produce no useful work.
In all radio receivers we connect our aerial to earth by means of a tuned circuit and then make this circuit
resonant to the particular frequency which we desire to receive. The result is that oscillatory currents of that frequency can scarcely pass through this resonant circuit, and so are available to work our receiving apparatus which is connected across the tuned circuit, whilst oscillations of other frequencies to which the circuit does not happen to be resonant pass unhindered to earth without affecting our receiver. By this means, thersfore, are we enabled to tune out unwanted stations, whilst retaining that one to which we wish to listen. But for the fortunate fact that resenance can be thus used, we should be unable to enjoy the benefits which radio confers on humanity, for, instead of clear and uninterrupted signals, we should hear, nothing but a veritable Babel of voices.