A circuit containing a certain amount of inductance, capacity and resistance tends to oscillate at a certain frequency. Therefore, the oscillations in every transmitting set have a certain frequency depending upon these factors. It is necessary to adjust the receiving apparatus so that it possesses the same frequency as the transmitter. The electro magnetic waves from the transmitting station will strike the aerial of the receiving station at a certain frequency and induce currence in it. If the receiving station is tuned to the same period as the transmitter each wave will give a slight impulse to the readily excited oscillations, which will grow in intensity just as small impulses given to a pendulum at the right times will make it swing violently.
The purpose of the tuning coil is to adjust the receiving circuit to the same period as that of the transmitter.
Tuning coils are wound of bare copper wire over a core composed of a specially treated cardboard tube. The wires are spaced apart so that they do not touch one another. Either one, two or three variable contacts or sliders are provided. The coils are consequently known as "single," "double" or "three" slide tuners.
A loading coil is a supplementary coil sometimes placed in series with the regular tuning coil to give a greater inductance to the circuit so that it may be given a much lower frequency in order to receive waves of greater length.
LESSON TWENTY-FOUR. LOOSE COUPLERS.
A Loose Coupler or Receiving Transformer is a tuning coil in which the coupling, as well as the inductance, is variable. We have already explained that an ordinary transmitting set throws off two sets of wave trains of slightly different length, one being somewhat weaker than the other.
FIG. 53. Loose Coupler.
The purpose of the loose coupler is not only to adjust the receiving set to the period of the transmitter in the manner of the tuning coil, but by varying the coupling to attract the apices of the weaker trains of waves to the same apex as the stronger waves and so really create a pure wave out of the other two.
This may be more easily understood from the accompanying illustration which represents diagrammatically a double train of waves and a pure train.