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1)  A radio receiver has two main functions to
perform: firstly, it has to select the required radiation from all the other
electromagnetic waves that are present over the surface of the Earth, and
secondly, it has to separate (or demodulate) the audio-frequency current from
the radio-frequency carrier.

2)  In the receiver these signals, which are close
to each other in frequency, have to be amplified and separated.

3)  The second function of a receiver is to
demodulate the earner-wave;

again,
this operation can be performed by using an electronic valve.

4)  They said that the separator contained two
resonant circuits, one tuned to the frequency of the sound carrier-wave.

5)  During this time the spot of light has covered
the whole screen twice, but owing to the persistence of the fluorescent coating
of the tube, the whole screen is illuminated.