I don’t propose to stop to tell you how a spark-set transmitter works. It is sufficient to say that when the key is depressed the set sends out radio signals at the rate usually of 1000 signals a second. Every time a signal reaches the receiving station the current in the telephone receiver is sudden reduced; and in the time between signals the leak across the grid condenser brings the tube back to a condition where it can receive the next signal. While the sending key is depressed the current in the receiver is decreasing and increasing once for every signal which is being transmitted. For each decrease and increase in current the diaphragm of the telephone receiver makes one vibration. What the listener then hears is a musical note with a frequency corresponding 173to that number of vibrations a second, that is, a note with a frequency of one thousand cycles per second. He hears a note of frequency about that of two octaves above middle C on the piano. There are usually other notes present at the same time and the sound is not like that of any musical instrument.
If the key is held down a long time for a dash the listener hears this note for a corresponding time. If it is depressed only about a third of that time 174so as to send a dot, the listener hears the note for a shorter time and interprets it to mean a dot.
In Fig. 91 I have drawn a sketch to show the e. m. f. which the signals from a spark set impress on the grid of a detector and to show how the plate current varies if there is a condenser and leak in the grid circuit. I have only shown three signals in succession. If the operator sends at the rate of about twenty words a minute a dot is formed by about sixty of these signals in succession.
The frequency of the alternations in one of the little signals will depend upon the wave length which the sending operator is using. If he uses the wave length of 600 meters, as ship stations do, he will send with a radio frequency of 500,000 cycles a second. Since the signals are at the rate of a thousand a second each one is made up of 500 complete cycles of the current in the antenna. It would be impracticable therefore to show you a complete picture of the signal from a spark set. I have, however, lettered the figure quite completely to cover what I have just told you.
If the grid-condenser and its leak are so chosen as to work well for signals from a 500-cycle spark set they will also work well for the notes in human speech which are about 1000 cycles a second in frequency. The detecting circuit will not, however, work so well for the other notes which are in the human voice and are necessary to speech. For example, if notes of about 2000 cycles a second are involved in the speech which is being transmitted, 175the leak across the condenser will not work fast enough. On the other hand, for the very lowest notes in the voice the leak will work too fast and such variations in the signal current will not be detected as efficiently as are those of 1000 cycles a second.
You can see that there is always a little favoritism on the part of the grid-condenser detector. It doesn’t exactly reproduce the variations in intensity of the radio signal which were made at the sending station. It distorts a little. As amateurs we usually forgive it that distortion because it is so efficient. It makes so large a change in the current through the telephone when it receives a signal that we can use it to receive much weaker signals, that is, signals from smaller or more distant sending stations, than we can receive with the arrangement described in Letter 14.
176LETTER 18
AMPLIFIERS AND THE REGENERATIVE CIRCUIT
My Dear Receiver: