The Separate Heterodyne Long Wave Receiving Set.--This is a better long wave receptor than the self heterodyne set described above for receiving wireless telegraph signals sent out by a continuous long wave transmitter. The great advantage of using a separate vacuum tube to generate the heterodyne oscillations is that you can make the frequency of the oscillations just what you want it to be and hence you can make it a little higher or a little lower than the oscillations set up by the received waves.
The Parts and Connections of a Separate Heterodyne Long Wave Receiving Set.--The parts required for this long wave receiving set are: (1) four honeycomb or other good compact inductance coils of the longest wave length that you want to receive; (2) three .001 mfd. variable condensers; (3) one .0005 mfd. variable condenser; (4) one 1 megohm grid leak resistance; (5) one vacuum tube detector; (6) one A battery; (7) two rheostats; (8) two B batteries, one of which is supplied with taps; (9) one potentiometer; (10) one vacuum tube amplifier, for setting up the heterodyne oscillations; (11) a pair of headphones and (12) all of the parts for a two-step amplifier as detailed in [Chapter IX], that is if you are going to use amplifiers. The connections are shown in Fig. 61.
In using either of these heterodyne receivers be sure to carefully adjust the B battery by means of the potentiometer.
[Footnote: The amplifier tube in this case is used as a generator of oscillations.]
[CHAPTER XIV]
HEADPHONES AND LOUD SPEAKERS
Wireless Headphones.--A telephone receiver for a wireless receiving set is made exactly on the same principle as an ordinary Bell telephone receiver. The only difference between them is that the former is made flat and compact so that a pair of them can be fastened together with a band and worn on the head (when it is called a headset), while the latter is long and cylindrical so that it can be held to the ear. A further difference between them is that the wireless headphone is made as sensitive as possible so that it will respond to very feeble currents, while the ordinary telephone receiver is far from being sensitive and will respond only to comparatively large currents.
How a Bell Telephone Receiver Is Made.--An ordinary telephone receiver consists of three chief parts and these are: (1) a hard-rubber, or composition, shell and cap, (2) a permanent steel bar magnet on one end of which is wound a coil of fine insulated copper wire, and (3) a soft iron disk, or diaphragm, all of which are shown in the cross-section in Fig. 62. The bar magnet is securely fixed inside of the handle so that the outside end comes to within about 1/32 of an inch of the diaphragm when this is laid on top of the shell and the cap is screwed on.