The dielectric between the plates of a variable condenser is air. There are no losses of energy due to hysteresis in a condenser having an air dielectric. Rotary condensers employing silk or some such material are not to be recommended.
LESSON TWENTY-SEVEN. TELEPHONE RECEIVERS.
Telephone receivers employed for wireless telegraphy are the same in principle as the ordinary telephone receiver but differ in construction and detail slightly.
They are always of the watch case type, this style being small and light, and consist of a ring or horseshoe shaped permanent magnet upon the poles of which are mounted two small bobbins containing many turns of fine insulated wire. Over the magnets, very close to but not quite touching, is placed a circular diaphram of thin sheet iron. The lines of force created by the permanent magnet pass through the cores of the little bobbin and exert a constant pull on the diaphram.
The little bobbins of wire or electromagnets are connected in series. If a current of electricity is sent through them they will create a little field of force of their own which will strengthen or decrease that of the permanent magnets according in which direction the current flows. Each change in the pull exerted on the diaphragm causes it to move and send out little sound waves which may be heard when the receiver is held close.
We have already learned that the strength of a magnet depends upon the ampere turns. Suppose that a current of one ampere passed through a coil containing 100 turns x 1 amp. = 100 ampere turns. If only one-tenth of an ampere was available and we wished to retain the same magnetic strength in the coil, the number of turns would have to be increased to one thousand in order for the ampere turns to remain equal; 1/10 amp. x 1.000 turns = 100 ampere turns.
FIG. 57. Types of Telephone Head Sets.
The currents passing through the receiver from the detector are exceedingly weak, and so in order to produce the maximum effect on the diaphragm, the electromagnets must be wound with a large number of turns of very fine wire. The resistance of fine wire is very great and for this reason wireless telephone receivers are usually termed high resistance receivers.
Winding a receiver with many turns of fine wire does not make it more sensitive in the true sense of the word or from the standpoint of efficiency, but makes it better suited to the minute fluctuations of a weak current.