The potentiometer is an instrument for carefully regulating the voltage of the battery supplying a detector of the electrolytic or carborundum types with current.
It is necessary to bring the potential of the battery to a certain critical point where it is just insufficient to "break down" the detector, that is, overcome the resistance which it offers to the oscillatory currents. In construction, the potentiometer usually consists of a small rod wound with German silver wire and provided with an adjustable contact. Graphite resistance rods are merely a cheap method of making a potentiometer and are to be avoided as entirely unsatisfactory for the purpose.
FIG. 59. Potentiometer.
LESSON THIRTY. DEAD END LOSSES AND "NO DEAD END" SWITCHES.
Practically every radio circuit includes an adjustable inductance of some sort, usually consisting of a layer of wire wound over a tube and arranged so that the amount of wire in the circuit can be varied by means of a switch, a plug or a slider. These methods of variation are familiar in the ordinary tuning coil, loose coupler, loading coil, etc.
FIG. 60. Diagram representing the effect of Distributed Capacity.
If the plug, switch or sliding contact, depending upon the method of variation employed, is at E as in Fig. 60 so that only the portion of the coil A E B is in the circuit, then the portion E F together with A E may form an oscillator which, in order for the reader to obtain a better conception, may be likened to a sort of secondary winding, with A E considered as the primary. The oscillations of this part of the system may produce some very undesirable disturbances, especially so when the frequency of the currents in the circuit A E bear a certain relation to the natural frequency of the oscillator or E F. The losses due to the disturbance of these undesirable oscillations and also those resulting from eddy currents induced in the free portion or oscillator E F by the magnetic flux of A E are known as "dead-end effects."
These losses are very much more noticeable in receiving circuits than in transmitters on account of the very weak currents in the former and the importance of preserving all the energy when it is already very small.