Radio Broadcast (Nov 1922-Apr 1923)

490 Radio Broadcast wiring is an appreciable percentage of the voltage drop across the filament. Now, assume that the telephone plug is inserted in the detector jack and all adjustments have been carefully made. On attempting to use the amplifier, by shifting the plug to the amplifier jacks, it may be found that the added voltage drop due to automatically cutting in the additional amplifier tubes may be sufficient to decrease the detector tube filament current below its most sensitive spot. It is well, therefore, to readjust all filament rheostats after changing the number of stages of amplification used in any filament control jack set. Various other uses of jacks will suggest themselves. In small tube transmitters, the favorite key or telephone microphone may be plugged into any one of more sets by suitable jack connections. Experimental sets can well afford to use jacks and plugs to facilitate the trial of any connections that may be desired. Two-circuit jacks should seldom be used in radio-frequency circuits on account of the losses in the insulation at these high frequencies and also on account of the capacity added. When it is desired to use jacks in high-frequency circuits, a separate jack should be used for each wire. TYPES OF PLUGS AND JACKS ALTHOUGH both plugs and jacks are , comparatively simple devices, yet there are several points in their design that very largely determine their efficiency. One of the FIG. I i prime requisites of any electrical connection, of course, is that it shall give good electrical conductivity at all times. This requirement is particularly important in circuits employing telephone receivers because the slightest variation in contact resistance, even though the average conductivity of the circuit may be good, can be heard. And with vacuum-tube amplifiers, any slight variations in circuit resistance in the detector or first stages of amplification are heard in the head set as loud responses. It is thus important that the design of jacks for radio work be such as to remove the slightest possibility of poor contact. The points constituting good design of both plugs and jacks can perhaps be best illustrated by showing photographs of actual apparatus. Fig. 1 1 illustrates a jack of a type commonly used. The frame is made of sufficiently heavy metal to give rigidity and to permit the engagement of enough threads of the screws to prevent stripping of the threads. Micarta, bakelite, and hard rubber are satisfactory for insulation between springs. Fibre should never be used, as it absorbs moisture, which reduces the insulating qualities and furthermore its physical dimensions change with variations in atmospheric conditions, which may either strip the threads or lessen the springs. Springs should be either of German silver or of phosphor bronze, and should be heavy enough to exert from one to two pounds pressure against the plug. The springs should in all cases be long enough so that when the plug is inserted they will not be bent in any one position sufficiently to exceed the elastic limit of the material from which they are made. Otherwise, the spring pressure will gradually diminish. All auxiliary contacts should be designed so that in addition to moving in a direction perpendicular to their surfaces, they will have a wiping effect. This is necessary to keep the surfaces clean of dust and to insure good contact at all times. As all plugs are standard and as the dimensions of the tip must be small, the tolerances in the distance from the surface A to the centre of the tip spring B in Fig. 1 1 must be small. In this particular design, washers are provided