Radio broadcast .. (1922-30)

534 Radio Broadcast mary, changes in the variometer cause changes in the wavelength of the entire grid circuit of which the secondary of the variocoupler forms a part and the inductive relation between the primary and secondary is therefore altered. The major adjustment of the "coupling," as it is called, is generally accomplished by changing the position of the secondary with relation to the primary. This may be done by rotating the secondary, which changes the plane of its winding with relation to that of the primary, or by moving the secondary away from the primary, which has the same effect, that is, reducing the coupling or the influence of one winding on the other. When the secondary winding lies in the same plane as the primary winding, the transfer of energy from one to the other is most pronounced and the coupling is "tight," and the same holds true when the secondary is brought close to the primary, provided the planes of the two windings coincide. The relation of the primary to secondary — in this case, the grid circuit — changes with the number of active turns in each, as well as with the placement of their windings with relation to each other. Most circuits of the character you refer to are provided with variocouplers having a secondary winding which is "fixed," that is, the number of turns can not.be changed, and the wavelength of the grid circuit is regulated by adjusting the variometer. However, the primary of the vario-coupler is generally provided with taps to permit the use of any number of turns, and the turns actually in use are termed the "active turns." Although the relation of the two windings may not be altered, there is a change in coupling with every change in the number of active turns in the primary winding. Having disposed of this relationship, we come to the explanation of the regeneration or feed-back, as it is commonly called here, or reaction, as it is termed in England. Here the relationship between the circuits is of a different nature and one is affected by the other through the vacuum tube and there is no direct inductive relationship between the variometer in the plate circuit and the elements which comprise the grid or secondary circuit. The term, reaction, used by the English, by the way, is much less a misnomer than either of our names and smacks less of mystery. It has been found — and there are many, many claimants to the finding — that a receiver employing a vacuum-tube detector will produce a greatly augmented signal when the electrical period of the wiring in the plate circuit is identical to that in the grid circuit, and a correct voltage is supplied the plate. The variometer in the plate circuit is merely a simple means for altering the electrical length of the circuit at will. to compensate for wavelength adjustments in the primary and secondary, for all three must be identical if the maximum signal is to be had. When the current passing through the turns of the roiarv element of the variometer opposes the current in the stationary winding, the wavelength of this unit is shortest, but when the current in each half of the unit is flowing in the same directions, the wavelength is longest. The range of a given variometer depends upon the number of turns in the elements as well as the distance between the rotary and stationary elements and the distance between the turns in the windings of each, as well as the particular kind of wire employed and the insulating material or varnish used. Some forms of regenerative circuits are made with a fixed inductance — the number of turns can not be altered — mounted so that its position with relation to the secondary circuit may be altered. In this instance the coil takes the place of the variometer and the reaction effect is obtained by altering the position of the feed-back or "tickler" coil, as it is commonly called. In this case there is, of course. and inductive relation between the secondary and plate circuits, whereas in the variometer-tuned arrangement this indicative relation does not exist and the reaction occurs directly through the vacuum tube when the current in the plate and grid circuits is in phase, that is, when it is passing through the same electrical length in each circuit and when the rising and falling of current in each circuit occurs simultaneously. Perhaps the case may be better understood from consideration of the action of the vacuum tube functions for this particular purpose. We know that the greatest energytransfer, from the antenna to ground circuit, which includes the primary of the variocoupler, occurs when the secondary or grid circuit is of the same wavelength and the coupling between the two circuits is properly adjusted. We also know, from a study of what happens when the vacuum tube is used as a detector, that the current imposed on the grid controls the current flowing in the plate circuit, but 'the current in the latter is greatly increased because it is drawn from a local reservoir, the "B" battery. By adjusting the wavelength of the plate circuit, it is possible to have the current flowing in the grid circuit and that in the plate circuit in step with each other. By doing this, we find thai a greater current finds its way to the grid, and as the current in the plate circuit depends upon the variations in the grid current, a greatly augmented signal results.