Radio Broadcast (May-Oct 1925)

456 Radio Broadcast coupling in the primary and secondary circuits except between two coils, one of which was necessarily in the field of the other. Then it occurred to me that I could minimize this coupling by partially shielding one of these coils through an auxiliary coil wound over it. I realized that this coil would pick up some current, and I quickly saw that this current might be employed to neutralize whatever capacity coupling remained. This was the first thought of capacity neutralization that I had, and I did not realize at the time that it was destined to be what one might call the keystone of the neutrodyne. The neutralization was actually incorporated in the Navy receiver although it was of the nature of a refinement rather than of a necessity. : "Later I attempted the design of an audiofrequency amplifier which would give a particularly high amplification, but after a time I came to the conclusion that such an amplifier would oscillate persistently on account of the capacity coupling between the plate and the grid of the vacuum tube, in its circuit, for the plate and the grid circuits would be connected to similar transformers and would therefore be in resonance — a condition particularly conducive to oscillation. Almost at once I saw the solution — the deleterious capacity coupling. I suppose that my experience with the Navy receiver helped me to reason out the method which I thus evolved. My experience seems to me to be an illustration of the adage that the realization of a problem is frequently more important and more difficult than its solution. This neutralization of capacity coupling in vacuum tubes was the basis of the neutrodyne circuit, the practical development of which came several years later. THE WAY TO SOLVE PROBLEMS — SOLVE THEM IN 1 9 1 9 I started to devote my time io a study of the application of three-electrode vacuum tubes to the various problems of power conversion, with efficiency the primary object. This work was to some extent a continuation of my earlier work on oscillating circuits, for the form of conversion which I first investigated was from direct current to high-frequency current as used [for radio transmission. The work was carried on much further, however, in order that it might include conversion of alternating current power into direct current power, of one frequency into another frequency, of direct current into alternating current of controllable frequency, and so forth. This new subject was a valuable background for my other work and made me realize that although two problems might not be closely related, they might, nevertheless, have a common ground in their respective solutions. " In my college work I constantly see many boys who seem quicker than I in absorbing mathematical theories, but they have not the fondness for work that leads to original investigations. I have long believed that the prime requisite for success along mathematical lines— and this applies to all scientific progress—is not so much a natural ability, as it is a certain fondness for the subject. The only way to learn to solve problems is to solve them. " I was engaged in the development of radio receivers during the fall of 1922 when my attention was directed to the immense possibilities of a receiver employing tuned radio frequency amplification. I knew that the great limitation of this type of receiver, which had thus far prevented its successful introduction, was in its strong tendency to oscillate because of the feed-back of the capacity coupling of the vacuum tube. This feed-back was accentuated by the tuned input and output circuits. I realized that my earlier work on the neutralization of this capacity coupling was directly applicable. A model receiver was constructed to incorporate these ideas and it was christened the neutrodyne. " During this period, several manufacturers were eager to obtain a receiver of this sort, and Mr. I. P. Rodman, an officer of the present Garod Corporation who had become convinced of the great value of a tuned radiofrequency amplifier, had much to do with its development. The neutrodyne was first brought before the public at a meeting of the Radio Club of America in March, 1923. HOW INVENTIONS ARE MADE. THERE is much curiosity as to how inventions are made. In the earlier development of an art, most inventions are the results of experimental discoveries, and this is often the case even in their subsequent growth. For example, Armstrong's inventions of regeneration and super-regeneration come under this category. My inventions, on the other hand, have all been the result of theoretical studies, verified and modified by later experimental work. Again some inventions are the result of mathematical analysis as, for example, Pupin's and Campbell's loading coils on electrical filters in telephone lines. Although I have used mathematical analysis