Radio Broadcast (Nov 1926-Apr 1927)

Record Details:

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DECEMBER, 1926 THE CONDENSER TRANSMITTER 183 he would make you clarify your thought, by a method of his own. He never told a student anything, simply in order to get through with the job and on to something else, but, by asking questions in a strictly logical sequence, he would lead him to the correct conclusion. This took time, and when the subject of such a Socratic inquisition happened to be stupid, Parmly would become impatient, but this never impelled him to abandon his method. The only effect was to make him shout his questions in a voice that reverberated through the College halls, while the cowering student stammered his replies in barely audible tones. Finally the correct answer would be drawn out, and then the professor's stern face would break into a smile of startling sweetness, under the influence of which the incipient inferiority complex of the sophomore or junior was transformed into a determination to do better next time. When Parmly spoke, he fairly etched his words. His clarity of diction and articulation expressed his nature. So did his handwriting, the accounts he kept, and his conduct in all the tasks and responsibilities he had at the College. He had, I believe, a feeling like that of Spinoza for the eternal laws of nature, and the exact quantitative relations of electrical phenomena gave joy to his heart. My radio friends who read this, and who never knew him, may form a picture of a severe and rectilinear character unlike the actual man. He was inflexible only insofar as he knew he had to be to turn out good engineers. He impressed us, by his speech, his precepts, and his methods, with that apprehension of undeviating principles in the behavior of physical things which distinguishes the engineer from the mere rule of thumb, hit-or-miss worker. At heart he was kind and gentle, but he did not shirk the task of the surgeon. He could have sent us on with much less than the pains he took. A man of means and ability, he could have enjoyed an easier and perhaps longer life than the one he chose. After his death, when I was assistant to Professor Goldsmith, 1 happened to get the task of disposing of some of Parmly's old records. Among other material, there were thousands of inventory cards in his handwriting, in several colors of ink, detailing the apparatus in his charge down to the last screw, with his customary lucidity and order. He did not have to do this. It was simply his way of going about his affairs, without haste, thoroughly, calmly, and efficiently. He made me detest a loose bolt, a poor connection, a sloppy diagram, as an offense, not only against safety and efficiency, but against the esthetic principles of the profession for which he prepared me. Often, in late years, contemplating a dirty shop, or some wretched manufacturing layout, or a test room converted into a shambles, I thought, "Oh for a Parmly, with his determination and his scorn, to tear into this and clean it out, like a wind from the sea!" If you have never seen such a situation in radio, perhaps you have not been in radio long. We have our Augean stables here and there. The electrical engineering laboratory at C. C. N. Y. in 1916 was not as well-equipped as it is now, but it served the purpose, and a man who could not learn anything from its motors and generators would not be better off at M.I.T. The machines were small, up to perhaps 15 kilowatts, controlled from two main switchboards, with arrangements for stopping and starting individual units by means of clutches in connection with chain drives. We made heat runs on transformers, determined load characteristics of generators, ran Prony brake tests on motors, and went through the whole Stage FIG. 2 standard list of experiments designed to prepare the student for the world of industry. At times we mixed up the connections and the breakers would go out with a bang which frightened the men as if they had been so many schoolgirls; it takes years to achieve the nonchalance of the fire-eating engineer who thumbs his nose at an arc and scorns to jump at the crack of the unchained lightnings. Then Parmly would come over and begin catechising us in a gentle and pleasantly sarcastic voice, which gradually rose until it dominated the noise of all the machinery in the building, while we sweated and fumbled for the right replies. When these were at length phrased to his satisfaction, he would look at us for an instant with the bright smile which was like the sun bursting suddenly through clouds, and pass on to the next squad. Not long after, in September, 191 7, he died suddenly. If he teaches now in some other sphere, I hope he has better students, but they will not remember him with more deeply felt gratitude than the men he taught last on earth. All this is not radio, but to those who think of our work as nothing but frequency characteristics, cost data, and the life of power tubes, I should like to remark that such things are based on much else that is outside of radio — on men, in the last analysis. Technical Operation of Broadcasting Stations 13. The Condenser Transmitter t; GMD, THE CARDWELL TRANSMITTER AT THE BASE STATION OF THE DYOTT BRAZIL EXPEDITION HIS article is not intended in the strictly practical sense of previous numbers in the series, for very few stations in the United States employ condenser microphones, and the men at those stations have nothing to learn from my description. Nor will any ambitious broadcaster find it possible to build a usable condenser microphone in his home workshop, unless he is a combination of jeweller, toolmaker, and expert electrician; and if he is all those things, his reasons for remaining in broadcasting are quite beyond me. The object of the present discussion is merely to acquaint technical broadcasters with the general theory and characteristics of a form of telephone transmitter in actual use for broadcast purposes, and second only to the common carbon microphone, in practical importance, not only in broadcasting, but in the allied field of phonograph recording. A valuable description of the condenser transmitter is found in a paper by E. C. Wente: "The Sensitivity and Precision of the Electrostatic Transmitter for Measuring Sound Intensities," in the Physical Review, Second Series, Vol. XIX, No. 5, May, 1922. Wente wrote about the condenser transmitter in the same journal for July, 1917; but the later paper gives more up-to-date data. The condenser transmitter, according to Wente, consists essentially of a thin metal diaphragm under tension, separated by a small distance from a plane metal plate, the plate and the diaphragm forming the two electrodes of an air condenser. The arrangement of these parts is shown schematically in Fig 2, together with the instrument to the associated amplifier. The thickness of the diaphragm is of the order of 0.001 inch (1 mil), and the spacing between the diaphragm and the back-plate is about the same distance. Obviously this construction requires the finest sort of machine work in the first place, and perfect freedom from foreign particles in the space between diaphragm and plate after assembling. A tiny thread of lint or metal cutting will put the transmitter out of commission. In the design of condenser transmitters, the natural frequency and damping are controlled, among other factors, by annular grooves cut in the back-plate, facing the diaphragm. The sensitivity to ordinary sounds increases inversely as the natural frequency. For broadcast purposes, the diaphragm is generally stretched to a natural frequency of about 8000 cycles per second, a figure sufficiently above