Exhibitors Herald World (Oct-Dec 1929)

32 EXHIBITORS HERALD-WORLD December 14, 1929 SOUND PICTURES Blue Book School Impedance By F. H. RICHARDSON BLUEBOOK SCHOOL QUESTION NO. 34.— What two properties are found in all truly elastic bodies? Describe a simple experiment which will serve to illustrate action in mechanics very similar to electrical action in a condenser. Describe a simple form of electric condenser, with drawing if preferred. Under what state is a condenser said to be "charged?" ANSWER TO BLUE BOOK SCHOOL QUESTION No. 30 ANSWER TO QUESTION NO. 30.— The question was: Which wire, positive or negative, connects with the silver lining of the photo-electric cell bulb? What does the other wire connect to? What is deposited upon the inner surface of the silver lining of the photo-electric cell bulb? 'X'RUE some of these questions were anJ swered by those replying to question No. 29, but it is no real waste of time to go over it again, for any one of several reasons. Please print your signature. Written signatures often are entirely illegible. The following made correct answers, with the notation that almost always some right answers come in too late to be included: W. C. Budge, I. T. (or maybe it is J. T.) Sanders, Bernard A. Omlor, Harlan B. Fox, Gayle Grubaugh, Thomas L. Sinclare, G. B. Lavenburg, William G. True, "Bill" Doe, T. R. Bankerton, George Thompson, A. D. Henley, G. L. Doe, B. T. Lounsbury, P. Schmidt, T. H. Dringe, T. L. Allred, Bryan T. Higgins, Frank Cary, S. R. Pearlmann, G. T. Baldy, Edward Hodson, James T. Mitchell, Joe R. Sidney, H. Brandt and G. W. Wooten. I have, after some hesitation, selected the answer of George Thompson as best suited to the needs of publication, though there were many most excellent replies. It reads: "The negative wire of the photo-electric cell connects to the silver lining of the photo-electric cell, while the positive of the photo-electric cell circuit (90-volt storage battery) connects to the round anode suitably positioned in the center of the cell or globe. The positive anode connects to the grid of the first amplifying tube. The negative connects to the negative side of the battery, and through it to the battery to the filament of the amplifying tube, in order to complete the circuit. The material deposited on the inner surface of the silver lining of the cell is a special form of metallic potassium, which is very, very sensitive to the action of light." Correct! Comb you hair and go up to the head of the class, Friend Thompson ! I have a letter from a St. Louis projectionist who says, "I am clipping out all the questions, pasting them at the head of a blank sheet in a suitable acount book. Under this I write my idea of the correct answer, and then under that I paste the right answer when published. Believe you me, as you say, already this book is very valuable, though I'd not show it to many folks because of the answers I thought were right, but !' A corking good idea, if you ask me? I advise all of you to do the same thing. ERRATA — Several have directed my attention to an error in the answer of Brother G. L. Doe, Chicago, in his published answer to question No. 28, November 30 issue, in which he says, "Permits the current to flow from the round, central anode to the silver cathode." I cannot imagine how I managed to overlook that slip. The action is the other way, so say the savants — from silver to ring. Please make pencil note of that fact, you who are preserving the Bluebook School answers. THE IMPEDANCE PUZZLE FROM the number of inquiries received, I am led to believe that what is termed "Impedance'' by the engineers, still has a great many projectionists considerably puzzled. They do not seem quite to grasp as yet the full meaning of the term. Also, they do not, in many instances, appear to understand just in what way it applies in practice. I am in receipt of the following questions from Crawford Johnson, projectionist at the Capitol theatre, Aylmer, Ont. This series of questions seemed to cover the ground pretty thoroughly, so I referred it to the ERPI engineering staff for a complete, official reply, which was supplied by that very capable engineer, H. B. Santee, director of theatre engineeringfor that organization. I shall place the question before each answer, thus avoiding any possibility of confusion or misunderstanding. (1) Does the amplifier have to have a certain input impedance to work right? Answer. — It is not necessary that every amplifier have the same input impedance in order to function properly, but it is necessary that the_ input impedance of the amplifier have a certain value relative to that of the apparatus from which it derives its input, whether this be a microphone, an electrical pick-up, or some preceding amplifier or other piece of equipment. Just what these relative values should be, will depend upon the electrical characteristics of the apparatus and is therefore up to the designer to determine. [Editorial Note. — From this it is apparent that adjusting or altering impedance is a matter with which the projectionist has no concern. It is, in fact, a matter with which it would seem impossible that he can interfere with benefit to the apparatus, and while it is perfectly proper and right that you seek to understand what it is all about, still you must NOT adjust or alter amplifier impedance. (2) How may I tell what the impedance of an electrical pickup is if it does not happen to be marked? Answer. — The term "Impedance" means the combined effect of the resistance, inductance and capacity present in any given circuit. The impedance of electrical apparatus, other than straight resistance [Santee said, "pure resistance," but I believe "straight resistance" is the better term] usually varies with frequency. In most cases it is not easy to measure with any degree of accuracy In fact, in the case of an electrical pickup, amplifier, loud speaker or other comparable piece of apparatus, the problem of impedance measurement is much too complicated and involved to be here discussed, because of the fact that it requires the use of a considerable amount of laboratory equipment and a knowledge of advanced electrical theory. (3) Do the pickup amplifier and speaker have to be "matched" in order to work right? How may I tell whether or not they are matched? What does the term "matched" mean in such a case? Answer. — The pickup, amplifier and loud speaker need not all have the same impedance in order to work properly, but as already indicated, the impedance of the pickup should be related to that of the amplifier input circuit, and the impedance of the loud speaker should be related to that of the amplifier output circuit. Usually the two latter impedances are made equal. The only way in which to determine whether these units of the equipment are properly matched is to measure their individual impedances and other electrical characteristics. Inasmuch as IT IS QUITE IMPRACTICABLE FOR THE PROJECTIONIST TO ACCOMPLISH THIS, and as the matter has already been considered and taken care of by the manufacturer of the equipment, we believe the projectionist had best not concern himself with it at all. In cases where an individual has purchased units of equipment from different manufacturers, and is attempting to combine them into a system, data on impedance and advice regarding the proper way of combining the units, should be sought from proper authorities, who may be one of the various manufacturers of the equipment in hand. The term "matching" merely refers to the proper balancing of impedance, as described. (4) Is there any way of ascertaining the impedance of various items of equipment? Answer. — This question has been dealt with in the answers to questions considered. There, gentlemen, not only does that seem to me to set forth the whole matter of impedance, in so far as it concerns the projectionist.