Radio Broadcast (May 1928-Apr 1929)

296 RADIO BROADCAST ADVERTISER New Tubes the perfect tonic for your radio IS your set "run down" after long, constant use? The finest radio tonic known is to install a correct new tube in every socket of your set. One inferior or old tube may be crippling your set and causing poor reception. Have your dealer test your tubes and recommend new, correct, wide-awake Cunningham Radio Tubes to replace the old ones. E. T. CUNNINGHAM Inc. New York Chicago San Francisco Manufactured and sold under rights, patents and inventions owned and/or controlled by Radio Corporation of America. No. 222 Radio Broadcast Laboratory Information Sheet September, 1928 Measuring Instruments THE AMMETER T ABORATORY Sheet No. 214 in the August ^ issue explained the operation of a simple measuring instrument. An instrument of the type illustrated in that sheet can only be constructed to handle small currents, for to handle large currents the moving coil, L, and the leads to it would have to be made of ve.y heavy wire. Since the coil, L, is part of the moving element, it must be kept light in weight ; it is possible, therefore, to use only a fine wire on the coil. For larger currents the arrangement indicated in the sketch on this sheet is used. R is a resistor called a "shunt" consisting of one or more strips of a special alloy. The[current in the circuit divides, most of it going through the shunt because its resistance is small in comparison with that of the moving coil of the meter. The current through the coil, however, is a certain definite fraction Case of Meter Moving Coil I Shunt R of the total current and therefore if we know the current flowing through the coil we can readily determine what the total current in the circuit is. As an example, if the resistance of the shunt, R, is 0.01 ohms and that of the moving coil of the meter 0.99 ohms then the current divides in the same ratio. Out of every unit of current flowing through the circuit into which the meter is connected 99 parts flow through the shunt and one part flows through the meter. The current in the meter is therefore an accurate measure of the I total current in the circuit and therefore for any one shunt the scale on the meter is calibrated to read directly the total current. Meters with current ranges up to 50 or 75 amperes may be obtained I with the shunt built inside of the case. For higher ratings the shunt forms an extra piece of apparatus and the meter is connected across it by means of a pair of wires. I No. 223 Radio Broadcast Laboratory Information Sheet September, 1928 Radio Transmission HOW DISTANCE AFFECTS THE SIGNAL ■pROBLEM: A receiver is tuned to a broadcast •*• station located a certain distance away and signals from it produce sufficient power in the loud speaker circuit to make reception satisfactory. By what percentage will the power in the loud speaker be reduced if the receiver is removed to a point twice as far away from the transmitter, assuming, of course, that the sensitivity of the receiver remains unchanged. Solution: To solve this problem we must know how the output power of a receiver varies with the r.f. input at the antenna and we must know how the received energy varies with the distance between the receiver and the transmitter. (a) The power in the plate circuit of the power tube (and therefore the power supplied to the loudspeaker) varies as the square of the signal voltage on the grid of the power tube. (b) The voltage output of a detector tube varies as the square of the voltage on its grid. (c) Therefore, the power into the loudspeaker varies as the fourth power of the voltage impressed on the grid of the detector tube. (d) The voltage impressed on the detector tube is proportional to the voltage at the antenna. Therefore the power into the loud speaker varies as the fourth power of the voltage impressed in the antenna. (e) The voltage at the antenna varies as the field strength. (f) Therefore, the power into the loud speaker varies as the fourth power of the field strength. Statement (f) tells us how the power into the loud speaker varies with field strength. But the field strength surrounding an antenna varies inversely as the square of the distance between the transmitter and the receiver. Therefore, the power into the loud speaker varies inversely as the eighth power of the distance between the transmitter and the receiver. The problem states that the distance between the receiver and the transmitter has been doubled, i.e. the distance has been multiplied by 2. The eighth power of 2 is 256; therefore by doubling the distance between the receiver and the transmitter we have cut down the power in the loud speaker to 1 256 of what it had been. No. 224 Radio Broadcast Laboratory Information Sheet September, 1928 Text Books on Radio THERE are certain books and radio magazines that the serious radio experimenter should not be without and in this sheet we give a list of some of what we consider the more important of the publications. The short descriptive sentence following each title will help to classify the book in our readers' mind. Radio Instruments and Measurements. A 345-page book, presenting information regarding the more important instruments and measurements actually used in radio work. The contents is of interest to all radio engineers. The book is published by the Department of Commerce and is known as Circular No. 74. Obtainable from the Superintendent of Documents, Government Printing Office, Washington, D. C, for sixty cents. Principles Underlying Radio Communication. Another government publication to be recommended. This book is quite an excellent elementary text book of radio and general electricity and may be easily understood by anyone with a fair knowledge of algebra. Everyone should have it. It is known as Radio Communication Pamphlet No. 40, and the Superintendent of Documents, Government Printing Office, sells it for $1.00. Principles of Radio Communication, by J. H. Morecroft. This is probably the most complete book on radio engineering. The text deals with all phases of the art of radio communication and the treatment is very complete, the book containing about 1000 pages. Published by John Wiley and Sons, Inc., New York City. Price: $7.50. Thermionic Vacuum Tube, by H. F. Van Der Bijl. An excellent book setting forth the principles of operation of vacuum tubes. It is a very useful book for any radio engineer. Published by the McGraw-Hill Book Co., Inc., New York City. Price: S3.00. Radio Engineering Principles, by Lauer and Brown. A book less extensive than Morecroft's but excellent for those whose requirements are satisfied with a shorter and less expensive text. It is a very scholarly presentation. Published by McGraw-Hill Book Co., Inc., New York City. Price: $3.50. Radio Frequency Measurements, by E. B. Moullin. A book dealing with the theory and practice of radio measurements. A handbook for the laboratory and a text book for advanced students. Many of the measurements are made with the aid of the vacuum tube voltmeter. Published in England but it can be obtained from the J. B. Lippincott Co., in Philadelphia. Practical Radio Construction and Repairing, by Moyer and Wostrel. This book aims to be of service to the amateur constructor and radio service man. It is essentially practical in its treatment. Published by McGraw-Hill Book Co., Inc., New York City. Price: $1.75.