Radio Broadcast (May 1928-Apr 1929)

144 RADIO BROADCAST JULY, 1928 crease in spacing. Variations of line voltage up to 25 per cent, can be tolerated with this tube. Forty-eight months ago we watched Austin G. Cooley transmit pictures across the Radio Broadcast Laboratory and wondered when it would be possible to turn his work over to the thousands of experimenters in this country. We now have a circular from the Radiovision Corporation, 62 West 39th Street, New York City, which announces that the Cooley Rayfoto kit is ready for sale and that pictures are being regularly transmitted from wor. and was free not only from standing waves, but from harmonic distortion as well. Publications W orthy of ^Hpte These booklets have come to our attention and deserve the praise of a review. However, their titles indicate enough of their contents to be of interest to those working in any of the subjects covered : Generator for Audio Currents of Adjustable Frequency with Piezo-Electric Stabilization, by August Hund. Scientific Paper of the Bureau of Standards, No. 569, Price 10 cents. Hew to Operate Any Receiver from the House Current without Batteries. Arcturus Radio Company, 255 Sherman Ave., Newark, New Jeisey. A Radio-Frequency Oscillator for Receiver Investigations, by George Rodwin and Theodore A. Smith. Institute of Radio Engineers, (reprint!. February 1928 issue. Calculation of Capacities of Radio Antenna and their Resultant Wavelength, by Frederick W. Grover. Scientific Paper of the Bureau of Standards No. ;68, price 20 cents. National Electrical Safety Code. Fourth Edition 1926, Handbook Series of the Bureau of Standards, No. 3. Price Si. Data on the Voltage Amplification of Radio Frequency Transformers, by Burr K. Osborn. Bulletin No. 15, Michigan Engineering Experiment Station, Michigan State College, East Lansing, Michigan. II hat B FAimmalor Shall 1 Buy? Electrad, 175 Varick Street, New York City. "Skim Milk Masquerades As Cream" One of our service-dealer friends sends us what he considers a good joke on himself. Recently he gave a wholesale firm a trial order for some output transformers which would retail at $1.50. When the transformers— so called — arrived he attempted to measure the resistance, to get an idea of the inductance, and found that the coils would pass one ampere at 85 volts, and when the cover was torn off he discovered a solid pig-iron core and two coils connected as shown in Fig. 2. When the coils were so connected between tube and loud speaker that energy transfer was inductive rather than conductive, nothing got through because of the extremely low inductance. In other words instead of a transformer, our friend had bought two coils which were to be placed in series with the loud speaker. The iron was thrown in. How many of Radio Broadcast's readers have been taken in by such devices' W ill any technically mindTechnical Smoke ed reader interpret this quoScreen tation from the April, 1928, Popular Radio (page 303), During tests that were conducted in the Popular Radio laboratory it was found that this cone could easily take an output of two 210 amplifier valves arranged in a push-pull stage. The output current during the test ran as high as 49 milliamperes, with voltage on the lower end of the frequency range varying between 1000 and 1280. The speaker performed remarkably well with this great load Recent Interesting Technical Articles The following articles in technical radio publications are to be recommended: Measurement of Choke Coil Inductance, Proceedings I. R. £., March. Mutual Inductance in Radio Circuits, Experimental Wireless, April. Calculations for Resistance Amplifiers, Experimental Wireless, April. Low-power Crystal-Control Transmitters, QST, April. Design of Iron-Core Inductances, QST, April. Photoelectric Cell Applications, Popular Radio, April. Geophysical Prospecting, Scientific American, May. The Stabilized Oscilloscope, Radio Engineering, April. The Neutroheterodyne, Radio News, May. Regeneration, What it is, Radio News, May. Present Compression Type Resistors We are glad to quote the following from a letter of Mr. Austin C. Lescarboura, apropos of an article on page 421 of the April Radio Broadcast in the department "Our Readers Suggest." Because of my extensive experience with all types of resistors, I wish to point out that the "additional precaution" of Mr. Harries is somewhat misleading. There are variable resistors to-day quite as reliable and as silent in operation as the wire-wound type. While it is true that the carbon-pile type is often noisy and microphonic, due to loose contacts between the carbonized paper disks made necessary in attaining a wide resistance range, there is none of this feature in the compression type utilizing graphite and pulverized mica. In the latter type there is always sufficient compression of the resistive material to insure perfect contact free from microphonism, although the widest possible resistance range is obtained in several turns of the knob. While in the past there may have been limited use for the compression type variable resistance, to-day, with various sizes up to and including a power type capable of handling 40 watts of energy, there is no reason to turn to fixed wire-wound resistors even in the large power packs and A-B-C power units. The following letter from From a Lab A. S. Penoyer of Saginaw, Circuit Fan Michigan, pleased the Labor atory Staff a lot; fortunately it is but one of many in a similar vein: To you this letter will mean just another from a new R. B. Lab. circuit fan; but to me FIG. 2 the circuit is little less than a revelation. I am well repaid for having disassembled a mighty fine seven-tube super to acquire parts for the Lab. circuit. I have been an ardent super fan since the days when you published Haynes' super article four or five years ago, and during that time have built and rebuilt several. Your last article in the April issue convinced me that you have something good. Yet it was with misgivings that I removed the first wire from Old Faithful; and when, due to the omission of a wire to ground from by-pass condensers the Lab. circuit motor-boated mightily, I cussed my temerity in proportion. But now, well it's a "wow" and I wouldn't trade it for anything I have seen. The coils I am using are home-made, but judging from results must be pretty good. All of the other parts are of the best and the layout is approximately as suggested in your April article. Selectivity is excellent and I am listening to stations I haven't heard in some time. The volume equals the super and the tone is round, full and very real and I'm critical too. Just why more than four tubes are necessary, I can't understand. I am fully "sold" and wish to express my appreciation of your April article. The May issue has not as yet arrived so I don't know what your constructional article is to be, but am sure it can't improve my set. We often wonder who the W ho Our thousands of Radio Broad Readers Are cast's readers, from whom we never get letters, are; what their business is, how they are interested in radio; how it is possible to get better acquainted with them. The following tabulation of one thousand readers is one yearold but givesa good idea of who the magazine's audience was in February, 1927. What we should like is for one thousand present readers of these pages to write us a note, letting us know where on this list they are. Occupation Per cent, of 1 000 What was actually inside an "output transformer" purchased at a low price by a radio service man. Pig iron was used to give the unit weight! Radio dealers 26 7 Miscellaneous skilled workers . . . 23 .8 Engineers 11.0 Clerks 5.4 Executives 4.5 Salesmen 4.3 Electricians 4.1 Draftsmen 3.3 Accountants 2.5 Mechanics 2.3 Machinists 2.1 High school students 1.8 Technical institute students . . . . 1.3 Physicians 1.2 College students ....... .9 Lawyers .9 Bookkeepers .8 Tellers .7 Chemists .7 Dentists .4. Pharmacists . .3 Miscellaneous 1 . o Users of nichrome and A Flux for other resistance wire and who Nichrome Wire have had difficulty in soldering it will be relieved to know that John Firth of the Firth Radio Company, 25 Beaver Street, New York City, has developed a special flux with this difficulty in mind. It is known as Perfecto No. 2 and may be obtained in 25 and 50 cent bottles from the Firth Company. — Keith Henney