Radio Digest (Oct 1923-July 1924)

October 27, 1923 RADIO DIGEST — Illustrated Simple Explanation of Radio for Everybody Chapter IV, Part I— Inductances— Fixed and Variable THE following article is the fifth of a series for Radio beginners, written by Marvin W. Thompson, well known in airphone circles for his understandable style of approaching his subject, and his ability as a Radio engineer. Mr. Thompson is now an associate editor on the staff of Radio Digest. A preliminary outline of the Chapters to follow is: Chapter V — Condensers — Fixed and Variable. Chapter VI — Vacuum Tubes. Chapter VII — Antennas and Grounds. Chapter VIII— Regeneration. Chapter IX — Audio Frequency Amplification. Chapter X — Loud Speakers. WHEN considering inductances we must, first of all, divide them into two groups — those used in circuits in which the current alternates or oscil Figures 15 and 16 — Bank-wound inductance, showing how turns overlap, and single-layer inductance tapped and wound on solid tube lates at Radio frequencies, and those in which the current varies at audible frequencies. Taking up the coils used to handle Radio frequencies, we must again subdivide and consider these inductances as those used to provide inductance in a circuit and those used to transfer energy from one circuit to another. The end toward which all manufacturers of coils are striving is to produce a unit that will provide the greatest inductance in the smallest possible space. At first thought this sounds very simple — merely use fine wire, a small core and wind the wire compactly — but there are several factors which make this method impossible. Opposing the passage of Radio frequency currents through a coil of wire is the element known as "effective Radio frequency resistance." Skin Effect This element is made up of the ohmic resistance of the wire and the "distributed capacity" of the coil. Every piece of wire offers ohmic resistance to the passage of a simple direct current; if alternating current passes through it, the resistance increases as the frequency is raised due to "skin effect," a term used to indicate Figure 17 — Correct or bank-wound method of making inductances the fact that the current travels on or near the outside surface of the wire. Since at very high Radio frequencies the current utilizes only a small fraction of the total cross-sectional area of the wire and disregards the center, it is equivalent to having a wire of small diameter. Because Radio Bargains for THIS WEEK Each BAKEUTTE DIALS, 3", V*" Shaft.. •$ .29 23 Plate Condenser .0005 1.43 Special Rheostat, 6, 25, 40 ohms 69 "B" Battery Clips 05 199 Tube Adapter 49 Bead-Em Marked Binding Posts, Set of 8 59 Gold Grain Crystals 79 Erla Grid Leak— Pencil Mark Type 18 Mercury Loud Speaker with Baldwin Unit 9.95 Special 180 degree Varlocoupler or Variometer $1.28 Hegehog Transformers — Any Katie 3.15 Ear Joy Phone Cushions 89 Indoor Spring Aerial, Solid Brass, 100' 98 lack Switch for "A" Batten''. . . .69 Clip this List Check Items You Want Enclose With Monpy Order Your Order Will 8e Filled By Return Mail Postage FreeSatisfaction or Moneq Back KADIOBARGAIMSi 17 N.LA SALLE ST-CiilCAoS By M. W. Thompson the Radio frequency current, will travel only near the circumference, as great a circumference as possible should lie provided to keep down resistance, To provide more surface or "skin," braided or stranded wire is frequently used and in labora Figure 18— Incorrect method of winding inductances of more than one layer tory apparatus a type of wire called "litzendraht" is employed. Ljtzendraht is made up of many fine individually insulated wires twisted into a cable. At wave lengths between 150 and 600 meters, however, the difference in efficiency between a coil wound with solid wire and one wound with "litz" is so small that the average experimenter may disregard it and use solid wire on his inductances. Distributed Capacity We come now to "distributed capacity," which is the second detrimental factor to be reduced to a minimum. When winding a coil it is intended that the currents travel in at one end, follow the wire around and around the coil, and out at the opposite end. Could a coil be made in which the high frequency current would do this, many problems would be solved. As stated in a previous chapter, any two Figure 19 — Bank-wound coil divided into sections conductive surfaces placed close together but not touching, form a condenser. Any two turns of a coil which are close to gether constitute a condenser and, because there is a difference in voltage or • trical pressure between turns, we actually have current (lowing across from turn to turn all along a coil. Thus, much of the ncy Is lost, as our coil provides Inductance only because of the current traveling around and around. That current which jumps across does no work and is lost. Since every inductance and capacity form a circuit which has a "natural period'' or frequency at which it oscillates easily, our distributed capacity, in conjunction with the inductance of the coil forms such a circuit. This unintended circuit absorbs energy from the incoming signals, wastes it and adds to the "effective frequency resistance." Miniature condensers are formed not only by the Figure 20 — Honeycomb duo-lateral winding, in which A is bottom, B middle and C top layer insulation between turns but by the material on which the coil is wound; so, to reduce resistance as much as possible, three things should be done: First, the wire used should be as large in diameter as possible, compatible with compactness; second, turns should be spaced, and turns with a large difference of pressure (potential) should not lie close to one another; third, as little solid material such as wood, bakelite, formica or other compound, as possible should be used to support the wire. Single-Layer Coils The simplest form of inductance is the short single-layer coil. Such a coil is shown in Figure 15. In this coil all turns are parallel and close together and the distributed capacity between turns is high, but as there are few turns, the total ca 10,000 Western Electric 5 Watt "E" Tubes, Type CW-931 or VT-2 Tubes! WITH A HIGHER AMPLIFYING FACTOR THAN ANY OTHER TUBE MADE! Almost a third of these 10,000 Western Electric VT-2 Tubes have been sold! A brand new genuine Western Electric VT-2 Tube for 27.45 isn't a purchase that can be made every day — that's evident from the way orders are piling in. Remember there only was 10,000 of them — so you'd better hurry if you want your tubes. These genuine Western Electric S-watt "E" tubes, type CW-931, or commonly known as VT-2 tubes were designed and built specially to conform to the specifications of the U. S. Signal Corps. They are built to be used both for Radio and Audio Frequency Amplification and for CW and phone transmitting. The characteristics of the tube are as follows: Filament: 4.8 to 7 volts Power Output: 5 watt* Plate Voltage: 45 to 350 volts Filament is of pure platinum Equipped with standard base to fit standard socket All merchandise GUARANTEED by us to be of BEST QUALITY All mail orders shipped to you PROMPTLY L "The largest Radio Store in America" 509 S. State St. Dept. RD-1127 Chicago, 111. not very great and such a coil only ny turns Figure 21— The Ciblin-Remler winding in which A is bottom, B middle and C top layer are necessary for the reception of signals on wave lengths over 500 meters. It will be noticed on this coil that taps are taken off every few turns. Taps are bad pracIf we are using only that amount of turns between one end and the fourth tap, there will be absorption of energy by that part of the coil between the fourth tap and the other end of the coil, due to the oscillating circuit formed by distribuapacity and Inductance. Whei • ble, only those turns necessary to reception should be wound on a coil. In everyday practice, this loss of energv in unused turns is accepted as a necessary evil. Distributed capacity may be kept down and inductance kept high by winding a coil so that the diameter is greater than the length of the winding. The best ratio is 2.5 to 1; that is the diameter should be two and one-half times the length of the coil for maximum efficiency, but there (Continued on page 18) Z&&fr Your Choice of one bound volume with each annual subscription MniM, Greatest Collection of Radio Information Ever Published Reinartz Long Distance Circuit Ultra-Reinart2 Circuit Flewelling "Flivver" Circuit Grimes "Inverse Duplex" Circuit Single Tube Reflex Circuit Armstrong "SuperHeterodyne" Circuit Two, Three and Four Tube Reflex Circuits A. B. C. 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