Radio mirror (Jan-Oct 1923)

KADI O DIGEST 11 First Steps for Beginners in Radio Chapter IV, Part II — About Condensers and Inductances AX INDUCTANCE is characterized by its ability to produce currents by induction within itself or self induction. All current carrying conductors ss self Induction due to the fact that all currents are surrounded by a magnetic field of force. Any variation in the strength of the field will induce a current in any conductor in that field. A straight wire "will be found to have self induction due to the fact that any variation in the field around that wire while carrying a current will induce current in the wire since it lays in the field of the magnetic lines. Inductive Effect When the wire is wound into a coil the inductive effect resulting from a change in the lines of force is increased many fold and we then have what is termed an ^ «—■ ■ Figure 19— Effect of Connecting Inductance Into a Circuit inductance. And to understand how an inductance tunes a Radio circuit we must have a knowledge of what takes place in the inductance. Let us then consider Figure 19 which shows an inductance I connected to an aerial A and ground G, the latter two forming a condenser as described previously. Let us assume that a Radio wave induces a positive charge on the aerial and current starts to flow. As the current starts through the coil it begins to build up a magnetic field. As this field takes form it cuts some of the turns of the coil. We have learned that when a magnetic field cuts or passes over a conductor it induces a current, hence, a current will be induced in the turns of the coil during the formation of the field around it. However, it will be found that the induced cur 5INCLE TURNS /)A EIGHT TURN TAPS // O Figure 20 — Two Methods of Varying Inductances with Taps rent opposes the current in the circuit and thus delays it reaching its full value immediately. Now let us assume that the charge induced on the aerial has finally all flowed through the inductance to ground. The magnetic field will then begin to collapse and the movement of the lines will cut the turns of the coil again and another current will be induced. However, the latter current will be in a direction similar to that from the aerial and will tend to prolong the current flow. Oscillating Circuit The effect of this last current surge By Thomas W. Benson, A. M. I. R. E. BEGINNERS will find the accompanying series by Mr. Benson very helpful in learning the rudiments of the popular science of Radiophony. The articles yet to appear are: Chapter V— Tuners and How to Tune Your Set. Chapter VI— About Crystal Detectors. Chapter VII— Tube Detector Theory and Operation. Chapter VIII— The tector. Chapter IX— Radio plifi cation. Regenerative DeFrequency Am Chapter X— Audio Frequency Amplification. Chapter XI— How Super Regeneration Is Accomplished. Chapter XII— Reflex Circuit Operation. Chapter XIII— About Headsets and Loud Speakers. Chapter XIV— Batteries Used in Radiophony. due to the self induction of the coil is to overdischarge the aerial-ground condenser and the current will start to flow in the reverse direction. The above action of first holding back the current and then helping it along will be repeated and an overdischarge results again but to not as great an extent. In this way the current will flow back and forth and the circuit is said to be oscillating. It is quite apparent that the time it takes for the current to reach its full value after starting to flow and the time the current flow is prolonged by the self induction will depend upon the amount of this self induction. This in turn will depend upon the number of turns in the coil cut by the rising and falling of the magnetic field. We see then that increasing the turns increases the self induction in the circuit and thus it takes longer for one complete cycle of charging and discharging of the condenser. Tuned Circuit Now going back a little. We considered that a wave induced a positive charge on the aerial and this flowed through the inductance and the aerial became over discharged and a negative charge was built up on the aerial. Now if the incoming waves are of such a lengtl. that they tend to put a negative charge on the aerial at the same instant that the currents due to self induction does, the total current will be increased and each succeeding oscillation of the circuit will be in time with the incoming waves and we say the circuit is tuned to that wave. Were we to add inductance the time for one discharge of the aerial would be increased and before it was entirely discharged the waves would have induced a negative charge on the aerial which would neutralize the positive current still present and the current flow reduced. The circuit is then out of tune. Reducing the inductance would have a similar effect only the aerial would have discharged once and started to oscillate back again before the wave had a chance to put the negative charge on the aerial and the current would never reach its maximum value because the circuit is out of tune New CARTER Vernier Control RHEOSTAT Simple, positive, distinctive, reliable. Satin silver finish ; clock spring pigtail connection insures positive and reliable operation. Code 1, 6-ohm. for V. V. 200 tube $1.50 ea. Code 1-C. 20 ohm, for U. V. 201 -A tube.. 1.75 ea. Code 1-D, 30-ohm, for U. Y. 199 tube 1.75 ea. Write for Bulletins of Carter Products CARTER RADIO COMPANY 209 S. 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Here again we have a unit too large for practical purposes so use is made of the millihenry or one thousandth of a henry. A smaller unit is also in common use, the centimeter or as it is abbreviated cm, which is the one-millionth part of a millihenry. Forms of Inductance Inductances take many forms and it is advisable to be acquainted with the va rious forms they take and their suitability for a given purpose. The simplest form of inductance is a single layer of wire wound on an insulating form, usually a tube. This form of construction is entirely suitable for short wave lengths but in order to vary the amount of inductance in the circuit taps are taken from the winding. There are two methods of tapping, one being to take taps evenly spaced over the length of the coil for rough tuning and relying on a variable condenser to get sharp tuning as shown in Figure 20. The condenser may be connected in parallel or series to get the fine variation. Another method of tapping is to tap single turns for 8 or 10 turns and then tap in sets of 8 or ten turns for 7 or 9 sets, respectively. AVith this arrangement rough tuning is done with the 8 or 10-turn switch and fine tuning with the single turn switch to get intermediate values. As a rule a variable condenser is unnecessary when a single turn tap switch is used in tuning. Winding's on a Coil For the sake of compactness it is often desirable to make the inductance smaller and a number of methods are employed to accomplish this without adding distributed capacity to the coil. We have learned in the preceding part that two conductors with a difference of potential form a condenser. Were we to wind a coil as shown in Figure 21 with the turns wound as numbered there would be an appreciable voltage difference between say turns 1 and 10 resulting in a capacity being formed that will bs-pass some of the current and reduce the strength of the magnetic field of the coil. 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