Radio Digest (July 1924-Apr 1925)

20 RADIO D I G E S T— Illustrated April 4, 1925 A. B. C. RADIO COURSE (Continued from page 17) electrical pressure acting: in the circuit and the electric current produced in the circuit when the circuit was closed. His experiments disclosed the facts that, first the electric current increased and decreased with an increase and decrease in the electrical pressure, and -second, that there was a decrease in current if there was an increase in resistance and conversely there was an increase in current if there was a decrease in resistance. The value of the units in which current (the ampere), resistance (the ohm) and electrical pressure (the volt) are measured are so chosen that the current in an electrical circuit in amperes is equal to the electrical pressure in volts divided by the resistance in ohms. This relation may be written as follows: volts amperes = I ohms The current is usually represented by the symbol I. the electrical pressure by the symbol K and the electrical resistance by the symbol R. The above relation may be written by using: these symbols and we have the following:: E I — II R This relation between current, electrical pressure and resistance is known as Ohm's law because, as stated above, it was discovered by Ohm. The above relation may be written in several forces as follows: volts Ohms = Ill amperes E R = — IV I and volts = amperes X ohms .... V E = I X R VI Ohm's law is perhaps the most important law in the many applications of electricity and a clear understanding: of its application to a few fundamental circuits will be useful to the inexperienced. Instruments to Measure Current The instrument marked A in figure 5 is called an ammeter because it is used to indicate the value of the current in the circuit in which it is connected. An ammeter is usually of low resistance and its operation depends upon some effect of the electric current such as the heating effect, magnetic effect and chemical effect. The instrument marked. V in figure 5 is called a voltmeter because it is used to indicate the difference in electrical pressure between the points on the electrical circuit to which its terminals are connected. A voltmeter has a high resistance as compared to an ammeter and its operation depends upon some effect of the electric current as in the ease of the ammeter. A description of the common forms of ammeters and voltmeters will he given in a later chapter. If the resistance of a certain coil of wire is known to be % ohm and this coil is connected to the terminals of a storage battery as shown in figure 5, whose electrical pressure between its terminals is 6 volts, the current produced in the coil can be determined by making use of relation 1 above, namely the current in amperes is equal to the pressure in volts divided by the resistance in ohms. amperes = 6 divided by Vi = 12 A resistance is usually represented by a symbol such as the one marked R. in figure 5. Such a symbol represents what is known as a non-inductive resistance, and an Inductive resistance will be explained in a later chapter. If a storage battery having a pressure of 6 volts between its terminals produces a current of 2 amperes in a coil of wire connected to the terminals, the resistance nhe^mffic Cop of the Jlir r Makeyoureetselective -separate the interfering stations by simply puttinjj the Traffic Cop on guard. The Ferbend Wave Trap will tune out interfering stations, no matter how troublesome. Never reduces, but nearly always increases volume. Add a Ferbend Wave Trap t« your set and "police" your reception. Itefrulate the traffic! Designed and manufactured complete by us after years of careful experimenting. It is not to be confused with imitations, hastily aasembled fromordinaryparta. Price is $8.60. Shipment is made Parcel Post C. O. D. plus postage, or postpaid on receipt of price. Order today . _ _ FERBEND ELECTRIC CO. 17 E. So. Water St. Chicago, III. Send/or FREE Booklet Tt.KBF.HTt t 3g l'ouKhkf^paie N. Y. Gentlemen: Tbe Wave Trap ordered from you some time ago is all that you claim for it and then some. It not only keeps out the other station that is causing the interference, but it also BeerriH to act " a clarifier of //<<; incoming station. (Signed) W. E. Wiltw. mm* I^AWAAAN of the coil can be determined by making use of relation 111 above, namely, the resistance in ohms is equal to the pressure in volts divided by the current in amperes, ohms = 6 divided by 2 = 3 If the resistance of a certain circuit is 10 ohms and it is desired to produce a current of 2 amperes in the circuit then the electrical pressure required can lie determined by making use of relation V above, namely, the electrical pressure in volts is equal to the current in amperes limes the resistance in ohms. volts = 2 times 10 = 20 The resistance of any conductor depends upon the kind of material composing the conductor, the dimensions of the conductor and also the temperature of the conductor. The resistance offered by a conductor depends upon the nature of the current it is carrying, that is, whether the current is a direct current or an alternating current. The resistance of a conductor to an alternating current is greater than it is to a direct current. The following discussion will be confined to direct curcent considerations and the alternating current effect will be discussed at a later time. Conductors of Electricity Silver is the best conductor but on account of the cost, and lack of supply it is not generally used. Copper offers very little more resistance to the electric current than silver and since it is much cheaper and more plentiful it is generally used. Other metals, such as tungsten, zinc, tin and iron, offer a greater resistance to the electric current than copper does. The greater t h e length of a conductor the greater its resistance and conversely the shorter the conductor the lower its resistance. The resistance of a conductor increases as the area of its cross-section decreases and conversely its resistance decreases as the area of its cross-section increases. Thus a conductor 100 feet long and \'s inch in -diameter will have onefourth the resistance of a conductor of the same material and same length but only 1/16 inch in diameter, because the area of the larger conductor is four times as great as the area of smaller conductor. The resistance of materials changes due to a change in temperature. Some materials experience an increase in resistance with an increase in temperature such as silver, copper, iron, tungsten, etc., while some other materials experience a decrease in resistance with an increase in temperature the best example of which is carbon. For example a tungsten-filament lamp has less resistance when it is cold than it has when it is hot, and a carbonfilament lamp has a greater resistance when it is cold than it has when it is hot. "When a tungsten-filament lamp is turned on it lights up almost instantly and it is more brilliant at first than it is later when the resistance of the filament has had time to increase and thus reduce the value of the current. The carbon-filament lamp lights up less rapidly as the resistance of the filament is a maximum when cold and this resistance decreases with an increase in temperature allowing a greater current to pass through the lamp. Series Electrical Circuit A series electrical circuit is one in which all the various parts of the circuit Figure 7 10 TUBE BRINGS IN THE WORLD* ATLAS MATCHED TUBES INSTRUMENT TESTED Made in all standard types. Guaranteed to function efficiently In the most exacting circuits. Every Atlas Tube is individually instrument tested. Try them 30 days. Money refunded if you are not delighted. At best dealers or direct from us. Our guarantee includes safe delivery. $ 32-° Special Offer Ai no extra charge, we will furnish selected Atlas Instrument Tented Tubes In matched sets for Keflex. Neutrodyne, Kadlo Frequency, Superheterodyne or other sets. They will improve the performance of any set. DEALERS: Atlas Matched Sets are hi great demand. Write or wire for proposition THE R-S-K CO. 311 Caxton Building 771 Ellicott Square 609 Chamber of Commerce CLEVELAND. 0. BUFFALO. N. Y. PITTSBURGH, PA. are connected together in such a manner that there is only one path for the current. Thus in figure 6 there are two tube filaments, Ti and T2 and a rheostat R connected in series to the terminals of the storage battery, and the same current passes through each part of the circuit. The resistance of such a circuit is equal to the combined resistance of the various parts of the circuit such as the filaments, rheostat connecting leads, etc. The two important facts about a series circuit are Figure 8 as follows: First, the electrical current at any instant is exactly the same in every part of the chtruit. If an ammeter be connected in circuit at various points in the circuit it will indicate the same current provided there is no change in the resistance of the various parts of the current or the electrical pressure acting in the circuit. There is no accumulation of electricity at any point in the circuit and just the same amount of electricity returns to, the storage battery or source of electrical pressure as leaves it. Thus in figure 6 there is the same current through the filaments of the two tubes. Second, the electrical pressure acting in a series circuit is distributed around the circuit in direct proportion to the resistance. If the filaments of the two tubes in figure 6 have the same resistance the electrical pressure of voltage between the terminals of the two filaments will be the same. All of the electrical pressure produced' in the battery is not used in producing current in : the two filaments but a part of this pressure is required in overcoming the resistance of the. rheostat, the connecting wires, etc. , The resistance of the connecting wires is usually -very small in comparison to the resistance of the remainder of the circuit and may be neglected in the majority of cases without series error. If the resistance of the rheostat be increased there will be an increase in the total resistance of the circuit and hence a decrease in the current. Likewise a decrease in the resistance of the rheostat will result in a decrease in the total resistance of the circuit and hence an increase in the current. As the resistance of the rheostat is decreased a. larger part of the total electrical pressure of the battery will exist between the terminals of the tube filaments. In all Cases the electrical pressure between the terminals of any part of the series circuit will have the same relation to the total electrical pressure as the electrical resistance of this portion of the circuit bears to the total resistance of the entire circuit. A parallel electrical circuit is one in which the various devices are so connected that there are as many paths for the current as there are devices in series. ^K w W Whether you plan to build or to buy a receiving set, it will pay you to know something about the "insides" of radio. This booklet gives you the "inside dope" on some of the recent inventions embodying the latest ideas of radio ~ engineers. ~ In this bulletin is full informa ~ tion about the E PREMIER 1 "HEGEHOG" jj Audio — Transformer — Ratios, I to 3, I to — 4 and I to 5, $3.50. ~ Ratio I to 10, $4.50. S This lightweight audio trans ZZ former lias earned a place in the Z very frpnt rank for its remarkable _ volume, and pure, natural tone. It — is 100% self-shielded against _ foreign noises. ~ lis small size is ;i surprise to everyone. It — cuts the space requirement)! for audio trans2 formers in two, This is a hin advantage In — portables' and makes it ideal for neat and — compact wiring. 2 FREE HOOK-UP DIAGRAMS also sent on — request. AH popular types. Address I frtmitr Jlpririr fompany | C 3810 Ravens wood Avenue, CHICAGO ^ Quality Radio Potts Thus in figure 7 two resistances Ri and Rj are connected in parallel to the terminals of the dry cell. In figure 8 the filaments of the two tubes, Ti and T2 are connected in parallel to the terminals of the storage battery B. The current in each branch of a parallel or divided circuit is independent of the current in the other branches and depends upon the resistance of that branch, and the electrical pressure acting on that branch. In order to easily determine the resistance of a parallel circuit, use is made of a property of a circuit called its conductance which is equal to the reciprocal of the resistance of the circuit. Conductance is measured in a unit called the mho which is the ohm spelled backward. For example, if two resistances of 4 and 6 ohms, respectively are connected in parallel their combined resistance may be determined as follows: The conductance of the 4-ohm branch will be % mho and the conductance of the 6-ohm branch will be 1/6 mho. The combined conductance of the two branches will be equal to % plus 1/6 or 5/12 mho. The reciprocal of the total conductance of 5/12 mho will be the value of the resistance of the parallel circuit which will be equal to 12/5 or 2 2/5 ohms. The resistance of a parallel circuit of any number of branches may be determined in this way. (Magnetism, Electromagnetism and Electromagnetic Induction will he the subject under discussion in the next chapter. — Editor's Note.) A. F. TUBE COUPLINGS (Continued from page 19) former from its primary to its secondary this steady current from the B battery does not of itself cause any effect in the secondary. If incoming signals cause this current to vary above or below the constant voltage, the changes cause currents to be induced in tlie secondary winding. In a 4 to 1 ratio transformer, where the secondary has four times as many turns of wire as the primary, the voltage in the secondary will be four times that of the change which caused it, originating in the primary. A two stage transformer coupled audio frequency amplifier is shown in figure 1, page 19. The points to be especially noticed are the C battery and the .5 condenser from the B plus lead to the negative A. With tubes of the 199 and 201-A class a C battery is necessary to perfect reproduction when B battery voltages greater than 45 volts are applied. Since it is desired to keep the grids of the amplifier tubes negative in relation to the filament, the negative side of the C battery is always connected to the grid, while the positive side of the C battery is connected to tin negative side of the filament circuit. Tin .5 mfd. condenser has been found to be of great aid in keeping Radio frequency oscillations out of the audio frequency stages. The fixed condenser of .00025 capacity will be noted across the secondary of the second audio frequency transformer. The use of this condenser .may very slightly cut the volume, bllt Clears up whatever distortion or "fuzziness" there may still be in the loud speaker. (Continued on page 22) I &ASHKP1MI A FIVE TUBE TUNED RADIO, FREQUENCY RECEIVER/ Because at the wonderful performance and immense popularity of this wonder set, imitations and counterfeits axe being offered by unscrupulous) dealers and distributors. Brery genuine Freshman Masterpiece has ■ serial Dumber riveted 00 the sub-panel. Be sure It appears 00 the set you bur. It is four protection and guarantee fertile Oteatest Value Ever Offered in a Radio Receiving Set. AtYatrDcala*