Radio broadcast .. (1922-30)

298 RADIO BROADCAST ADVERTISER The Heart of a Good "B" Eliminator ELECTRODYNE "B" BLOCK Get This Point Electrodyne engineers have contributed a new "B" Block for "B" Eliminators. Utmost care is taken to drive out every particle of mois- ture, thus making it "moisture-proof." The condensers are rugged and made with the greatest precision which gives the "B" Block long life. Engineers and many other satisfied users have endorsed it because it is technically right, because it makes a "B" Eliminator operate with steady power. Radio fans like it be- cause it satisfied their requirements and makes a telling effect on the tone quality of their sets. They vary in capacity and price from $9.00 — $11.00 Electrodyne By-Pass and Filter Condensers Guaranteed to stand 600 and loco volts respec- tively according to our laboratory standard. Elec- trodyne condensers are guaranteed to hold their charge for days. Prices according to capacity from 6oc to $7.50 Electrodyne Fixed Mica Condensers Electrodyne fixed mica condensers are sealed in a moisture-proof insulating compound which guaran- tees absolute freedom from moisture. This means clarity in reception. Prices according to capac- ity vary from 25C to 95C. If your nearest dealer cannot supply you write to us direct Manufacturers write us for our special offer ELECTRODYNE CO., Inc. 2378 Third Ave., N. Y. C. The Radio Broadcast SHEETS TNQUIRIES sent to the Questions and Answers department of RADIO BROADCAST lave until recently •*• been answered either by letter or in " The Grid." The latter department has been discontinued, and all questions addressed to our technical sen-ice department are now answered by mail. In place oj " The Grid," appears this series of Laboratory Information Sheets. These sheets contain much the same type of information as formerly appeared in "The Grid," but we believe that the change in the method of presentation and the wider scope of the information in the sheets, will make this section of RADIO BROADCAST of much greater interest to our readers. The Laboratory Information Sheets cover a wide range of information of value to the experimenter, and they are so arranged that they may be cut from the magazine and preserved for constant reference. We suggest that the series of Sheets appearing in each issue be cut out with a ra^or blade and pasted on filing cards, or in a note book. The cards should be arranged in numerical order. Several times during the year, an index to all sheets previously printed will appear in this department. The first index appeared in November. Those who wish to avail themselves of the service formerly supplied by " The Grid," are requested to send their questions to the Technical Information Sen-ice of the Laboratory, using the coupon which appears on page 318 of this issue. Some of the former issues of RADIO BROADCAST, in which appeared the first sets of Laboratory Sheets, may still be obtained from the Subscription Department of Double- day, Page Sr Conpany at Garden City, New York. No. 57 RADIO BROADCAST Laboratory Information Sheet What is Resonance? January, 1927 AN ELECTRICAL ANALYSIS TT HAS frequently been said that, in order to re- •*• ceive any particular station, we must tune the various circuits of our receiver to resonance with the incoming frequency. We shall endeavor, in this sheet, to explain what is meant by resonance. In the accompanying sketch, first refer to "A." Diagram No. 1 is a simple circuit consisting of some source of alternating voltage, marked E, in series with a condenser. The reactance or impedance offered by the condenser to the flow of current de- pends upon the frequency, and, if we plot a curve showing the change of reactance with frequency, we will get a curve similar to that marked C, in "B." As the frequency increases, the reactance of the condenser decreases, and the curve approaches the zero line. Capacitive reactance is usually considered negative, as shown, in order to indicate that it is opposite in effect to the inductive reactance. If the condenser in the circuit is re- placed by a coil of wire, or inductance, as shown in No. 2, "A," we find that the inductive reactance increases with an increase in frequency which is shown by the curve marked L on "B." Now, if we connect both a con- denser and an inductance in a circuit as shown in No. 3, "A," we will have a combination of the effects produced by both of them. Remembering that the effective resistances or impedances are opposite in sign, we may add the two curves together and the result will be a curve such as that marked "Total Reactance" in "B." At one point we notice that the line passes through zero, this point being indicated by the dotted line marked R. In other words, at this point, the total reactance in the circuit is zero, the reactance which is due to the condenser cancelling out the reactance due to the inductance. When a circuit is tuned to resonance, the capacity and inductance are so proportioned that their effect in the circuit are nullified, and, as more current will flow in a circuit of least resistance or reactance, the combination will offer very little opposition to the currents having a frequency of R in the diagram but will offer considerable resistance to any other currents having a different frequency B- No. 58 RADIO BROADCAST Laboratory Information Sheet January, 1927 The UX-171 and UX-210 A COMPARISON TDOTH of these tubes are suitable for use in the •O last stage of audio amplification, but under certain conditions one tube is to be preferred over the other. By glancing at the table, it will be seen that the output resistance is lower for the 171 than the 210 for all values of plate voltage. The greatest trans- fer of energy occurs between a tube and a speaker when their impedances are matched. As most loud speakers have very little impedance at low fre- quencies, it is advantageous to use a tube such as the 171 which has a very low plate impedance. This compensates to some extent the low amplifica- tion factor of 3. From the figures given for the maximum undistorted output, it can be seen that, with 180 volts on the plate, the 171 will deliver to the load 700 milliwatts of power. This is about the same power as can be obtained from a 210 with about 300 volts on the plate. However, at 425 volts, the 210 is capable of delivering more than twice the undistorted power of a 171. It is quite evident then, that the 171 is somewhat to be pre- ferred for ordinary signal strength such as is needed in the home, and that for unusual volume, such as concert work in large halls, the 210 would prove more satisfactory. Examined and approved by RADIO BROADCAST