Radio Broadcast (May 1928-Apr 1929)

280 RADIO BROADCAST SEPTEMBER, 1928 FIG. 3 Thus we obtain directly a value for Z in ohms. If then the d.c. resistance is obtained on a Wheatstone bridge the inductance is easily obtained from well known laws. In the case of large condensers the resistance may be safely neglected. For inductances: Z = \/ 0)2 U + R* orL = jE±F \2itf At 60 cycles L = where z=impedance in ohms <i)=2H:f =6.28 times the frequency in c.p.s. L = inductance in henries R = resistance of the coil in ohms In the case of a condenser the following holds true: _ IO6 106 At 60 cycles *~ 2lzk ~ 377Z Where z = impedance in ohms c = capacity in microfarads THE CONSTRUCTION OF THE UNIT \ A / E SHALL now describe this set-up, Fig. 2, " * in greater detail. Resistance R is a variable decade resistance box (such as those made by General Radio Company). This box has steps of units, tens, hundreds and thousands. The tubes used in the vacuum tube voltmeter are of the CX-301A type and should preferably be matched to give identical readings on the calibrating set-up so as to facilitate measurements. A go-volt B battery is used in conjunction with 13! volts of negative C bias. This bias drives the operating point well down to the lower knee of the plate-current-grid-voltage characteristic of the tube so that a small applied voltage gives a relatively large change of plate current. The meters. Mi, and M2, are of Weston or Jewell 0-1.5 d.c. milliampere type. If the a.c. voltage from the transformer is about 12 volts the plate current of each tube will be about 1 milliampere and this is a convenient value to use. The transformer T which furnishes the measuring voltage should deliver about 12 to 14 volts, and a 5000ohm heavy duty Bradleyohm or similar variable resistance is inserted in its primary. The photograph at the beginning of this article shows a typical layout of the testing apparatus described above. The vacuum tubes, meters Mi and M;, exciting transformer, T, bypass condensers and C batteries are housed in the box. The box should be so arranged that the C batteries may be easily replaced when necessary. The knob of the 5000-ohm variable resistance which is used to control the exciting voltage is placed in a convenient position outside of the box between the two tubes. The cable at the left goes to the 110 volt a.c. source, and the cable at the left to the batteries. The impedance to be measured is connected in the circuit by means of the two binding posts on the front. Two similar terminals in the back are used for connecting in the decade resistance box. The resistance used at X for calibration should be about 1000 ohms and unless it is a standard Book Review Experimental Electrical Engineering and Manual for Electrical Testing. By V. Karapetoff. Third Edition, Revised and Enlarged, in Two Volumes. Vol. I, xxxii and 795 pages, 6 by 9, 391 figures, $6.00. Vol. 11, xxxii and 618 pages, 280 figures, $5.00. John Wiley & Sons, New York. IN ISSUING the third edition of Karapetoff 's famous work on " Experimental Electrical Engineering" in a revised and reset printing John Wiley & Sons are performing about the same service for the electrical engineering profession at large as when they made Morecroft available in a second edition to radio engineers. About the book little remains to be said. The author has "been Professor of Elecfical Engineering at Cornell University since 190^ He is one of the group of American electrical engineers, like Steinmetz, Pupin, Pender, Ryan and others, who have advanced the profession by their work both in the universities and outside, and who have in several cases combined notable literary and artistic talents with engineering ability. The two volumes of " Experimental Electrical Engineering" contain about all that the engineer wants to know about electrical testing and the practical behavior of electrical machinery. If he is interested in the process of getting the magnetization curve of an iron sample by the bismuth spiral method he will find it, in the same chapter with the Thompson permeameter, the Du Bois balance, the Koepsel permeameter, and the Ewing Permeability Bridge, in the first volume. If he happens to have gone into the talking movie field to find, perhaps to his surprise, that oscillograph galvanometers are used in some of the sound-movie recorders, he can read up on the applications and physical behavior of such devices in the second volume, where the electro resistance it will have to be measured carefully on a good bridge and labeled. If greater accuracy is required higher grade meters than the ones described will have to be used; however, the small meters are accurate enough for all practical purposes. when biasing current is present IT HAS been pointed out that it is frequently necessary to find the impedance or inductance of a filter choke coil, an impedance coil, or the primary of an audio-frequency transformer when certain values of direct current are flowing through the windings. Or it may be necessary to study the effect of various values of direct current on the inductance of a coil when the turns are varied or the air gap changed. A circuit which is used for this purpose is shown in Fig. 3. In this case a network consisting of two identical coils, L and L (under test), a milliameter, M3, and a storage B battery is connected at X-X. The terminals of the B battery should be accessible so that various voltages may be tapped off. It should also be in good condition and all cells should show the same terminal voltage. As may be seen from the diagram the battery produces a circulating current around through the' two coils, but since terminals X-X are connected to the midpoint of the battery and of the choke coils no potential exists between these points. When changing the amount of circulating current it is very important that this condition be maintained, otherwise thegrid bias on the vacuum tube voltmeter, M2, will be changed and a serious error will result. In case there is any doubt it is well to check the voltages on each side of the midpoint with a high grade voltmeter. Another way to check this is to take a reading of the vacuum tube voltmeter, M2, when no a.c. is impressed and when no biasing current is flowing (battery cut out of circuit). Another reading is then taken with no impressed a.c. but with battery voltage of the desired amount in the circuit. If the vacuum tube voltmeter reading, M2, does not change then the battery is not affecting the voltage balance, and the test may proceed. Storage B batteries will deliver up to about 150 milliamperes. If more current is desired through the coils under test it will be necessary to use regular storage batteries. Impedances measured in the manner indicated in Fig. 3 will be equal to one half the impedance of a single choke coil. Multiply the measured value by two and proceed to compute the inductance as outlined in the equations above. magnetic oscillograph, its galvanometer, optical system, recording apparatus, electrical circuits, transient visualizer, and possible accessory apparatus, such as vacuum tube amplifiers, are quite completely described. The radio engineer will find a chapter on high-frequency measurements. Everybody in the profession will find something, and usually a lot, about what he wants to know. The treatment is packed with valuable references and numerical data. On assuming a new job recently, one of the first things I did was to order copies of both volumes. I knew perfectly well that before long one of the engineers would refer to it and find out in short order somethi ng worth a great deal more to the company than the price of the books — and the 14 1 3 pages would remain for more use of the same kind indefinitely, unless somebody who is not as honest as he is wise steals them for his private library. — Carl Dreher.