Radio Broadcast (May 1929-Apr 1930)

MODEL 339 TRANSFORMER Fig. 1 — Schematic diagram showing method of connecting the miniature current transformer. C batteries or one 22-volt tapped C battery) gives plenty of "poke" to make the last named range useful. The fundamental wave is then almost too strong. Oscillations will occur when only 6 or 9 volts are used on the plate. Too much plate battery brings out the unwanted odd harmonics. A battery potential of 22.5 volts is more than should be used. Harmonics more remote than those named can be used but they are rather weak." CURVE SHEET When using a variable condenser having two rotor sections and two stator sections, each of 4 plates and double-spaced, the tuning range shown on the curve sheet is obtained with a 23-turn coil of No. 28 wire spacewound on a ux tube base. The coil is split, hence each section has llf turns. In case the meter is to cover a higher band of wavelengths the ux base will prove too small. Its size can be increased by means of a tube that will just slip over the base, and such a tube may be secured with small screws. However, readymade forms of sufficient size are now obtainable. By plotting the dial readings against both wavelength and frequency we have a combination wavemeter and frequency meter. The curve chart is also an instrument for converting wavelengths to frequencies, and vica versa. For example, suppose we wish to know the frequency corresponding to a wavelength of 43 meters. We find (wavelength) 43 meters on the left of the chart, follow the ordinate to the right until it intersects the wavelength curve, then follow downward until we intersect the frequency curve, then to the right again, and read off the frequency — 7000 kilocycles. For such conversions the dial reading does not concern us. The curve sheet shown is for a particular meter. It covers only the one band since, when the meter is used in the band above or below, it is only necessary to multiply or divide the reading by 2. CALIBRATION To calibrate such a meter it is necessary to secure a sufficient number of points from the transmissions of standardfrequency stations such as 9xl and wwv. Commercial stations whose wavelengths are known may also be used. The method is as follows: first tune-in the standard signal on an oscillating receiver and adjust the tuning controls to zero beat. Then (without touching the receiver) tune the oscillator until it produces zero beat with the receiver and standard signal. Record the wavelength and oscillator setting and proceed to the next point. Since the second harmonic of both receiver and oscillator is easily distinguished by its strength, points may be obtained also from signals of half or twice the wavelength at which calibration is desired. The receiver, of course, is tuned to the signal and the oscillator to half or twice the wavelength so that the fundamental of one beats with the second harmonic of the other. USES OF THIS METER Besides serving most of the ordinary purposes of an oscillator, the meter is of aid in locating stations whose wavelengths f -i 1 1 <— 1 1 1 1 1 1 / / 1' §/' f / / / / 4 / / Sw Fig. 2 — Diagram of oscillator -AA^VC^ — 1 1 f J j M / V ♦ A Rheostat (optional) 3 Volts 0 10 20 30 40 50 60 70 80 90 100 DIAL DEGREES Fig. 3 — Calibration curves of Mr. LeucWs oscillator. are known, it comes in handy when building or altering a short-wave receiver, and is useful in adjusting the wavelength and tone of a transmitter carrier. In doing the latter it is useful to listen to a harmonic of the transmitter while beating it again.st the receiver and one of the frequencies of the meter. Mr. Leuck suggests, however, that "if one feels it imperative to listen on the transmitter fundamental, CorrfenslTrRotoF tn'S may ^ done after ipoint b> . the oscillator has been equipped with a pair of phones and the sensitivity sufficiently reduced by enclosing the oscillator bodily in a metal shield such as a ~f!v\ | ^ bucket. The phone cord ix. s\ to stator may emerge through a hole in the bucket lid and," Mr. Leuck dryly adds, "it is necessary to devise an extoSk, light tension to the tuning knob to reach outside the pail or else put an assistant inside to do the tuning." A and point., B A convenient wire-wound potentiometer for 110-volt circuits. Miniature Current Transformer For the particular use of the experimenter, seemingly, Weston has just offered a miniature current transformer (type 539) . With a single one-ampere a.c. ammeter, such as the type 528, it makes possible measurement of any current from 0.2 to 200 amperes. For currents below 1 ampere the meter is used alone. For currents from 1 to 20 amperes the meter is connected as shown in Fig. 1 and the primary switch is set to the proper range. For larger currents — up to 200 amps. — the line itself is treaded through the "window " of the core. If the line is put through once the maximum of the meter range represents 200 amperes, if twice — 100 amps., and if 4 times — 50 amps. Even if we assume that we will use only the upper f of the meter scale we can obtain the following ranges, which have liberal overlaps. 0.2-1, 0.4-2, 1-5, 4-20, 10-50, 20-100, 40-200. The rather awkward range of 13.3-66.6 amps, (obtained with three conductors through the window) may be omitted. Most current transformers have their ranges disturbed and their insulation burned if the secondary is left unloaded (meter off) while current flows through the primary. The size and design of the 539 largely prevents this but a shorting switch (See Fig. 1) has been provided as a precaution. A 110-Volt Potentiometer There is much peace of mind in being able to apply voltage gradually instead of "slamming it on." A recent temporary need for a variable a. c. voltage resulted in the acquisition of a Ward Leonard "Vitrohm" potentiometer of a pattern which may be left continuously across a supply potential as high as 125 volts a.c. or d.c. It is known as catalogue No. 64304a and has a resistance of 250 ohms. There are 22 contact points and the slider covers two at a time, thus providing 20 steps with sparkless transfer. Obviously, the device may also be used as a rheostat. It costs less than a single ux-210 and may easily save much more. The I. R. E. Patent Digest It is regrettable that the patent digest has gone from the pages of the I. R. E. Proceedings. Having asked some dozen or so of the members, I find confirmation of my feeling that the ordinary member read this digest with interest, found it stimulating, and looks forward to the time when it will again be possible for Mr. Brady to furnish this review. Letters are Welcome Letters or informal papers to be read or discussed in "The Experimenter's J Armchair" are welcome. They should be addressed to Robert S. Kruse. % Radio Broadcast, Garden City, N. Y., and should refer to these pages by title. I 04 • • JUNE • 1929 •