Radio broadcast .. (1922-30)

272 bring in a beat note of 1000 cycles per second. The width of the region of inaudible (subaudible) beats just passed through is determined by the limitations of the human ear and of the apparatus used. In most cases a beat note below about 50 cycles per second would not be heard. In practice it is posible to approximate closely to jero beat (that condition existing when the two notes are of exactly equal frequency and no beat note is heard) by setting the dial at the middle of the silent region which is confined, often narrowly, between the two regions of clearly audible beats whose frequencies decrease as the inaudible band is approached from either side. Examine Fig. 2. If the occasion demands, it is possible to actu- ally make the zero beat frequency audibly evident by employing a third oscillator to give an auxili- ary constant audible beat note whose intensity varies except when the variable frequency oscil- lator gives zero beat with the first oscillator. All that has been said above regarding fre- quencies is true of every frequency, whether fundamental or harmonic, having the requisite amount of energy; when we hear a beat note, therefore, we are not at once certain which pair of frequencies from the two harmonic systems of the oscillators is producing the beats. We can be certain, however, that the audible beat frequency is the difference in frequency between some harmonic of one oscillator and some har- monic of the other oscillator, and, by methods described in part farther on, we can determine the respective harmonic numbers in question. The important thing to remember is that the audible beat note is used to in- dicate a very definite relation be- tween frequencies which in them- selves may be far above the range which is audible. These indicators bridge the gap between such frequencies as fall above a few thousand cycles per second, which by any stretch of the imagin- ation could not be measured by simple mechan- ical means, and those frequencies in the few hundred cycles per second class which are readily measured by mechanical-photographic means. An adaptation of the familiar telephone receiver, foi example, makes it possible to photograph oscillations of a few hundred, or say for instance, a thousand cycles per second. It is a familiar fact that the metal diaphragm just back of the hole in the ear piece of the telephone receiver can be made to vibrate audibly at this frequency. If the vibratory motion of the diaphragm were made to operate a small mirror suitably, a spot of light reflected from the mirror would be de- flected with a vibratory motion. A photographic record could be made by the spot of light, simul- taneously with a similar record from the motion of the pendulum of an accurately regulated clock, upon a moving photographic film. The re- sulting print would look like the illustration of Fig. 3. The time represented by the distance be- tween any two lipes made by the swinging pen- dulum could be read from the clock. The number RADIO BROADCAST 99,000 100,000 101,000 FREQUENCY OF VARIABLE OSCILLATOR CYCLES PER SECOND FIG. 2 of oscillations of the mirror within the boundaries set by any two time-lines can be counted, just as the number of box-cars between the locomotive and caboose of along freight train can be counted. The number of oscillations per second can then be computed from the information so obtained. CALIBRATING CRYSTALS TO PUT the facts outlined in the preceding paragraphs to work to tell us the frequency of piezo-eiectric or other electric oscillating cir- cuits, a series of oscillators are set up as suggested made from pendulum-^ Record made by excitation from oscillator FIG. 3 by Fig. 4. Suppose that "A" is the crystal oscil- lator which we had suspected of giving oscilla- tions of frequency in the neighborhood of 6,000,- 000 cycles per second. "D" is a variable low- frequency oscillator which can be photographic- ally calibrated as above described. " B" and " C" are two other variable oscillators. The funda- mental frequency of "B" may be adjusted until its harmonic number 10 gives zero beat with the fundamental of "A"; harmonic 20 of oscillator "C" may be similarly set against the funda- mental of "B"; harmonic 30 of oscillator "D" may be set to equal the fundamental of "C." Under these conditions we now know that the fundamental frequency of "A" is 10x20x30 = 6000 times the fundamental frequency of " D." 1 f a photograph of " D " shows its frequency to be 1019.8 cycles per second it is safe to believe that the frequency of A is 6000 x 1019.8 = 6,118,800 cycles per second, which would certainly justify the nickname "six-million cycle crystal." Thus we can measure extremely high frequencies by first measuring in an electro-mechanical way a much lower frequency which has some simple and definite relation, as indicated by beat notes, to the very high frequency. _20 CRYSTAL OSCILLATOR A 6,000,000 Cycles per Sec. FIG. 4 SEPTEMBER, 1927 For an instructive and very interesting article which describes unusual results of some recent experiments of Professor Wood of Johns Hop- kins University with piezo-electric crystals, readers would do well to read "A New Magic," by Frank Thone, in the Century Magazine, for February, 1927. A scientific report on methods of using the piezo-electric crystal for precision wavemeter calibration was published in 1923 by Prof. G. W. Pierce of Harvard University in the Proceedings of Ihe American Academy of Arts and Sciences. Since that time frequency meters covering the range from more than i 50 million cycles per second down to audible frequencies have been built and crystal-calibrated at Cruft Laboratory, Harvard. Another paper by Pro- fessor Pierce on the measurement of the velocity of sound at high frequencies, published in October, 1925, by the American Academy of Arts and Sciences, includes a detailed description of some of the methods noted here for calibrating crystals and for using them in precision work. When once a crystal has in the above man- ner been calibrated it is a standard against which other instruments for measuring frequency may be checked by methods much simpler in procedure but which in many instances again make use of the whistle of beat notes. Other crystals, by chance or by careful cutting, giving beats with the standard, may be precisely cali- brated in short order. The case, however, of precision wavemeter cali- brations against a standard crystal is typical of what may be done with, and represents one of the earlier laboratory uses for, the piezo-electric crystal; methods for such work de- veloped in the university laboratory a few years ago have now become common and are regarded as stand- ard practice. CALIBRATING WAVEMETERS A WAVEMETER is essentially an instru- f\ ment for measuring frequency and there- fore may better be called a frequency meter. Consisting merely of a coil connected to a con- denser, the latter usually variable, a wavemeter gives a marked response at each setting of the condenser to a particular frequency. The re- sponse takes the form of a very rapid increase in the current circulating in the wavemeter when the condenser setting is adjusted toward a value at which the wavemeter is in tune to the fre- quency of the oscillator inducing the current. The evidence of the response may be given by a current meter built into the wavemeter, by a meter in a circuit of the oscillator to which the wavemeter is being tuned, or by other means. Although for many wavemeters calculation will give the approximate range of frequency, or wavelength, covered by the scale of the instru- ment, only an experimental comparison against frequencies already known can give a dependable calibration. The frequencies for comparison may be determined by using another wavemeter which has already been standardized, but in any case the frequencies must at one time or another have been measured by some such methods as described for the case of the crystal. Because the frequency of the piezo-electric oscillator depends almost wholly upon the physical dimensions of the crystal, it is especially well adapted for meet- ing the requirements of a standard of frequency. So, ultimately, if our needs demand frequent measurement of frequency with high accuracy we shall find it necessary to calibrate a wave- meter against a piezo-electric crystal. The wave- meter is the instrument of greater utility in radio work in general, while the crystal is the more highly dependable as a standard of frequency. In