556 BELL SYSTEM TECHNICAL JOURNAL 



This was done by measuring the duration of each beat by a photographic 

 method. By means of a modulator, a relay, and induction coil, a spark was 

 produced at the spark gap at a definite phase of each beat period. The 

 spark illuminated the edge of a transparent scale rotating 10 revolutions per 

 second under control of one of the oscillators, Oi. The transparent scale 

 contained 100 numbered divisions, which therefore represented milUseconds 

 in any time interval so measured. Each time a spark occurred, the portion 

 of scale illuminated was registered on photographic film. Thus, the dura- 

 tion of each beat was registered photographically with an accuracy of one 

 part in ten thousand. Since the beat frequency is one millionth of the high 

 frequency, the resulting comparison of high frequencies is precise to one part 

 in ten thousand million, or 1 in lO^*'. Actually, it was possible to estimate 

 fractions of a scale division which gave greater precision of measurement 

 than was required in the study of oscillators of that date. 



L. A. Meacham in 1940 improved upon this method of frequency com- 

 parison by substituting an electronic relay for the mechanical relay, and by 

 using a discharge lamp instead of a spark for illumination. He used the 

 improved apparatus^^ for studying the behavior of the then new and highly 

 stable bridge stabilized oscillators. 



Still further improvements in the general method have been repqrted by 

 H. B. Law using a "phase discriminator" to trigger off a special chronometer, 

 consisting of a decimal scaling counter, and thus avoiding the photographic 

 process^ °°. The scaling counter as used here counts the number of cycles 

 of a 100,000-cycle input timing wave that occur during any one beat between 

 the two frequencies being compared, and registers that number, in scale of 

 ten, on a system of dials that can be read directly. In comparing frequencies 

 that are free from interference, the accuracy of comparison by this means is 

 limited chiefly by the precision with which the "phase discriminator" can 

 mark the beginning of successive beats. An accuracy of one part in 10^^ is 

 claimed. This is one of the rate comparison means employed in the fre- 

 quency and time standards of the British Post Office and in measurements 

 involving the quartz clocks of Greenwich Observatory and the National 

 Physical Laboratory. 



The scaUng counter is a particularly useful device for the precise measure- 

 ment of any time or rate phenomena that can be reduced to the measure- 

 ment of short time intervals. The counter idea originated some years ago as 

 a means for counting alpha-particles and other phenomena associated with 

 radioactivity studies, one of the original devices being the well known 

 Geiger-Muller counter. The basic scaling circuit, used in many counters, 

 was proposed in 1919 by W. H. Eccles and F. W. Jordan. An interesting 

 history of counting circuits as applied primarily to the counting of electron 

 and nuclear particles has been written by Serge A. Korff in his book on that 



