A HIGH PRECISION STANDARD OF FREQUENCY 



511 



an adding machine. Three of the counters are used as outlined above. 

 The fourth counter is to be used for recording the mean of these three 

 numbers, computed automatically by an auxiliary device. The re- 

 maining one is a serial counter which is used to record the time, 

 either directly, or by numbering the 1,000 second intervals con- 

 secutively. 



The five element counter is shown in Fig. 20 with the cover removed 

 to show part of the mechanism. The energy for actuating the counting 

 and resetting mechanism is obtained from a small motor running 

 continuously. These elements are operated at the proper times by 

 clutches controlled by electro-magnets which are selected by the relays 

 in the modulator circuits described above. The counting, printing. 



Fig. 20 — Automatic beat counter. 



and resetting operations are interlocked by means of cams and relays 

 so that no counts may be missed through superposition of operations. 

 The electrical circuit of one unit of the recorder is shown in Fig. 21. 



The 1,000 second intervals are determined by a cam operated by the 

 1,000-cycle synchronous motor. It might be questioned whether one 

 of the crystals being checked should be used to determine the 1,000 

 second intervals. No serious error arises from this, however, since 

 the percentage variation in the interval due to a change in rate of 

 the crystal is only one millionth of the percentage variation in the 

 recorded beat number. Thus, using one crystal to determine the 

 intervals, used in comparing its rate with other crystals, makes the 

 final measurement subject to an error from this cause of only about 

 0.0001 per cent. 



