554 BELL SYSTEM TECHNICAL JOURNAL 



generator. Extremely fine control of timing is possible by means of the 

 electrical phase shifter since it can be included in the circuit at any stage of 

 frequency subdivision. If, for example, it is used at the lowest frequency, 

 assumed to be 1,000 cycles, one complete turn of the phase shifter dial will 

 cause a progressive time adjustment of one millisecond. When used at a 

 higher frequency, the precision of adjustment is increased correspondingly. 

 Continuous phase shifters suitable for such purposes were proposed as early 

 as 1925.®^ The idea of utilizing continuous phase shifters for the purpose 

 of making controllable changes in the frequency or indicated time in a 

 standard time and frequency system®^ was first disclosed in a comprehensive 

 patent filed in 1934 and issued to Warren A. Harrison in 1937. The most 

 elegant type of phase shifting element suitable for such purposes was de- 

 veloped by Larned A. Meacham.®^ xhis has been used in many transmis- 

 sion systems requiring continuous variation of phase such as in variable 

 direction radio beam systems ^^ and LORAN. 



The conversion between mean solar time and sidereal time, or for that 

 matter between any time systems, may be accomplished very easily with the 

 quartz clock. Having a rotating device, such as a dial or commutator, 

 whose rate corresponds to mean solar time, it is only necessary to apply a 

 gearing or the equivalent to obtain another rate corresponding to sidereal 

 time. It has been shown by F. Hope-Jones*^, Ernest Esclangon*^ and 

 others how any desired ratio, such as the ratio of the rates of mean solar 

 and sidereal clocks, can be obtained with any required accuracy by gearing. 

 A combined mechanical and electrical method was proposed in the ''Crystal 

 Clock" paper by means of which this ratio can be realized with an accuracy 

 of one part in 10^^ using simple gearing and a continuous phase shifter. 



The potential value of the factors just discussed in precision time studies 

 was realized early in the crystal clock development. This was indicated in 

 the "Crystal Clock" paper written in 1930 which closed with the following 

 paragraph : 



"It would thus be possible to combine, in a single system mean solar and sidereal time- 

 indicating mechanisms, means for rating the clocks in terms of time star observations and 

 means for transmitting time and frequency signals with the absolute accuracy of the time 

 determinations." 



It is of some interest to compare this prediction with the present trend of 

 development. In describing the quartz clock installation at the Royal 

 Observatory in Greenwich, Sir Harold Spencer Jones stated^* in 1945: 



"The quartz clocks being installed at the Royal Observatory are all adjusted to give a 

 frequency of approximately 100,000 per mean time second. By suitable gearing, the syn- 

 chronous motor can give impulses every sidereal second and tenths of seconds. Thus, the 

 same clock can be made to serve both as a mean time and as a sidereal time standard. 

 W\ time signals are, of course, sent out according to mean time; the sidereal time is re- 

 quired only for the actual time determination so that it is not necessary for all the clocks 

 to have the gearing to give sidereal seconds." 



