DETERMINATION OF PRECISE TIME—SPENCER JONES 197 
natural resonance frequency is equal to the frequency of the oscillating 
circuit. Under these conditions a strong vibration is set up and the 
quartz crystal takes control and locks the frequency of the oscillating 
electrical circuit to its own resonance frequency. Quartz is a very 
stable substance and, provided it is maintained at a very uniform 
temperature and the drive circuit is properly designed, the frequency 
remains constant to a high degree of accuracy. It is usual for the 
crystal to be cut to give a frequency of 100,000 cycles a mean time 
second, the dimensions of the quartz then being conveniently small. 
This frequency is divided down in steps electronically, either by the 
use of multivibrators or by frequency subdivision until an output 
with a frequency of 1,000 cycles a second is obtained. The output 
of this frequency is used to drive a phonic motor, from which time 
signals can be obtained at any desired intervals. 
Such clocks have many advantages over pendulum clocks. They 
have proved to have very high short-period stability. Their erratic 
changes of rate are less than half a millisecond a day, and the clocks 
themselves can be relied upon to about 1 millisecond a day. For 
extrapolating between scattered time determinations they are there- 
fore much superior to pendulum clocks. They have, moreover, the 
advantage of the great flexibility inherent in dealing with 100,000 
vibrations a second instead of only a single one. Electronic methods 
can be used for quickly and accurately determining the relative errors 
and rates of the clocks. For such purposes at Greenwich, decimal 
counter chronometers are used. This device consists of a scale-of-ten 
counter, and is actuated by the 100,000-cycle output per second from 
one of the quartz crystals. When it is switched on, it will start 
counting these vibrations, recording the count on five decade dials, 
reading, respectively, tenths, hundredths, thousandths, ten- 
thousandths, and hundred-thousandths of a second. To compare 
two quartz clocks, a seconds signal from the phonic motor driven by 
the one clock is used to start the count and a signal from the second 
clock to stop it. The time difference between the two clocks, accurate 
to a hundred-thousandth of a second, is thus obtained in a fraction of a 
second. As a check, the second clock can be used to start the count 
and the first clock to stop it. The difference in frequency of the two 
clocks is obtained by feeding the 100,000 ¢. p. s. outputs from the two 
clocks into a comparator, so that they beat against one another, and 
timing the beats. It is possible to obtain an accuracy of one part in 
10”° in the measurement of the frequency difference. 
At Greenwich, the clocks are used in groups of three, one phonic 
motor being provided for each group of three clocks. One of the 
clocks is selected to drive the phonic motor, but regular comparisons 
are made between each pair of clocks in the group. Automatic beat 
