DETERMINATION OF PRECISE TIME—SPENCER JONES 195 
some measure of control over the periodic errors in right ascension of 
the FK3 system itself. 
Until about 25 years ago, pendulum clocks of the regulator type 
were used as the standard clocks in observatories. A considerable 
improvement in precision was brought about by the invention of the 
free-pendulum clock. In an ordinary pendulum clock the timekeeping 
is impaired by the variable friction involved in driving a train of 
wheels to move the hands and record the actual time on a dial. An 
appreciably higher accuracy is to be expected if the pendulum is 
allowed to swing freely, except when it receives periodically impulses 
to maintain its swing, and is thereby relieved from all extraneous 
work. To achieve this had been the aim of horologists for many 
years, but although many attempts were made it was not really 
successfully accomplished until the invention by W. H. Shortt of his 
free-pendulum clock. The master pendulum is enclosed in an airtight 
case, in which the air pressure is reduced to about 1 inch of mercury 
and which is maintained at constant temperature and swings freely, 
except for small impulses, given at half-minute intervals, to main- 
tain the amplitude at a nearly constant value. The slave clock 
is a normal synchronome electric clock, which is adjusted when swing- 
ing as an independent clock to lose about 6 seconds a day. The 
synchronizing action required from the master free pendulum is there- 
fore a one-way action—always an accelerating action. The slave 
pendulum itself releases electrically the impulsing lever of the free 
pendulum, which falls when the free pendulum is at the midpoint of 
its swing. The impulse arm falls on the top of a small pivoted wheel, 
mounted on the free pendulum; this being a dead point, and the 
impulse not commencing to be given until the pendulum swings 
outward from the central position, the amount of the impulse does 
not depend upon any slight variation in the synchronization between 
the two pendulums which may occur. 
The synchronization of the slave pendulum is effected by means of a 
light flexible spring carried on it. The impulse arm of the free 
pendulum, after it has fallen clear of the pendulum, actuates a device 
which closes an electric circuit and sends a current through a small 
electromagnet adjacent to the slave pendulum. If the slave clock has 
dropped sufficiently behind the master, the armature of this electro- 
magnet will, when the electromagnet is excited, engage the bent end of 
the light spring on the slave pendulum. The end of the spring is then 
held fixed and, as the pendulum swings, the spring is flexed and the 
pendulum is accelerated; if, on the other hand, the slave pendulum is 
closely in phase with the master, the end of the spring passes under 
the armature before the electromagnet is excited, and nothing happens. 
The length and strength of the spring are so adjusted that when the 
synchronizing action occurs the slave pendulum is accelerated by 
