The Shortt free pendulum clock was a quantum advancement in the field of electrical horology and after its trials in Edinburgh it was regarded as the most advanced timekeeper to date giving an astonishing accuracy of less than one second a year/
Frank Hope-Jones, Electrical Timekeeping op.cit.
...fortunately in 1921 he (Shortt) hit upon a means of two pendulums swinging together in PRECISE sympathy vis. 'the hit and miss synchroniser'......I rank this invention as one of the very few of outstanding importance in the application of electricity to horology; it was the last thing needed to produce a practical free pendulum since it enabled two synchronome clocks to be firmly held in synchronisation. .......The first free pendulum clock was made by Mr Shortt with his own hands and was erected by him in the Edinburgh observatory in the christmas of 1921.....the free pendulum itself being in a cylindrical copper case from which the air was exhausted. It is known in the astronomical world as SH.O and the report of Professor Sampson on its first year's run created a sensation
In 1912 the watch trials at Kew were transfered to Bushy House in Teddington where the new National Physical Laboratories were set up. In its basement was an old beer cellar which was converted into a rating room with small rooms constructed alongside for maintaining cold, normal and hot temperatures. In March 1927 the laboratory installed the Shortt free pendulum clock No. 13 to be used as the standard timekeeper. This was the replacement for a master clock by J. Morrison & Sons. It was bolted to the 3 ft. thick cellar wall and final regulation was attained by adjusting the air pressure, enabling rates of under one second a year to be attained. Its time was transmitted by the slave clock to the three rating room's impulse dials. Eventually it was replaced by a quartz clock about 1955 and the Shortt clock was dismantled. Later given to the Liverpool City Museum where it remains.
Seventy six Shortt Free Pendulum vacuum clocks were built, all for observatories all over the world. For decades they were the most accurate form of timekeeper known until they were superseded by Atom clocks.