An introduction to the Quartz Clocks of the Greenwich Time Service


page under construction


The Observatory's first quartz clock in 1939. From the 24 February 1946 edition of Le Patriote Illustré

Humphry Smith was the Head of the Obserrvatory's Time Department from 1936 to 1977. During his tenure, the accuracy of measurement increased by a factor of a million as the Observatory's standard timekeepers were upgraded from pendulum clocks to quartz clocks and then to atomic clocks. The Observatory's first quartz clock was acquired in 1939 and the last in 1965. The first atomic clock was acquired the following year.

Clocks, consist of an oscillator, a means of counting the number of oscillations, and a mechanism to display this information in a meaningful form. Until the arrival of the quartz clocks, the observatory's standard (main) timekeepers all used a pendulum adjusted in length so that its natural period of oscillation (from side to side and back again) was a nominal two seconds.

In 1937, the Observatory abandoned the concept of using a single clock as a time standard, switching instead to the concept of a mean clock based on a number of different clocks, including some from outside Greenwich. These initially included Shortt 4 at the Royal Observatory Edinburgh and Shortt 13 at the National Physical Laboratry (NPL) in west London. By 1942, two of the NPL Quartz clocks, Q2 & Q6, were included followed soon after by some of the Post Office Standards at Dollis Hill and later Banbury.

A typical quartz clock consists of four units:

  1. A crystal controlled oscillator and its maintaining circuit oscillating at 100 kc/s.
  2. Electronic frequency-divider circuits to reduce the frequency to 1 kc/s.
  3. An amplifier to provide the necessary power at 1 Kc/s to operate a phonic motor (a small synchronous motor).
  4. A phonic motor to operate a clock mechanism including ‘second’ contacts.

By monitoring the comparative rates of the different quartz oscillators at the observatory (which was easy to do), the best performing ones at any particular time could be connected to a set of dividers and phonic motors that could be switched from one oscillator to another as circumstances dictated. As a result, most ot the Observatory clocks did not have divider-circuits and phonic motors of their own.

The world's first quartz crystal oscillator was built by Walter Cady in 1921. In 1923, David Dye at the National Physical Laboratory and Warren Marrison at Bell Telephone Laboratories in the USA produced sequences of precision time signals with quartz oscillators. In October 1927 the first quartz clock was produced at Bell Telephone Laboratories and in the 1930s, Louis Essen produced NPL's first quartz clock based on Dye's design. The UK's General Post Office (GPO) was also in the vanguard of clock development.

In 1939, the Observatory obtained its first quartz clock from NPL. Most of the rest came, in whole or in part, from the GPO. The last clock to arrive was a commercial American clock by Sulzer, which was delivered in 1965.

The acquisition of quartz clocks by the Observatory can be broken down into seven distinct parts of phases. In the first three, the clocks were supplied as complete units. By the late 1940s, the Observatory had acquired the necessary skills and resources to build its own circuitry and build its own clocks using either new crystals supplied by the GPO or by reusing exisiting crystals from clocks already in the Observatory's possession. 

In total, the Observatory acquired around 33 quartz oscillators. These were installed over time in a total of around 50 different clocks on thee of the observatory's sites - Greenwich, Abinger and Herstmonceux; with one being sent to the Royal Observatory Edinburgh during the war. By far the most important site in terms of technical developments and methodology, was the site at Abinger.


Phase 1 (1939)

Installation at Greenwich of a clock made specifically for the Royal Observatory by the National Physical Laboratory (NPL). Given serial number Q3 by NPL, it was the third clock that they had made and was refered to by this name thereafter. It was moved to Abinger following the outbreak of WW2.


Phase 2 (1944)

Installation of three quartz clocks with GT-cut Crystals designed and built by GPO. Designated by the Observatroy as A1, B1 & B2, they were mains operated and commenced operation towards the end of WW2. During the war years, the time service had been evacuated to Abinger with a subsidary service operating from the Royal Observatory Edinbugh. Although for reasons of secrecy Abinger was referred to as Station A and Edinburgh as station B, the clocks B1 & B2 were assigned to Abinger and A1 to Edinburgh. Two of the three oscillators were later used in the Group B clocks of phase 3 (A1 & B2).


Phase 3 (commenced 1944)

Installation of eighteen oscillators with GT-cut Crystals designed and built by GPO to their Group IV design specification. Twelve oscillators were installed in four groups of three at Abinger (Groups B, C, D & E) and six in two groups of three (A & F) at Greenwich. The oscillators were rack mounted, with all three from a group mounted one abobve the other on the same rack. Two of the oscillators from Phase 2 were incorporated into Group B. The installation of the Abinger clocks was completed in October 1946 and the Greenwich clocks in November 1947.


Phase 4 (commenced 1950)

Upgrading of Phase 3 installation at Abinger by separating oscillators onto separate brick piers, modifying electrical circuity and replacing GT-cut crystals with Essen Ring Crystals. Group A at Greenwich was also rebuilt using the original GT-cut crystals retained and renamed as H1 & H3. Although the general practice was to rename the clocks when new crystals were substituted for existing ones, this was does not seem to have been the case with the later alteratins in the B and D groups.


Phase 5 (commenced 1951)

Construction of experimental Clocks at Abinger. These allowed new clocks and circuits to be designed and tested against the existing. Designated as Clocks G1 – G 4. Also includes test on clock PTR VIII which was removed from Germany after the Second World War as part of the German reparations.


Phase 6 (commenced 1957)

Installation of clocks at Herstmonceux, these were rebuilt versions of the nine best clocks from Abinger. Designated  H11 – H 19, the clocks were not renamed if a crystal was subsequently replaced.


Phase 7 (commenced 1964)

Installation of 3 commercial oscillators by the American company Sulzer. Unlike the earlier clocks which relied on valves, these used solid state components.


Clocks from elsewhere identified as having contributed to the Greenwich Time scales
or been mentioned in a Report

NPL: Q2, Q6, Q13, Q25, Q32, (Rb5), Sulzer S40, (Rb33),
GPO: (IVA, IVB, IVC), 5B, 5C, 49A, 9B, 9C, EA ...

Post 1964 clocks remain to be added.

The publication of clock comparisons was discontinued from January 1965 onwards.


Illustrated histories of the quartz clocks and control equipment

The Quartz Clock, Q3, installed at Greenwich in 1939 and re-erected at Abinger in 1943

The Quartz Clocks A1, B1 & B2 installed at Abinger and Edinburgh in February 1944

The Quartz Clocks installed at Abinger and Greenwich from mid-1944 onwards

The Quartz Clocks installed at Herstmonceux


Complete list of quartz clocks owned by the Observatory

Explanation of columns

  • PSE = Phase and corresponds to the phases outlined above
  • Maker: GPO=General Post Office (UK); STC=Standard Telephone & Cables (UK/USA); NPL=National Physical Laboratory (UK); Sulzer=Sulzer (USA); PTR=Physikalische Technische Reichanstalt
  • Locations: A42=Abinger 1942 building;  A44=Clock Cellars in Abinger 1944 building,;  E=Royal Observatory Edinburgh;  G(F)=Flamsteed House Greenwich;  G(M)=Meridian Building Greenwich; H=Herstmonceux
  • Date: is the date of installaton or when the clock was set going
  • Name: brackets were not used in the Observatory's documentation, but are used here to indicate when a clock was given a new crystal but its name was unchanged
  • Cryst.: The serial number of the crystal is given where it has been identified. Serial numbers were designated by the maker/supplier, not the Observatory. The GT-cut crystals and Ring crystals supplied by the GPO had different sets of serial numbers.
2 A1(1) GT-cut GPO 318 E 1944 Crystal later reused in B3 at Abinger
3 A1(2) GT-cut GPO 94 G(M) 1947
A2 GT-cut GPO 90 G(M) 1947
A3 GT-cut GPO 315 G(M) 1947
2 B1(1)
GT-cut GPO 312 A42 1944
3 B1(2) GT-cut GPO 495 A44 1945/6
2&3 B2 GT-cut GPO 320 A42/44 1944 Rebuilt as B2 into new B group 1945/6
3 B3 GT-cut GPO 318 A44 1945/6 Reused crystal from A1(1) Edinburgh Clock
4 B4 GT-cut A44
4 B5(1) GT-cut A44
4 B5(2) Ring 35 A44 1953/4
4 B6(1) GT-cut A44
4 B6(2) Ring 40 A44 1953/4 Crystal later transferred to H11
3 C1 GT-cut GPO 447 A44 By 1946
3 C2 GT-cut GPO 448 A44 By 1946
3 C3 GT-cut GPO 444 A44 By 1946
4 C4 GT-cut A44
4 C5 Ring 30 A44 1951
4 C6 Ring A44 1951
3 D1 GT-cut 441 A44 By 1946
3 D2 GT-cut 442 A44 By 1946
3 D3 GT-cut 443 A44 By 1946
4 ?D4(1)
A44 ???
4 D4(2) GT-cut 1954/5 Contained Crystal from G2
4 D5(1) GT-cut A44 ???
4 ?D5(2) GT-cut A44 1954/5
4 D5(3) Ring 47 ?
4 D6(1) GT-cut A44 1953/4 four-point supported crystal
4 ?D6(2) Ring Planned in 1955 to use Crystal from E6
3 E1 GT-cut 645 A44 By 1946
3 E2 GT-cut 646 A44 By 1946
3 E3 GT-cut 652 A44 By 1946
4 E4 A44
4 E5 Ring GPO 17 A44 1950 May
Z-cut ring-crystal mounted in a W6 holder
4 E6 Ring GPO 69 A44 1950 As above, but installed a few months later
3 F1 GT-cut 91 G(M) 1947
3 F2 GT-cut 93 G(M) 1947
F3 GT-cut 324 G(M) 1947
5 G1 GT-cut STC A42 1951 Stopped Dec 1953 and rebuilt
5 G2 GT-cut A42 1952 Jul
Soldered wire supported Cryst. (new or E1 or E2?)1
5 G3 GT-cut A42 1953/4 Contained Crystal from B5(1)
5 G4 Ring 39 A42 1953/4
4 H1 GT-cut 94 G(M) 1953/4 Contained Crystal from A1
4 H3 GT-cut 315 G(M) 1953/4 Contained Crystal from A3
6 H11 Ring 40 H 1957 Feb
Contained Crystal from B6
6 H12 Ring 39 H 1957 Feb
Contained Crystal from G4
H13* Ring 35 H 1957 Feb
Contained Crystal from B5(2)
H14(1) Ring 69 H 1957 Contained Crystal from E6**
H14(2) Ring 30 H 1957 Contained Crystal from C5**
H15 Ring 47 H 1957 Contained Crystal from D5
H16 Ring 21 H 1957 Jul
Contained Crystal from E6
H17 Ring 17 H 1957 Contained Crystal from E5
H18 Ring H 1958/59 Contained Crystal from C6
H19 Ring 69 H 1961 Sep
Crystal from H14(1)
7 H20 Lenticular
Sulzer H 1964 Mar AT-cut
7 H21 Lenticular
Sulzer H 1964 Mar AT-cut
7 H22 Lenticular 
Sulzer H 1965 Nov AT-cut
5 PTR VIII PTR A42 1949 Acquired from Germany in 1945/6***
1 Q3 Ring NPL G(F)/A42 1939 Re-erected at Abinger Jan 1943

* Taken out of service in 1961 and its its ring-crystal and oven used in an experimental, prototype, transistorized oscillator
**H14(1) built using Crystal from E6 which developed a fault soon after and was replaced by the crystal from C5
*** Removed from Germany after the Second World War as part of the German reparations
1 Crystal transfered to D4(2) in 1954/5

One of the Sulzers appears to be serial number S40



Quartz crystal time signal transmitting clocks for the Royal Observatories. Post Office Engineering Department, Radio Report No.1295 (1945 )

Quartz Clocks for the Royal Observatory. Post Office Engineering Department, Radio Report No.1752 (1948)

Precision quartz vibrator (100 kc/s. Z-cut ring) supplied to the Astronomer Royal. Post Office Engineering Department, Radio Report No.1990 (1950)

Overhaul of a quartz crystal oscillator for the Royal Greenwich Observatory. Post Office Engineering Department, Radio Report No.2192 (1952)

Comparison of the Royal Observatory Quartz Clock with the Post Office Frequency Standard by Means of a 2000 kc/s Radio Transmission. Post Office Engineering Department, Radio Report No.1471 (1946)

Comparison of the Royal Observatory Quartz Clock with the Post Office Primary Frequency Standard by Means of a 2000 kc/s Radio Transmission, 1946. Post Office Engineering Department, Radio Report No.1625 (1947)

Quartz Clocks of the Greenwich Time Service. Humphry Smith, MNRAS (1953)

Quartz Crystal Clocks. Humphry Smith & G.B. Wellgatge, Vistas in Astronomy, Vol 1, pp.438-446 (1955)

Sulzer manual

Facilities and services in the Time Department of the Royal Greenwich Observatory. Humphry Smith (1973)

ING group

International Coordination of Atomic Time. Humphry Smith, Vistas in Astronomy (1985)



1939: Q3

1944, February: A1 at Edinburgh (Station B) and B1 & B2 at Abinger (Station A)

1944 - November 1947, 12 clocks at Abinger and 6 at Grennwich:

At Abinger:

B1, B2, B3 (B3 used the A1 crystal formerly at Edinburgh and B2 the B2 crystal from the early Abinger Clock)
C1, C2, C3
D1, D2, D3
E1, E2, E3

At Greenwich:

A1, A2, A3
F1, F2, F3


1950, E5 in May, E6 a few months later. Two new standards, EA and EB at Dollis Hill, which employ ring-crystals suspended by threads, have been brought into operation. The phenomenon of ageing is noticeably diminished with this form of crystal, and these clocks are already of great value in the operation of the Time Service.


1951, end: C5 & C6

B5&B6 first mentioned in 1953 report

1953 Report: experimental clocks: G1, G2, G3 (crystal of B5),


1954 Report:

B4, C4, E4 mentioned for the first time. H1 & H3 installed at Greenwich incorporating crystals from A1 & A3

1957 Report: Three quartz clocks were set up in cellars in the new West Building at Herstmonceux early in 1957 and are now running. These have been designated H.11, H.12, and H.13, and employ the ring-crystals previously at Abinger in clocks B.6, G.4, and B.5, respectively.

1958 Report: three additional operation clocks set going since the last report are:- H.14, H.15, H.16, incorporating the ring crystals used in the Abinger clocks E6, D5 and D6 respectively. The crystal in clock H.14 developed a fault, and was replaced by that formerly used in Abinger clock C5.

1959 Report: Two more quartz clocks have been installed during the year, H17 and H18, incorporating the ring crystals used in the Abinger clocks, E5 and C6 respectively.

1962 Report: Ring crystal No. 69 was sent to the G. P. O. for investigation; after a complete renovation it was installed as oscillator H19 in 1961 September. Oscillator H13 was taken out of service in 1961 September and its ring - crystal and oven are being used in an experimental, prototype, transistorized oscillator.

1967 Report: The last [H12?] and best of the quartz crystal standards designed and built by the RGO, which incorporated an Essen ring crystal was taken out of service in 1967 May after an oven failure, having been in use from 1954.