1943 Cooke, Troughton & Simms Tavistock theodolite

The 1943 Cooke, Troughton & Simms Tavistock theodolite.
Figure 1: The 1943 Cooke, Troughton & Simms Tavistock theodolite.
'I Expect that some of us are unaware of the significance of the name "Tavistock Theodolite"; so I had better make it clear at the outset that the name applies to certain theodolites that are now being produced to a specification drawn up as the result of a meeting of British instrument makers and surveyors at Tavistock in 1926.'1 This is how an article from 1929 by E.R.L. Peake starts describing this instrument.

Peake continued:
'Since the war certain Continental instrument makers had put on the market new designs2 that appeared to be a great advance on the models we had always been accustomed to. The man largely responsible for these go-ahead ideas was Heinrich Wild. He had had an extensive experience as a surveyor in the mountains of his native country of Switzerland, and I have no doubt that the labour of "humping" heavy instruments about had set his mind to work on how he could cut down weight.

It was in 1905, while he was with the firm of Zeiss, that he endeavoured to improve on the idea of producing a theodolite, so constructed that it would not be necessary for an observer to leave his place at the telescope to read micrometers on either side of the instrument, but he would be able to observe two opposite portions of the circles at once with a single eyepiece and read them by means of a single-screw micrometer. ...

Though it will probably be some time before surveyors have complete confidence in instruments so revolutionary in design as the Wild, it is fairly certain that as time goes on there will be an increasing demand for them, not only by the Services but by those dominions and colonies where labour is expensive. It was for these reasons that it was decided to give a Wild theodolite a trial at Tavistock in 1926 and to compare it with existing British models. Officers from the Admiralty, the War Office, and the Ordnance Survey were present, and the opportunity had been taken of inviting the leading British instrument makers to send representatives in order that they might join in the discussion.


The 1943 Cooke, Troughton & Simms Tavistock theodolite from the other side.
Figure 2: The 1943 Cooke, Troughton & Simms Tavistock theodolite from the other side.
As a result of this meeting a specification for a British theodolite was drawn up, as we were all convinced that an all-British instrument could be produced that would combine the latest ideas in design with the well-known qualities of strength and reliability possessed by the existing types of British make. The chief difficulty the makers had to contend with was in producing an instrument that would not transgress existing patents. Messrs. Cooke, Troughton & Simms, Ltd., have succeeded in doing this, the optical arrangements of their model and the method of reading it being different in every respect from existing patterns. Messrs. E. R. Watts & Son, Ltd., on the other hand, came to an agreement with the firm of Zeiss and have incorporated in their model several of the Zeiss patents; thus it can hardly be said that theirs is an all-British instrument. So far nothing has materialized from the third firm of instrument makers, Messrs. C. F. Casella & Co., Ltd...'

Features1
According to Peake the Cooke, Troughton & Simms prototype for the Tavistock was a '...3½ inch Double-Reading Theodolite. The horizontal circle is 3½ inches in diameter and is flat divided on glass to 20'; the vertical circle is 2¾ inches in diameter and similarly divided.
The images of opposite sides of each circle are brought together and are viewed through a single eyepiece situated near the eye-end of the telescope. Readings are taken by means of a patented diagonal scale direct to 10", and by estimation to 5" or even 2½". The change over from horizontal to vertical circle or vice versa is made by means of a lever on the standard. ... The horizontal circle is carried on an independent axis with its own clamp and slow motion for repetition.
The telescope is of the "no constant" type and can transit at both ends even with the diagonal eyepiece in position. The focussing, which is internal, is actuated by a milled collar on the telescope. The object-glass aperture is 1.5 inches and the power 22.


The 1943 Cooke, Troughton & Simms Tavistock theodolite with illumination installed.
Figure 3: The 1943 Cooke, Troughton & Simms Tavistock theodolite with illumination installed.
The plate bubble has a sensitivity of 20" per 2 mm. division, and the altitude bubble 10" per 2 mm. division. The latter is fitted with a device to enable it to be read from the telescope eye-end in any position.3
The Tribrach is fitted with three levelling screws with dust covers.
The illumination of the circles ..., altitude bubble ..., and diaphragm ... for night use, is effected by standard 4-volt lamps permanently fixed but easily accessible for replacement. A switch on the cover plate controls each light separately, with an extra stud for illuminating both circles together. This switch is wired to terminals on the tribrach for the battery connection. A rheostat ... is fitted for controlling the diaphragm illumination.'


Above description fits the original prototype presented in 1929, But the '...method of reading [of the circle] has not received universal approval, and the makers are now at work on a second model, which, though built on the same general lines as the earlier one, incorporates a method of reading invented and patented by Captain T. Y. Baker, b.a., r.n., which is very ingenious.'1 Baker's method however needed a complete redesign of the reading microscopes and thus resulted in a complete new instrument, the one shown here, and which was launched in 1931.4

Accuracy
The circles remained the same as in above described prototype. Thanks to Baker's new circle reading method the circles can now be read directly to 1 arc second and estimated to tenths of an arc second (see figure 26 and figure 27).5
The plate vial on the horizontal circle of the instrument shown here (see figure 6) has a sensitivity of 40 arc seconds per 2 millimetres run as described in the Cooke, Troughton & Simms CV400A brochure,5 while another Tavistock that I have seen, that was despatched in 1945 (two years after the one shown here), was equipped with a 30 arc second per 2 millimetres run plate vial.
The coincidence vial has a sensitivity of 23 arc seconds per 0.1 inch (see figure 14) as indicated in the brochure,5 while at the 1945 version this had been increased slightly to 20 arc seconds per 0.1 inch.
It seems that Cooke, Troughton & Simms was experimenting with these parts ever since they designed the instrument.

The instrument
With serial no. 41274 the Tavistock shown here dates from August 1943. It was part of a large order of 228 Tavistock Theodolites with tripods T95 despatched to the Ministry of Supply (Contracts Dept.), Great Westminster House, Horseferry Rd., Westminster SW1 on 5/8/1943.6

When the ministry was set up under the Ministry of Supply Act 1939 it became responsible for the administration of the Royal Ordnance Factories and for design, inspection, research and experimental work in connection with the supply of munitions, clothing and other stores to the War Office and Air Ministry. It was also responsible for the supply of certain common stores to the Admiralty, Ministry of Home Security, Office of Works and subsequently to other government departments.7

How it ended up in the United States is still unclear, but it may have been sold during one of the sales of R.A.F. surplus stores in 1949.8

The instrument is remarkably complete and came with its original metal case (see figure 18 and figure 19) which can be carried as a rugsack (see figure 20). When it arrived it was in a sorrow state with broken and bend knobs and a lot of friction in the primary and secondary axis and the scale microscopes. All parts were however in the box, while cleaning and lubricating a Tavistock is relatively easy when compared to the Wild instruments. It took about a whole day to get the instrument back into working order again and now stands proud next to the instruments that it originated from (see figure 4).

Notes

[1]: E.R.L. Peake, 'The Tavistock Theodolite: A paper read at the Afternoon Meeting of the Society on 11 February 1929', in: The Geographical Journal, Vol. 73, No. 6 (Jun., 1929), (1929), pp.513-519.
[2]: These of course were the Zeiss Th1 and archetype Wild T2.
[3]: The Tavistock shown here has a plate vial with a sensitivity of 40" per 2mm (see figure 6) and an altitude vial of 23 arc seconds per 0.1 inch (18 arc seconds per 2 millimetres), see note 4 and the section on accuracy.
[4]: E.W. Taylor, 'The 'Tavistock' transit theodolite', in: Transactions of the Optical Society, Vol. XXXII, 1930-31, No.2., (1931), pp.45-60.
[5]: Cooke, Troughton & Simms, Tavistock Theodolite: Publication No. CV400A, pp.3,4,15.
[6]: Cooke, Troughton & Simms, Theodolite Despatch Book (1936-1946), VI/CTS/5/1/3/3 page 287. With many thanks to ms. Alison Brech, curator of the Vickers Collection at the Borthwick Institute.
[7]: See the Records created or inherited by the Ministry of Supply and successors, the Ordnance Board, and related bodies of The National Archives.
[8]: 'Sales of R.A.F. surplus stores', in: Flight, May 26th, 1949, p.629.

If you have any questions and/or remarks please let me know.

The Tavistock compared to the Zeis Th1 (left) and archetype Wild T2.
Figure 4: The Tavistock compared to the Zeis Th1 (left) and archetype Wild T2.
 
Rough aiming can be done using two sets of peep-sights.
Figure 5: Rough aiming can be done using two sets of peep-sights.

The 40" plate vial and indication 'Tavistock' on the theodolite.
Figure 6: The 40" plate vial and indication 'Tavistock' on the theodolite.
 
The serial indicates this Tavistock theodolite was made in 1943.
Figure 7: The serial indicates this Tavistock theodolite was made in 1943.

The ocular of the Tavistock theodolite has two grades of shades.
Figure 8: The ocular of the Tavistock theodolite has two grades of shades.
 
The circle microscopes have a single green shade.
Figure 9: The circle microscopes have a single green shade.

The Tavistock comes with two straight oculars.
Figure 10: The Tavistock comes with two straight oculars.
 
The Tavistock also came with its original zenith ocular.
Figure 11: The Tavistock also came with its original zenith ocular.

The Tavistock comes with two diaphragms, this is the webbed one.
Figure 12: The Tavistock comes with two diaphragms, this is the webbed one.
 
The etched diaphragm with the tool to insert it.
Figure 13: The etched diaphragm with the tool to insert it.

The Wild type coincidence vial of the Tavistock theodolite.
Figure 14: The Wild type coincidence vial of the Tavistock theodolite.
 
The little knob locks the circle microscope in its vertical position.
Figure 15: The little knob locks the circle microscope in its vertical position.

The original lens cap of the Tavistock (and sun shade next to it).
Figure 16: The original lens cap of the Tavistock (and sun shade next to it).
 
The original sun shade of the Tavistock (and lens cap next to it).
Figure 17: The original sun shade of the Tavistock (and lens cap next to it).

The Tavistock theodolite in front of its original case.
Figure 18: The Tavistock theodolite in front of its original case.
 
The Tavistock theodolite in its original case.
Figure 19: The Tavistock theodolite in its original case.

The container has straps to carry it like a rugsack.
Figure 20: The container has straps to carry it like a rugsack.
 
The base of the Tavistock has a chord to suspend the plumb bob from.
Figure 21: The base of the Tavistock has a chord to suspend the plumb bob from.

The pouch with plumb bob, tools, and etched diaphragm.
Figure 22: The pouch with plumb bob, tools, and etched diaphragm.
 
The pouch with all the illumination and oculars.
Figure 23: The pouch with all the illumination and oculars.

A look through the telescope fitted with the etched diaphragm.
Figure 24: A look through the telescope fitted with the etched diaphragm.
 
A look through the telescope fitted with the webbed diaphragm.
Figure 25: A look through the telescope fitted with the webbed diaphragm.

The vertical circle reading 97°-10'-23.8''.
Figure 26: The vertical circle reading 97°-10'-23.8''.
 
The horizontal circle reading 209°-24'-44.2''.
Figure 27: The horizontal circle reading 209°-24'-44.2''.

Surveyor's crosses Geodetic Sextants Theodolites Total Stations Levels Standards Tools Firms
19th C. SDL 1919 K&E 1926 Zeiss RThII 1924 Zeiss Th1 1929 Wild T2 1937 Wild T3 (astronomic) 1939 Wild T3 (geodetic) 1943 CT&S Tavistock 1948 Wild T1 1952 Wild RDH 1956 Wild T0 1961 Wild T1A 1961 Wild MIL-ABLE T2 1962 Wild T2 1963 Wild RDS 1966 Kern DKM2 1969 Wild T2E 20th c. Askania Tu400 1990 Wild T2 mod