E. R. Watts & Son. Ltd., London MK IV Balloon Theodolite.
This is the 4th version of an instrument in a series of theodolites that was the standard in the meteorological services of the British Commonwealth. They were designed at the Meteorological Office in collaboration with E. R. Watts & Son LTD. 5 different models were produced Mk I through MK V. Mark III through Mark V Models have detachable battery cases, and fixed focus telescopes. The models were produced as successive improvements over earlier models. In common with other land based balloon theodolites it has a telescope with a bent optical axis. A secondary wide-angle telescope using the same eyepiece selectable with a mirror. Follow this link for a more detailed description and Cutaway Illustration of a Watts MK I Balloon Theodolite.
Like the vast majority of balloon theodolites the image in inverted when viewed in a Watts theodolite. All Watts Balloon theodolites feature micrometer drums that drive the circles with tangent screws. The tangent screws and micrometers can be disengaged for rapid motion and positively reengaged for accurate tracking. I do not know the production dates of the MK IV series. I have an example produced in 1943. The Mark V is not mentioned in a text from 1961, so it can be assumed that the production run was at least 1943 to 1960 for this instrument.
These theodolites incorporate a lined gradicule to support the tail method of altitude determination. The spacing of the gradicule lines was changed during the evolution of the theodolite series. Slide rules manufactured as early as 1941 (and possibly much sooner) have notes explaining two procedures for calculations based on two gradicule line spacings. A Met. Office Instruction sheet dating May 1927 contains instructions for changing the length of the tail to match the line spacing of the gradicule, but I believe that this is to allow for small variations in the gradicule not, the change in manufacturing.
| Drive Type | Positively Engaging Tangent Screw | Compass | No |
| Display Type | Micrometer .1 degree | Levels | 1 tribrach base plate, 1 Inst, body bubble tube |
| Main Telescope | 40 mm 16x 2deg. | Mounting | Tribrach |
| Finder Telescope | 12.5mm
4.5x 8deg. single eyepiece |
Illumination | 2, 3
Volt lamps, scales reticule |
| Gun Site | Yes 1, on finder telescope | Weight (theodolite only) | 8.4 kg |