The Post Instrument was called ‘The Observer Instrument Mk 2A, made by R.B. Pullin & Co., originally in 1935. It was designed by an Engineer named Captain Ball who lived in Hampshire. Contrary to some writings, the instrument was generally called ‘The Post Plotting Instrument’ and not ‘The Micklethwait’. Also, the Micklethwait Operating Manual, sometimes referred to in post-war years, never existed. Part of the instrument was called ‘The Micklethwait Height Correction Attachment’ and was only a small part of the Post Instrument. This attachment allowed for heights to be corrected between two posts plotting the same aircraft. The ‘Micklethwait’ was actually invented by an Observer Micklethwait !
Eric Walter Eustace Micklethwait was born in 1906 and at the age of 33 years, in 1939, because he was in a reserved occupation, he joined the Observer Corps. Obs E W Micklethwait served on K1 Gower Street Post in No 17 Group Watford. Gower Street Post opened in 1939 and was located on the roof of the GPO Parcels Office at Mount Pleasant. However, in November 1939 the post was re-sited to London University roof. Obs Micklethwait left the Corps sometime before Stand-Down in 1945. In private life he was a patent agent in the early part of the war and was later employed by the Ministry of Supply and also the School of Tank Technology at the Military College of Science. He devised the ‘Micklethwait Height Corrector’ in 1940 and it was fitted to all subsequent Post Instruments. The attachment consisted of a second cranked grid pointer attached to a sliding height scale. Eric Micklethwait died in 1986. Rather strangely there was another Observer on the Gower Street Post named Micklethwait but with the initials - ‘R. G’.
The Correction Height Procedure was as follows :-
1. An observer estimated the height of an aircraft and set the height on the height bar.
2. The aircraft was sighted and the grid position shown by the instrument pointer was reported to the Ops Room. It should be noted that the aircraft grid position was only as accurate as the accuracy of the estimated height. It was necessary for at least one other post to carryout this procedure as well.
3. Using cross plotting the Ops Room plotter could ascertain the true position of the aircraft and he would quickly inform the posts of this grid position.
4. The post observer would then slide the Micklethwait cranked pointer on to the true grid position and read off the true height of the aircraft on the Micklethwait Height bar. This would then be set on the instrument height bar for all subsequent plots.