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Met. Office Pilot Balloon Slide Rule MK IV
This is a Mark IV Pilot Balloon Slide Rule with a date mark of '53 (1953) It was made of a heavy white plastic with a length of 24.5-in. and width of 2.5 inches. Exact dates of production are not known but the MK IV.A is known to have been in production as early as 1952. The printed scales are engraved on the white plastic face. The slide rule has 4 captive cursors - this example is missing two, that ride in slots on the top and bottom edges. Each can be moved independently of one another. Brass knobs at both ends of the slide facilitate its movement. The case wood with oiled natural wood finish. Follow this link for more detailed information on the Met. Office Mark IV Pilot Balloon slide rule
Pilot Balloon slide rules were made for the British Meteorological Office by different makers, this one was manufactured by James Farrow and Son. A later model MK IV.A was made by BRL- Blundell Rules Ltd. The MK IV and MK IV.A are functionally identical. The later MK IV.A is thicker and slightly wider. Its layout is identical to the IV.A rule except for the addition of a single reference mark ("SR. YDS" on the tangent scale at 1.85deg) on the IV.A.
Succeeding Mark IV.A, Mark V and Mark 5 Pilot Balloon slide rules were manufactured by Blundell Harling Ltd. and believed to be in production from 1964 until possibly as late as 1989. However there is no mention of a MK V rule in the "Met. Office The Measurement of Upper Winds by Means of Pilot Balloons Fourth Edition 1968 which leads to speculation that the MK V and MK 5 may have been introduced soon before or after 1968.