Bygrave slide rule

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{Bygrave A.M.L. Position line slide rule Mk. IIA serial No. 355}
{Bygrave A.M.L. Position line slide rule Mk. IIA serial No. 355}

The Bygrave slide rule is a slide rule named for its inventor, Captain L. G. Bygrave of the RAF. It was used in celestial navigation, primarily in aviation. Officially, it was called the A. M. L. Position Line Slide Rule

It solved the so-called celestial triangle accurately to about one minute of arc and quickly enough for aerial navigation.

It was developed in 1920 at the Air Ministry Laboratories at Kensington in London and was produced by Henry Hughes & Son Ltd of London until the mid-1930s.

The slide rule was constructed as two concentric tubes with spiral scales, like the Fuller slide rules, with yet another tube on the outside carrying the cursors.

The solution of the celestial triangle used the John Napier rules for solution of square-angled spherical triangles.

During the Second World War, a closely related version was produced in Germany by Dennert & Pape as the HR-1 (for the Luftwaffe), MHR-1 and HR-2 (both Kriegsmarine).