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This is a double sided rule made of German silver and glass by the J. Halden Co. Ltd. of Manchester. It comes with a booklet of instructions which tells us that: 'The Halden Calculex is a slide rule made up in circular form. The circular metal dial on which the scales are engraved consists of two pieces, an outer ring and an inner disc, the logarithmic scales being engraved on the inner edge of the outer ring and on the outer edge of the inner disc.' Halden produced the calculex slide rule from about 1910 to 1951 The Calculex is unique in not using a knob winder; instead the glass itself is rotaded . The purple velvet lined case measures 7 X 6.5 cm (2.7 X 2.5 inches) |
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