Polar planimeter in case, Allbrit, steel, /plastic / glass / leatherette, W.F. Stanley, London, mid-20th century.
This object is an Amsler-type polar Planimeter in a blue velvet lined leatherette case. A supporting arm is anchored to the work table with a short cylinder having a sharp point like a compass needle. It is linked to the tracing arm with a ball headed pin that forms an elbow hinge. The tracing arm has at one end a carriage consisting in a pair of supports that hold between them the accumulator wheel and vernier. This carriage may be positioned along the tracing arm for the scale of the curve being measured. The accumulator wheel is the moving part that touches the table. At the tracing end the arm has a magnifying glass cursor. As the pointer is moved around the perimeter of the curve whose area is being measured the steel rim rolls forwards or backwards and skids sideways. The scale is on the wheel itself, and indicates how far the wheel has turned. The final dial reading represents the sum of all the rim's rolling motions. This number is proportional to the area of the region.
The accumulator wheel and vernier carriage has the instrument number: 38870. The tracing arm has a scale marked from 21 to 68 in fine gradations of ten marks between each numbered scale. The case has a calibration chart glued into its lid.