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A Simple and Accurate Method of Deriving the Slope of a Graphed Function
Journal of Scientific Instruments, (1949)
  • Edmund Ramsay Wigan, BBC Research Department
The following method has been developed to obtain the tangents to a curve accurately and rapidly with no more than a i/4-3/ diameter cylindrical rod of Perspex or glass used as a lens. The rod is laid roughly tangentially to the curve near to the point P at which the slope is required. It is then rolled over the paper till the image of P appears on the axis of the rod. The rod is twisted in the plane of the paper until the image of the part of the curve near P lies exactly along the axis of the rod. The magnification of the cylindrical lens ( x 5 or 6) increases the accuracy v i t h which this can be done. But a further check is obtained if a short pencil stroke is made across the curve at P:this, viewed through the rod, will appear as a line almost exactly normal to the rod axis, however carelessly the stroke is drawn. This line, therefore, gives a secondary criterion for setting the slope of the rod: the image of the stroke must appear to cut the image of the curve at right angles.
  • tangent determination
Publication Date
Citation Information
Edmund Ramsay Wigan. "A Simple and Accurate Method of Deriving the Slope of a Graphed Function" Journal of Scientific Instruments, Vol. 26 Iss. 5 (1949) p. 162
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