GB1327220A - Calculating instrument - Google Patents

Abstract

1327220 Calculating devices R H BROMLEY 6 Oct 1971 [15 Oct 1970] 49008/70 Heading G4B A calculating device comprises a disc 3 bearing on its upper surface a graduated spiral scale, Fig. 1, and a transparent cursor disc 1 bearing on its lower surface a spiral cursor line (shown on the right hand side of Fig. 1) of opposite hand to the spiral scale line. The form of the cursor line spiral is given by the solution of where r is the length of the radius vector from the axis to a point on the cursor spiral and α(r) is the acute angle the radius vector makes with the curve at this point. The graduations along the scale spiral are marked with values proportional to r cos α; this results in a uniform (linear) graduation. For any relative angular position of the discs 1 and 3, the scale and cursor spirals intersect orthogonally. Relative rotation of the discs between positions in which the cursor spiral intersects the scale spiral at first and second values is through an angle of relative rotation which is equal to k times the difference between the natural logarithms of the values. By adding or subtracting such angles of relative rotation, multiplication or division calculations may be made. The ends of the spiral scale are joined by a further line (not shown), having the same shape as a corresponding length of the cursor spiral, and an outer ring 2 bears an extension line of the cursor spiral. This further line and extension line are provided to enable the discs to be returned to datum positions relative to each other and to the ring 2, during the performance of calculations.