Patent GB-657051-A (GB)
Title: Improvements in and relating to mathematical calculators
IPC: G06G1/00
Filed: 1947-07-18
Abstract: 657,051. Calculating-apparatus. BRITISH THOMSON-HOUSTON CO., Ltd., and SHURMER, H. V. July 18, 1947, No. 19339. [Class 106 (i)] A device for graphically calculating the candle power of an illuminant consisting of a light source and a reflector, is used upon a scale drawing of a section of the illuminant and comprises three linear oc-planar scales at angles to one another called (1) the " scale of angles," (2) " scale of reflector candle power and (3) "scale of source candle power respectively. Scale (1) is based on the equation d=r cosec # , where r is the radius of the section of the source, 2 # the angle over which an elemental strip of the reflector (extending at right angles to the section) appears bright, and d the distance from the centre of the light source to the reflector strip. Scale (2) gives the total candle power due to the reflector in a given direction which equals n.k.Cs/2r when Cs in the candle power of the source (assumed the same in all directions in the plane of the drawing), k the reflection factor (assumed constant for all angles of incidence) and n the length of the projection, perpendicular to the chosen direction, of the effective area of the reflector. Scale (3) gives the total candle power due to direct light from the source which equals m.Cs/2r, where m is the effective width of the source. As shown in Figs. 5 and 6, the light source is a fluorescent tube 3 placed at the focus of a parabolic reflector 4, and the calculating device comprises a plane right-angled triangular member with the linear scales (1), (2), (3) along its edges xy, xz and zy respectively, and a protractor scale within the right angle. In use, the device is placed on the sectioned drawing with the point x at the centre of the light source. The edge xy is then rotated about this centre until # 1 <SP>0</SP> degrees on the scale (1) lies on the reflector, at a point Q say, where #  1  is the angle the chosen direction makes with the axis of the parabola, Fig. 5. The point Q is then the extreme point at which the reflector is bright in the chosen direction, so that the effective area of one half of the reflector is from D to Q. A line SQT is drawn in the chosen direction and a perpendicular DE is drawn to this line. The length DE corresponds to the required reflector candle power which is then read off on the scale (2). If the edge of the reflector nearest the source is at D<SP>1</SP>, Fig. 6, so that the tangent FG in the direction # 1  cuts D<SP>1</SP>E at F the effective area of the reflector will correspond to FE and not D<SP>1</SP>E. Since the source is completely visible in the chosen direction the total source candle power will be read off on scale (3). If the source were partly obscured as shown in Fig. 3, the effective width KL is measured against scale (3) to give the source candle power. The total candle power for the chosen direction can then be calculated, and the process repeated for other directions thus obtaining points on a polar curve. In the above, it is assumed that r, k, and Cs are constants. To enable the calculating device to be used in problems with different values for these quantities, detachable interchangeable scales 7 may be used, Fig. 7, which are held by clips 8 in the recessed edges of the triangular member 5. The method of using the device when the reflector is not parabolic is also described.