Patent GB-650718-A (GB)
Title: Improvements relating to electrical computing instruments
IPC: G06G7/22
Filed: 1947-05-21
Abstract: 650,718. Electric calculating apparatus. FERRANTI, Ltd., and WALKER, D. F May 21, 1947, No. 13648. [Class 40 (i)] An electric computor for solving triangles comprises a uniform resistance 1, Fig. 1, with a variable control (tapping 3), and a centre tap 2, variable tappings 4, 5 ganged to a second control to maintain them equidistant from the centre tap 2, being connected by respective equal impedances 7, 8 to a common lead 6, a source of supply 9 connected across the tapping 3 and lead 6 to produce voltages v1, v2 across the impedances 7, 8 respectively, means whereby a voltage proportional to ¢ (v1+v2) is applied to a cosine potentiometer 14 having a third control (tapping 15) moved in accordance with the angle R and yielding an output voltage v3=¢ (v1+v2) (1-cos R), means for applying the difference voltage (v3 - v2) to a potentiometer 17 having a fourth control (tapping 18) for deriving an out-of-balance current which through a transformer 20 and amplifier 24 operates a motor 25 for adjusting one of said controls when the other three are preset, to the correct value conforming with the triangle represented by the value to which the preset controls are adjusted. For determining the side q, Fig. 1, in the triangle P, Q, R, when p, R and (r - q) are known, it can be shown that The tapping 3 is adjusted so that the resistance between the tappings 2, 3 is proportional to (r - q) on a scale in which the resistance between tap 2 and common lead 6 either by way of impedance 7 or 8 corresponds to the base length p, so that v1 and v2 are proportional respectively to Voltage v1 is reversed in polarity by a 1 to 1 auto-transformer 11 so that (v1+v2) is applied to the cosine potentiometer 14, the centre tap 16 of which corresponds to 90 degrees at zero position and the ends to 0 and 180 degrees respectively. The voltage between the centre tap 16 thereof and the tapping 15 moved in accordance with R is thus ¢ (v1+v2) cos R, and the voltage v3 between the tapping 15 and the right-hand end of the cosine potentiometer 14 is ¢ (v1+v2) (1-cos R). This voltage subtracted from v2 is proportional to 1/q and is applied to potentiometer 17. The fractions of the potentiometers 17, 22 between lead 6 and the tappings at 18, 21 being s, t respectively, then, when the motor 25 has driven tap 18 to achieve balance, i.e. no current flow in transformer 20, s/q=t.v where v is the supply voltage 9. Hence t being fixed s is proportional to q and the potentiometer 17 may be graduated to read the value of q. The potential of point 21 may alternatively be fed direct to tapping 18, the primary 19 being included in the lead from tapping 15 to a fixed tapping on the potentiometer 17 between tapping 18 and lead 6. The centre-tapped potentiometers 1, 14 allow positive and negative values for (r - q) and R. If, alternatively, the three sides of the triangle are known and the base angle R is to be determined, contacts 3, 18 are set by hand or continuously adjusted by the associated apparatus to represent (r - q) and q respectively and the balancing motor 25 arranged to adjust the tapping 15 on the cosine potentiometer 14 to indicate R. If p, q, R are known, control tappings 15, 18 are set accordingly and the motor adjusts tap 3 to indicate (r - q) from which the required r is obtained. The base p has been assumed fixed in the above examples but it may be continuously varied by the second control operating the ganged tappings 4, 5, Fig. 8 (not shown). The resistors 7, 8 may be replaced by chokes, Fig. 3 (not shown), and errors introduced by the reversing transformer 11 may be compensated by a correcting winding thereon, Fig. 4 (not shown), or by a resistance, Fig. 5 (not shown), connected across the points 4, 5. The reversing transformer may be dispensed with by replacing the impedances 7, 8 by transformer primaries, Fig. 6 (not shown), and connecting the cosine potentiometer 14 across their secondaries. In another embodiment, Fig. 7, the impedances 7, 8 of Fig. 2 are replaced by oppositely disposed cosine potentiometers 30, 31 having their variable tappings 32, 33 ganged so as to be moved in opposite directions by a control 34 so that both tapping positions are always appropriate to the angle R the cosine of which is proportional to the distance of each tap from the associated centre tap. The two tappings 32, 33 are connected by the primary 37 of a transformer the secondary 39 of which is connected across potentiometer 17. The operation is the same as for Fig. 1 except that the ganged controls 32, 33 are adjusted in lieu of the tapping 15.