760,336. Comparing apparatus; indicating and recording apparatus. NATIONAL RESEARCH DEVELOPMENT CORPORATION. Oct. 22, 1954 [Oct. 23, 1953], No. 29322/53. Classes 106 (1) and 106 (4). [Also in Group XL (b)] An electrical code comparator for two sets of digit signals representing two numbers coded in a cyclic permuting binary code comprises means for determining which of the two numbers, in coded form, contains a " 1 " in the most significant place at which disparity between the digits occurs, and counting means which provides an output if the number of pairs of " 1 " digits, in places of significance above that at which disparity occurs, is odd. General.-The device described translates the readings of an instrument recorded in a cyclic permuting binary code as holes in transverse lines on a tape 1, Fig. 1, into decimal indications of a counter 8. The counter is coupled to a tape 5 having the code values recorded in consecutive order, and a further " calibration " tape 9 has recorded thereon in the same code successive instrument readings for which a change in the calibration error occurs, and on a line adjacent each such reading an indication of the direction of change. The tapes are read by photo-electric heads 3, 6, 10 associated with polygonal rotatable scanning mirrors 7, 11 so that each line of holes is translated into a series of timed electrical pulses. The head 3 has a photo-electric cell on each side whereby a further pulse is obtained at the beginning and end of each scan. In operation, the tapes 5, 9 are first set with a datum representation (e.g. 0) in front of heads 6, 10, the counter 8 being correspondingly set. The data on tapes 5 and 1 are then scanned by mirror 7 and compared, and the tape 5 and therefore counter 8 moved until comparison agreement is obtained. Similarly the tapes 1 and 9 are scanned by mirror 11 and the tape 9 moved until the difference between the readings on the tapes changes sign, the error increments on tape 9 causing a further movement of counter 8. The corrected counter reading may be printed on paper. The tape 1 is then moved to the next reading and the operations are repeated. Motion control mechanism.-The tapes, mirrors and counter 8 are driven by a motor 20, Fig. 2, through clutches 21-28, e.g. as described in Specification 699,367, [Group XXIV], and differential gearing. Engagement of clutches 21, 22 produces forward and backward movement respectively of tape 1, Fig. 1, through differential gear 29 and shaft 30. Clutches 23, 24 and 25, 26 similarly produce forward (i.e. in the direction of increasing values) and backward movement of tapes 5 and 9 respectively and corresponding rotation of mirrors 7 and 11. The shaft 33 for tape 5 drives also the counter 8 through shaft 34 and differential gear 35. Clutches 27, 28 produce the further calibrationerror-correcting movement of the counter. Comparing and control circuits.-The circuit (Fig. 3, not shown), for comparing the recordings on tapes 1 and 5 or tapes 1 and 9, and controlling clutches 23, 24 or 25, 26 accordingly, comprises an arrangement of bi-stable trigger circuits, gates and other elements similar to those described in Specification 717,114. The digit pulses from the reading heads are temporarily stored on triggers (to allow for possible lack of coincidence between corresponding pulses) which control the setting of a trigger 51 (not shown) to one state or the other according to which of the compared coded numbers has a " 1 " in the most significant position in which disparity occurs. A further trigger 55 (not shown) is switched by each pair of " 1 "s above that position so as to indicate whether such number is even or odd. In the cyclic permuting code employed, the greater number has either a " 1 " in the disparity position and an even number of higher " 1 "s, or " 0 " and an odd number. The triggers 51, 55 condition a gating " mesh circuit (Fig. 4, not shown) accordingly, whereby it produces an output in response to an end-of-scan pulse, whenever a change of direction of tape 5 or 9 is required. This output switches over, with delay, trigger tubes 56, 57 (not shown), e.g. as described in Specification 745,833, which conduct alternately and control reverse and forward movement respectively. For forward movement, digits are scanned from the most to the least significant, and tube 57 opens a gate to cause the input circuits for triggers 51 and 55 to be inhibited after the first disparity in digits. For backward movement, tube 56 opens a gate to cause trigger 55 to be reset for each disparity, so that the odd-even count is recommenced. When during the comparison of tapes 1 and 5, no disparity is detected, a trigger 73 (not shown) is operated by the next end-of-scan pulse to switch control over to clutches 25, 26 for commencing the comparison of tapes 1 and 9. When the mesh circuit then produces an output, it initiates the end of an operation, the trigger 73 being reset, and tape 1 stepped on. The end-of-scan pulses are employed to switch the data from alternate lines of tape 9 to the comparing circuit and an auxiliary circuit (not shown) which controls clutches 27, 28.