431,961. Calculating-apparatus. JACKSON, L. MELLERSH-, 28, Southampton Buildings, Chancery Lane, London.-(Felt & Tarrant Manufacturing Co.; 1735, North Paulina Street, Chicago, U.S.A.) June 19, 1934, No. 18128. [Class 106 (i)] In a key-responsive, power-operated machine for the four rules an error control means is provided for locking the keys against depression upon an incompleted key stroke being made. The lock is effective in all the banks of keys except that in which the key is partially depressed. Complete depression of the operated key results in the locking of that bank and all the keys remain locked until a special key is depressed to release the locking means. The link 145, Figs. 1 and 2, of the parallel motion device of the differential actuating mechanism is provided with a latch 393 adapted to cooperate with a ratchet tooth 397, Fig. 3, on a member 398 arranged on a lever 399 that is pivoted to a yoke 402 mounted on a cross rod 403. Upon depression of a key the link 145 is moved towards the front of the machine and the latch 393 moves with it past the tooth 397. If the key is not completely depressed the rearward movement of the link will cause the pawl to rock the lever 399. A link 409 attached to the lever 399 will be pulled to the rear and by the action of a projection 417 thereon engaging an arm 418 will rock a shaft 419. This shaft is provided with an arm 421, Fig. 6, that cooperates with an arm 424 on a shaft 425 carrying a number of projecting pins 426, one for each column of differential actuators, extending over the tails of locking dogs 428 adapted to cooperate with shoulders 431 on the parallel motion bars 138, 234 of the multiplicand and multiplier key banks. When the shaft 419 is rocked the above described mechanism is operated to throw the locking dogs 428 into engaging position except in that column in which the key was depressed. In this column the link 145 is prevented from returning fully by the latch 393 engaging the tooth 397 and by an arm 415 pivoted to the link 409 engaging a pin 430. Complete depression of the key allows the parts of the actuators to return to normal position whereupon the dog 428 of the column affected moves to locking position. When a full key stroke is accomplished the tooth 397 is moved out of the path of the latch 393 by the action of a roller 433 on the link 121 of the actuator engaging an arm 432 of the yoke 402 to raise the yoke and lever 399 out of effective position. At the same time a rearwardly extending arm 434 on the yoke is moved to engage a shoulder 435 on a latch lever 407 to hold the lever 399 in raised position until near the end of the return movement of the link 145 whereupon a lug 436 on the link meets the end of the lever 407 which is rocked to free the lever 399 which returns to normal under spring action. Redepression of the mis-operated key is prevented before the completion of the previous operating stroke by providing the member 398 with a number of ratchet teeth 437 arranged in a position to be engaged by a latch 438 on the link 145. As the lever 399 is raised on the key being fully depressed an attempt to depress the key again before the adding operation is completed will cause the latch 438 to engage the ratchet teeth. In order to release the locked keyboard a key 451, Fig. 7, is provided depression of which rocks a lever 454 that co-operates with a projecting pin 457 on the shaft 425 which is rocked to restore the looking dogs 428 to inoperative position. The key 451 is raised when the shaft 425 is again operated in the direction of locking movement.