Parkinson's gun director

The invention of an electromechanical system for directing antiaircraft guns is described. The inventor, David B. Parkinson, was working on improving an instrument called an automatic level recorder, which used strip-chart paper to plot the logarithm of an applied, rapidly varying voltage. A critical component was a small potentiometer, which controlled a pair of magnetic clutches that in turn controlled the pen. After reading about the evacuation at Dunkirk in 1940, he had a dream in which his control potentiometer was mounted on an antiaircraft gun. Eventually more than 3000 of the gun directors, designated the M-9, were built. Simple arithmetic, differentiation, and integration were performed in the M-9 by 28 operational amplifiers (op amps). The output of the op amps went to three 50-cm-diameter cast-iron potentiometers, the output voltage of which in turn drove the gun controls. The M-9 brought the number of shells fired to bring down a single aircraft down from thousands to about 100