Photoelectrically Balanced Recording Potentiometer

Abstract

The problem of stopping a moving light‐beam on the edge of a phototube without any tendency to oscillate or ``hunt,'' and with speed and precision has been solved by a new method and a recording potentiometer is rapidly and accurately balanced. By (1) artificially canceling the generated back e.m.f. of the moving galvanometer thus advancing its return movement from a deflected position, and (2) delaying the grid potential change of a triode amplifier in the photoelectric circuit, galvanometer lag is counteracted and the potentiometer is balanced with an accuracy of a few hundredths of a percent of the range. The total power input to the instrument is about 35 watts and the contact carriage speed is made as high as 10″ in 20 seconds.¹ The fundamentals of the complete instrument are discussed and equations describing the galvanometer movement are included.