ELECTRICAL MEASURING AND CONTROL APPARATUS AND INSTRUMENTS TO SUPERVISE AND CONTROL PHYSICAL CONDITIONS SUCH AS PRESSURE, [ VELOCITY, FLOW, MIXTURES, TEMPERATURES, ] POSITIONS, MOTIONS AND FORCES, AND COMPONENT PARTS THEREOF-NAMELY, DEFFERENTIAL TRANSFORMERS THEREFOR, AMPLIFIERS THEREFOR, [ INDICATORS, RECORDERS, AND CONTROLLERS, ] SUCH APPARATUS BEING DESIGNED FOR BOTH SCIENTIFIC USE AND IN INDUSTRIAL PLANTS