26 ELECTRICAL MEASURING AND INDICATING INSTRUMENTS [ ELECTRONIC COUNTER AND TIMER INSTRUMENTS, INSTRUMENTS AND APPARATUS FOR DETERMINING HUMIDITY AND THE MOISTURE CONTENT OF MATERIALS; THERMOMETERS; LIGHT-MEASURING INSTRUMENTS; STANDARD CELLS; ELECTRICAL CIRCUIT ANALYSERS; TACHOMETERS, TACHOMETER GENERATORS, A.C., D.C. ] AND [ FREQUENCY GENERATORS, ] ELECTRICAL RESISTORS AND POTENTIOMETERS [ ; ELECTRICAL APPARATUS FOR TESTING RADIO AND TELEVISION EQUIPMENT; AND INSTRUMENT CALIBRATORS, ]