26 MEASURING AND TESTING APPARATUS-NAMELY, [ ELECTRICAL INSULATION TESTERS, ] MAGNET CHARGERS, [ PHOTOELECTRIC DETECTORS AND INTEGRATORS OF RADIANT ENERGY, ELECTRONIC METAL DETECTORS, ELECTRO-MAGNETIC GEOPHYSICAL EXPLORATION APPARATUS, ] ORE TESTERS, [ POTENTIOMETER CALIBRATION STANDS, MECHANICAL SHOCK TESTERS, ] AND TEMPERATURE SENSITIVE SWITCHES