3825 Instruments for Measuring & Testing of Electricity & Electrical Signals Establishments primarily engaged in manufacturing instruments for measuring the characteristics of electricity and electrical signals, such as voltmeters, ammeters, wattmeters, watt-hour meters, demand meters, and equipment for testing the electrical characteristics of electrical, radio, and communication circuits and of internal combustion engines. Establishments primarily engaged in the manufacturing of electronic checkout, monitoring, evaluating, and other electronic support equipment for electronic navigational, radar, and sonar systems are classified in industry 3812, and those manufacturing similar equipment for communications systems classified in industry group 366.