21 Electronic Amplifiers, Electrical Alarms, and Electric Signalling Devices for Use in Connection with Manual, Automatic, Pneumatic, Electric and Electronic Gaging Apparatus; Dimensional Comparators; Process Measuring and Control Equipment; Sorting Devices; Apparatus for Measuring Proportion, Liquid Flow, Liquid Level, Temperature, Pressure, Hardness, Tensile Strength, Durability, Density, and Distance