Indicating instruments can be classified based on various criteria, including their function, mode of operation, and the type of physical quantity they measure. Here's a classification based on function and operating forces:

1. Analog Instruments:

   • Mechanical Instruments: These instruments operate based on mechanical principles such as the movement of springs, levers, or diaphragms.
     • Examples: Analog pressure gauges, analog thermometers.
   • Electromechanical Instruments: These instruments combine mechanical elements with electrical components for measurement and indication.
     • Examples: Moving-coil meters, moving-iron meters.

2. Digital Instruments:

   • Electronic Instruments: These instruments utilize electronic circuits to measure and display values digitally.
     • Examples: Digital multimeters, digital oscilloscopes.
   • Computer-Based Instruments: These are instruments where data is processed by a computer for display and analysis.
     • Examples: Data loggers, virtual instrumentation systems.

3. Operating Forces:

   • Passive Instruments: These instruments do not require external power sources for operation.
     • Examples: Analog thermometers, mechanical pressure gauges.
   • Active Instruments: These instruments require an external power source for operation.
     • Examples: Digital multimeters, electronic pressure transducers.

The choice between analog and digital instruments often depends on factors such as accuracy requirements, ease of reading, and compatibility with other systems. Additionally, the selection of passive or active instruments depends on whether continuous power is available and the level of precision needed in measurements.