Page 805 - Basic Electrical Engineering

Figure 11.3 An analog instrument measuring pressure of liquid

Figure 11.4 Active instrument illustrated

11.3 PASSIVE AND ACTIVE INSTRUMENTS

Instruments can either be active or passive. In active instruments the output is

produced entirely by the quantity being measured.
For example, the instrument shown in Fig. 11.3 is an active instrument
because the deflection of the indicator is produced entirely due to the pressure

created by the heated liquid.
Passive instruments are provided with some external source of supply. The

quantity being measured simply changes the magnitude of the external source
of supply as shown in Fig. 11.4. As shown in the figure, the pointer moves
due to, say movement of some liquid level in a tank. The output voltage can