What does the measuring instrument G in the circuit mean?

Measuring instrument G stands for galvanometer, which is used to check whether there is current and the direction of current, and cannot measure the specific current value. It is a high-sensitivity magnetoelectric instrument, which is used for bridge measurement, thermocouple temperature difference, electromagnetic induction and photoelectric effect.

When instruments are used for extended calculation of electric meters, such as ammeters and voltmeters, galvanometers are also called meter heads and sensitive galvanometers. This instrument can be used to calculate the full bias current. At this time, G is an element of the electric meter, and it does not appear in the circuit as a measuring instrument.