What is the zero error of voltmeter and ammeter?

The term "zero error" in the context of voltmeters and ammeters refers to a calibration or measurement error that occurs when the instrument does not read zero when it should ideally do so in the absence of any voltage or current being measured. In other words, it's the error associated with the instrument not starting from a true zero point on its scale.

Zero Error in Voltmeters: When a voltmeter is connected to a circuit, it should ideally read zero when there is no voltage applied across its terminals. However, due to various factors like manufacturing imperfections or aging of the instrument, it may sometimes show a nonzero reading when connected to a voltage-free circuit. This nonzero reading is referred to as the voltmeter's "zero error."

Positive Zero Error: If the voltmeter reads a voltage greater than zero when connected to a voltage-free circuit, it has a positive zero error.
Negative Zero Error: If the voltmeter reads a voltage less than zero (i.e., in the opposite direction) when connected to a voltage-free circuit, it has a negative zero error.
To compensate for zero error, the actual voltage reading is calculated by subtracting the zero error from the indicated reading.

Zero Error in Ammeters: Similar to voltmeters, ammeters can also have zero errors. An ideal ammeter should read zero when there is no current flowing through it. However, due to factors like resistance in the instrument's wiring or shunt resistor, a nonzero reading may occur when there is no current.

Positive Zero Error: If the ammeter indicates a positive current (current in the direction of measurement) when there is no current flowing, it has a positive zero error.
Negative Zero Error: If the ammeter indicates a negative current (current in the opposite direction) when there is no current flowing, it has a negative zero error.
To correct for zero error in ammeters, the indicated current is adjusted by adding or subtracting the zero error value, depending on its sign.

It's essential to account for and correct zero error in precision measurements to obtain accurate results. Manufacturers may provide information on how to adjust or calibrate the instrument to eliminate or minimize zero error, and periodic calibration of these instruments can help maintain their accuracy over time.