ERRORS COMMON TO ALL TYPES OF INSTRUMENTS

The errors that are common to all types of instruments are given as follows:

1. Temperature error: Apart from room temperature, the rise in temperature occurs due to the heating effect of current flowing through the operating coil. This causes the change in resistance of the working coil and resistance of the other resistors connected in the instrument. Consequently, an erroneous reading is obtained while measuring an electrical quantity. Such an error is not serious in the case of ammeters, but it causes serious effect in the case of voltmeters. This error can be reduced by providing sufficient ventilation and cooling. This error can be further minimised by providing a series resistance coil of material having very small temperature co-efficient of resistance and working coil is made of copper wire.
2. Friction error: When deflecting torque occurs, the spindle of the moving system pivoted in the jewel bearing rotates and this causes friction; in this manner, it affects the instrument reading and hence it is called friction error. This error is more prominent when the deflecting torque is very small. To minimise this error, a moving system of light weight is designed.
3. Observational error: This error is usually due to misreading of the scale. It can be minimised by placing parallax mirror with the scale and using it effectively.