All electrical measuring instruments depend for their action on any of many physical effects of electric current or potential. The following are the effects generally used in the manufacture:
i. Magnetic effect …. Voltmeters, ammeters, wattmeters, power factor meters etc.
ii. Thermal effect …. Ammeters, voltmeters, maximum demand meters etc.
iii. Chemical effect …. D.C. ampere hour meters (integrating meters).
iv. Electrostatic effect …. Voltmeters which can indirectly be used as ammeters and wattmeters.
v. Electro-magnetic induction effect …. Voltmeters, ammeters, wattmeters integrating meters used in A.C. only.