(i) Indicating instruments
(ii) Integrating instruments
(iii) Recording instruments (i) Recording instruments Those instruments which provide a constant record of variations of electrical amount to be calculated are call recording instruments. Recording instrument is simply an indicating instrument by a pen attach to its pointer. Pen rests evenly on a chart wrap above a drum affecting by a slow regular speed. The motion of drum is in a way perpendicular to direction of pointer. The path traced out through pen indicates manner in which quantity, being calculated, has mixed during time of record. Recording voltmeters are use in supply station to record voltage of supply mains during day. Recording ammeters are working in supply stations for registering current taken as of batteries. (ii) Indicating instruments Those instruments which openly point out value of electrical amount at time as it is being calculated are call indicating instruments e.g. voltmeters, ammeters and wattmeters. In such instruments, a pointer moving above a graduate scale directly gives value of electrical amount being calculated. Such as, when an ammeter is joined in circuit, pointer of meter openly indicates value of current flow in circuit at that time. (iii) Integrating instruments Those instruments which calculate total amount of electricity (in ampere-hours) or electrical energy (in watt- hours) in a specified time are call integrating instruments e.g. ampere-hour meter and watt-hour meter. In such instruments, there are sets of dials and pointer which register total amount of electricity or electrical energy supplied to load.