When a ampere meter's range is extended the shunt resistor is used to bypass the maximum amount of current. It is a precision resistor having very low resistance. Shunt resistor are low resistance connected in parallel with ampere meter whose rating is to be extended. shunt resistance bypass the maximum amount of current but the power rating of shunt must be large enough to desipate the power. These shunts are used with DC and AC both meters. It is an electrical conductor which is used with Analog /Permenant magnetic moving coil meters as well as digital meters.

Shunt resistancae must be connected in Parallel & it must be lower than meter resistance so that maximum ammount of current will be bypassed through this external resistance. If accidently shunt resistance is disconnected or loosed then the lasrger amoun of current will by passed through the meter which causes the meter internal coil to be burnt and meter will be useless.

Formula is based on ohms law where to calculate the resistance is based on voltage divided by current. In this formula we have two currents one is internal full scale current of meter and another is required current which is always higher than internal fulll scale current.

Note:

- I
_{m}: Full scale Deflection current of Meter - R
_{m}: Internal Resistance of Meter - I
_{req}: Required Current; must be higher than Full scale current of Meter