A galvanometer with a full-scale sensitivity of 500 microA and a coil resistance of 60 ohms is to be used to build an ammeter designed to read 7.5 A. What is the required shunt resistance?

Question

A galvanometer with a full-scale sensitivity of 500 microA and a coil resistance of 60 ohms is to be used to build an ammeter designed to read 7.5 A. What is the required shunt resistance? 
Rs = milliA

bobpursley · Answer

AT full scale, 500microA is going through the meter 60 ohm, or a voltage of 30mV. That same 30mV is carrying 7.5 amps (yes, less the 500microamps). R= .030/7.5 ohms

Henry · Answer

E = 5*10^-4 * 60 = 0.03 Volts. I1 = 7.5A. = 7500 mA. I2 = 0.5 mA. I3 = ? I1 = I2+I3 = 7500mA. 0.5 + I3 = 7500. I3 = 7499.5 mA. = 7.4995A. Rs = E/I3 = 0.03/7.4995 = 0.0040 Ohms. = 4.0 Milliohms.