What is a galvanometer called when it has been calibrated to read current?
Galvanometers are analog devices that read only current. A very small current of about 10 -100 microamps (from 10 x10-6 to 100 x10-6 A) . By shunting or placing a resistor in parallel to its coil ( 20,000 – 100,000 ohms ) we get an ammeter By placing a resistor in series with the galvanometer coil we make a voltmeter. To compute the values of shunt and series resistors it is customary to assume that maximum voltage drop across the galvanometer terminals should be about 50 mV. Parallel or shunt resistor is then for 1.0 A of current is Rp= 50mV/1A= 50 mOhms A series t resistor must create a voltage divider with a maximum voltage drop across the galvanometer terminals should be about 50 mV V0= Vin (Rs + Rg)/Rg Where Rg= 50,000 ohms; V0= 50 mV and Vin=10 V Then Rs= (Vo/Vin) Rg -Rg Rs= (10/50 x10^-3)x 50 x10^+3 – 50 x10^+3 Rs= 9950000 ohms = 9.
