posted by Jp .
The ammeter your physics instructor uses for in class demonstrations has a resistance R(g)= 60 ohms and requires a current of 1.0 mA for a full scale deflection. The same ammeter can be used to measure currents of much greater values by wiring a shunt resistor of relatively small resistance R(p) in parallel with the ammeter. The values of the shunt resistance that allows the ammeter to be measure a current of 12 A at a full scale deflection is
Vf = If*Rg = 0.001 * 60 = 0.06 Volts.
Rp = Vf / If = 0.060 / 12 = 0.005 Ohms.
Vf = Full-scale voltage.
If = Full-scale current.