When Test Meter Range is 10 amp range we can calibrate a current shunt (piece of heavy wire like battery cable) to determine high amperage range.
Top circuit we measure known amperage within 10 amp meter range.
Middle circuit we change to millivolt meter and pierce through battery cable insulation(shown as resistor) to show voltage drop in millivolts at same value as above amp meter. This calibrated our shunt.
Bottom circuit we series connect our calibrated shunt in high current circuit like a starter circuit. Our 200 millivolt voltage drop tells us 200 amps is current going through starter circuit.
Now adjust bottom circuit amperage values and watch reading of millivolt meter match amperage reading.
This is from Internet video :
https://youtu.be/Q849H1s-g1o
