[tac]

1. Most ammeters that measure over a few amps use a shunt, and are actually millivoltmeters, not ammeters. A shunt is no more than a precision, high wattage, low resistance resistor. They are large in mass, and often finned, so the heating effect of large currents on the resistance is minimal. Standard shunts are sized to give a voltage drop across this resistance of 50 millivolts for a given full scale current. A 100A/50mV shunt would have 50 millivolts out with a current of 100 amps through the shunt. Plugging this into Ohms law gives shunt resistance of R = .050V/100A, or .0005 ohms.

2. Analog meters used with a shunt are sized so the meter scale reads in amps the same as the shunt, and that scale maximum is when the input voltage is 50mV. As in the above shunt example, an analog meter matched to a 100A/50mV shunt would be marked, generally on the back, "50mV full scale", and the meter scale on the front would be marked 0 to 100 amps. This means you can buy a Simpson 0-100Amp/50mV meter to go with a Yokagawa 100A/50mV shunt.

3. Because an analog ammeter is measuring the voltage drop, or the differential voltage, and that can be a floating voltage (not referenced to ground), across the shunt, it doesn't care if it is on the positive side or negative (return) of the circuit. Nor does it care if neither side of the shunt is connected to ground.

4. Most low priced digital ammeters with shunts are different. Again, they are actually millivoltmeters. But they do not actually measure the voltage drop across the shunt, they measure the voltage of the shunt with respect to ground (battery return). Therefore, most (but not all) can only be used in the negative side of the circuit where one side of the shunt is connected to battery negative, with the other side connected to all the return wires for the circuits you want to measure. They are sized the same way as the analog meters.

5. The reason for this is that digital meters contain one or more instrumentation operational amplifiers, and analog-to-digital converters. They are expensive and can be susceptible to noise, so design and construction of micro/millivolt signal conditioners and A/D converters is much more expensive than analog meters, which contain no electronics. These electronics require a good battery return as a reference, which an analog meter doesn't. The simplest and cheapest answer to these problems is to require one side of the shunt to be grounded. Good industrial digital ammeters that will measure a floating differential voltage do exist, but are much more expensive.

6. Some nice things about using shunts, and either analog or digital meters:
a) Generally you can use anyone's shunt with anyone's meter as long as you match the amp scale and shunt millivolts. Don't like the Blue Sea meter face? Get a Yokagawa
b) The shunt is a resistor and has no polarity, so how you connect the main power leads is unimportant.
c) For a digital meter the two small leads to the meter can be hooked so that the meter reads negative when the circuit is drawing current, and positive if there is a charger. Or hook it so it reads a positive number when current is drawn. You can do that with an analog meter only if you have a center-zero scale meter. They exist, but are expensive and hard to find.