Lesson 3/Learning Event 1
The second type of ammeter has the zero located at the left side of the dial and numbers
increasing to the right side of the dial. This ammeter is found on many battery chargers. Its
purpose is to show the rate at which the battery is being charged.
The third type of ammeter is found in most automotive testing equipment. It can be identified by
the zero located close to the left end of the scale and the numbers increasing on each side of the
zero. The right side increases much more than the left. Let's find out how this ammeter is used.
An ammeter resembles a voltmeter in many ways. It may contain one or more scales on its face or
dial. Ammeters with only one scale, like voltmeters, are limited in use. Ammeters with two or
more scales generally have a terminal connection for each scale instead of a selector switch,
especially if the ammeter is made to measure high amperage. Ammeters with terminal connections
for each range have one common terminal. One test lead is connected to this terminal; the other is
connected to the terminal of the range desired. The ammeter, like the voltmeter, may be a
separate meter or a part of a test instrument that contains other test equipment.
The electrical portion of the ammeter must be constructed to handle the amperage it must test. An
internal shunt or resistor is built into many ammeters to help carry the current. Some ammeters
are used to test very high amperage, and these meters are made to connect an external shunt or
resistor to make the tests. This is the purpose of the exterior-shunt terminals on many ammeters.
When the electrical leads are connected to these terminals, the external shunt assembly is included
in the meter circuit. All ammeter leads must be connected to the meter in the right polarity (red to
positive, black to negative). As you now know, the circuit must be broken and the ammeter must
be connected in series so that all current will pass through the ammeter or the external shunt.
An ammeter is easy to use, if you follow proper procedures:
You should never try to connect the ammeter into a circuit under load. This will cause
arcing, when the test leads are connected, which could injure you as well as damage the
equipment. This is especially true if the circuit carries a heavy amperage load. The load
can generally be disconnected by opening a switch.
The ammeter should never be connected while the vehicle engine is running.