How to Read an Ammeter

By Edwin Thomas
Antique ammeter.

If you have ever seen an ammeter, it was probably in the movie "Apollo 13." The device might be obscure, but it is an important measuring instrument for electrical systems. It measures one of the two basic aspects of electric current, namely amperage. Reading an ammeter is simple, as is using the data revealed.

Zero-center ammeter.

Look at the gauge and determine if your ammeter is a normal ammeter or a zero-center one. The zero-center ammeter will have what looks like negative and positive readings on both sides of the zero. This is actually what it does: It reads the strength of the current in both negative and positive polarities.

Read the gauge after the ammeter is connected to an electric circuit. How an ammeter is connected varies. A typical example might have screw connectors: Wrap the exposed leads around the positive and negative screws and tighten the screws to clamp the wires into place. With an ordinary ammeter, this connection is enough. The gauge will indicate the number of amps in the current.

Note both the negative and positive numbers for a zero-center ammeter. This is useful for revealing the strength of a battery within a circuit. A battery with much higher negative amperage is running on a low charge.


With amps and one other number, all three basic numbers needed for describing normal electrical systems can be determined. Those numbers are amps, watts and volts. Use these two equations to determine the missing third number: Amps x Volts = Watts and Watts/Amps = Volts

The typical 60-watt light bulb uses 0.54 amps. A laptop computer uses between 0.8 to 1 amps. A power-hungry space heater or hair dryer, on the other hand, can consume as much as 13 to 15 amps.