Investigations with a multi-meter.


To Do and Notice

Set up:

Connect a black lead to the COM or common input of the meter and a red lead to the volt ohm input.

Make sure the meter reads zero when it is turned off.

You might enjoy looking inside the workings of an old dead meter, go to Inside a meter.

Batteries, Ideal vs. real meters

An ideal meter when it is used to measure the voltage between two points, will have an infinite resistance. When placed across a battery the meter is an open circuit and so it measures the open circuit voltage of the battery, (if nothing else is connected to the battery).

An ideal meter when it is used to measure current will have zero resistance. Connect an ideal current meter to a battery and it will measure the short circuit current delivered by the battery.

A perfect battery will deliver infinite current to a short circuit. A real battery has internal resistance which limits its short circuit current. So we model all real batteries as an ideal battery which produces a constant voltage in series with a resistor. (In the model, both the ideal battery and the resistor are inside the battery case.)

When you place a meter in the circuit to measure voltage, you change the voltage. To estimate the error introduced by using a meter to measure voltage, add a second meter in parallel with the first. The percentage change in the reading of the first meter when the second meter is added is an estimate of the percentage change produced by adding the first meter to the circuit. To estimate the error in a current reading add a second meter in series with the first, the change in the reading of the first is an estimate of the error.

Going Further

Look inside a meter

If you have an old dead meter take it apart. Notice that there is a coil attached to the meter needle. The coil is pulled down to the zero reading by a weak spring. The coil is inside a magnet, use a compass or a steel paper clip to find the magnet. When an electric current flows through the coil, the magnet pushes on the current, and so on the coil, which moves the needle. The equilibrium position is determined by the balance of the magnetic force and the spring force. The deflection of the needle is proportional to the electric current which flows through the coil.

Scientific Explorations with Paul Doherty

© 2000

2 August 2000