Experiment 3 - The Real Ammeter

An ideal ammeter does not exist. In order to describe a real ammeter, you make a model...

                             

Real Ammeter The model of a real ammeter consists of an ideal ammeter  and an resistor Ri in series.

                 

Equivalent circuit diagram real ammeter.

Circuit diagram with a real ammeter.

This has a direct influence on the measurement. The total resistance R_total of the circuit is...

R_total  =  R_L + R_i

and is always greater than the load resistance R_L alone. Thereby is...

• A real ammeter shows a smaller current than would flow in the circuit without ammeter!

The value of the internal resistance is dependent on the measuring device (digital, analog multimeters, etc.) and the selected measuring range. Typical values ??range from a few Ω to several kΩ.

You can examine this by connecting the analog and digital ammeter and measuring the current at different measuring ranges. Thereby you see that the measured values differ due to the internal resistance.

In the next experiment, you look at the voltage measurement...