My name is Alexander FufaeV and here I write about:

Ideal and real Current Source

A load resistor with resistance $R$ is connected to a real current source (it has an internal resistance), which specifies the current $I$. With a real current source, the current $I$ depends on the choice of load resistance:

$$ \begin{align} I ~=~ \frac{U}{R} \end{align} $$

In order to measure how much current flows through the load resistor $R$, a current meter (ammeter) must be connected in series with this load resistor. In the case of a real ammeter, this has an internal resistance $R_{\text A}$.

Circuit for a Current Measurement

However, this distorts the current $I$, as it is reduced by the connected ammeter:

$$ \begin{align} I ~=~ \frac{U}{R + R_{\text A}} \end{align} $$

The ratio of Eq. 1 and Eq. 2 indicates the factor $F$ by which the measured current 2 deviates from the actual current 1:

$$ \begin{align} F = \frac{R + R_{\text A}}{R} \end{align} $$

In order to have a reliable current measurement, the internal resistance of the ammeter must be as low as possible. Or, equivalently, the load resistance through which the current is measured should be much greater than the internal resistance of the ammeter: $ R \gg R_{\text A}$.

Difference between real and ideal ammeter

In contrast to a real ammeter, an ideal ammeter has no internal resistance: $ R_{\text A} = 0 $. It therefore does not falsify the current measurement.