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Resistance measurement

Procedure for resistance measurement:

1. Set the measuring device to resistance measurement.
2. Connect the resistor to be measured to the corresponding sockets on the measuring device (the sockets labelled COM and $\Omega$).
3. Read the measured value.

There are different types of resistance measurement:

• direct resistance measurement
• indirect resistance measurement

Direct resistance measurement

Determine the nominal and measured values of the resistance for $R_{\rm 1}$ (brown, green, orange), $R_{\rm 2}$ (yellow, violet, red), $R_{\rm 3}$ (red, violet, red), and the incandescent lamp $R_{\rm L}$. Also measure the approximate resistance $R_{\rm K}$ of your body from your right hand to your left hand.

How do you explain the deviation between $R_{\rm L,nominal}$ and $R_{\rm L,meas}$?

What consequences can $R_{\rm K}$ have?

Now determine the series and parallel combinations of resistors $R_{\rm 1}$, $R_{\rm 2}$ and $R_{\rm 3}$.
State the formulae used:

$R_{\rm series} = R_{\rm a} + R_{\rm b}$
$R_{\rm parallel} = R_{\rm a} \parallel R_{\rm b} = \frac{R_{\rm a} \cdot R_{\rm b}}{R_{\rm a} + R_{\rm b}}$

Indirect resistance measurement

Resistance can also be determined by measuring current and voltage.

Ohm's law: In an electrical circuit, the current increases with increasing voltage and decreases with increasing resistance.
$ I = \frac{U}{R} $

Build the measurement circuit shown in figure 1 for each of the three resistors and set the voltage on the power supply to $12 ~{\rm V}$.

Fig. 1: Indirect resistance measurement

Measure $U_{\rm n}$ and $I_{\rm n}$. Calculate $R_{\rm n}$ from these values.