User manual

Hardly any current flows through the very high 2.2 MΩ resistor. A measurement
current of approx. 1 mA, however, suggests a very high measuring error. To
actually have a current of 1 mA flow through it, a voltage of 2,200 V would be
needed. About 2.4 ma each flow through the two 330 Ω resistors. The total of the
individual currents measured is a little less than the overall current
determined. The reason is in the unavoidable measuring errors.
Figure 61: In this circuit of several parallel and series resistors, wire
bridges are required. The wire bridges may be replaced by the multimeter for
current measurement.
Figure 62: To receive suitable measuring points for current measurement at the
paths of the parallel resistors, the three resistors must be bent to different
lengths.
Figure 63: The total current determined for this circuit is at 4.87 mA. It is
the total current that flows through the three parallel and then through the
series resistor and the LED.
Figure 64: Hardly any current flows through the very high 2.2 MΩ resistor. The
measured value of 1 mA is, however, only due to a high measuring error. In fact,
it is much lower.
Figure 65: About 2.4 A each flow through the two 330 Ω resistors.
Check Measuring Results
You have already seen some formulas in the exercises above. This chapter deals
with the most important formula: Ohm's law. It describes the interrelation
between current, voltage and resistor and once again shows by calculation what
you have already found in several measurements: For example, that very low
currents flow through high resistors and that there are high voltage drops there.
Ohm's law for direct currents/direct voltages
R = U / I
I = U / R
U = I * R
U ... Voltage in volt (V)
I ... Current in ampere (I)
R ... Impedance in Ohm (Ω)
helps to support measurements by calculation as well. Calculations can also help
recongnise possible measuring errors that may happen when, e.g, a mistake has
been made when reading the display's decimal digit.
Ohm's law also helps you save measurements. For example, if the voltage and
resistance are already known, you can use the formula I = U / R to calculate the
current that flows through the circuit. Of course, you may also determine
partial currents or voltages in a circuit. Even resistors can be calculated.