Ohmmeter Design

Ohmmeters are designed to measure resistance in the low, middle, or high range. The most common is the series ohmmeter, designed to read resistance levels in the midrange. It uses the series configuration in [Fig. 1]. The ohmmeter design is quite different from that of the ammeter or voltmeter because it shows a full-scale deflection for zero ohms and no deflection for infinite resistance.
Series ohmmeter.
Fig. 1: Series ohmmeter.
To determine the series resistance $Rs$, the external terminals are shorted (a direct connection of zero ohms between the two) as shown in the above figure to simulate zero ohms, and the zero-adjust is set to half its maximum value. The resistance $Rs$ is then adjusted to allow a current equal to the current sensitivity of the movement ($1 mA$) to flow in the circuit. The zero-adjust is set to half its value so that any variation in the components of the ohmmeter that may produce a current more or less than the current sensitivity can be compensated for. The current $I_m$ is,
$$I_m \text{(full scale)} = I_{CS} = {E \over Rs + Rm +{zero-adjust \over 2}}$$
$$ \bbox[5px,border:1px solid grey] {Rs = {E \over I_{CS}} - Rm - {\text{zero-adjust} \over 2}} \tag{1}$$
If an unknown resistance is then placed between the external terminals, the current is reduced, causing a deflection less than full scale.
If the terminals are left open, simulating infinite resistance, the pointer does not deflect since the current through the circuit is zero.
Fig. 2: Nanovoltmeter.
An instrument designed to read very low values of resistance and voltage appears in [Fig. 2]. It is capable of reading resistance levels between $10 mΩ$ ($0.01 Ω$) and $100 mΩ$ ($0.1 Ω$) and voltages between $10 mV$ and $100 V$. Because of its low-range capability, the network design must be a great deal more sophisticated than described above. It uses electronic components that eliminate the inaccuracies introduced by lead and contact resistances. It is similar to the above system in the sense that it is completely portable and does require a dc battery to establish measurement conditions. Special leads are used to limit any introduced resistance levels.

What is megohmmeter or Megger?

The megohmmeter (often called a megger) is an instrument for measuring very high resistance values. Its primary function is to test the insulation found in power transmission systems, electrical machinery, transformers, and so on.
Fig. 3: Megohmmeter.
To measure the high-resistance values, a high dc voltage is established by a hand-driven generator. If the shaft is rotated above some set value, the output of the generator is fixed at one selectable voltage, typically $250 V$, $500 V$, or $1000 V$-good reason to be careful in its use. A photograph of a commercially available tester is shown in [Fig. 3]. For this instrument, the range is $0 Ω$ to $5000 MΩ$.

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