Ohmmeters

Basically , an ohmmeter consists of an internal battery, the meter movement, and a current limiting resistance, as shown in below figure (a).  For measuring resistance, the ohmmeter leads are connected across the external resistance to be measured.  Power in the circuit being tested must be off.  Then only the ohmmeter battery produces current for deflecting the meter movement.  Since the amount of current through the meter depends on the external resistance, the scale can be calibrated in ohms.

The amount of deflection on the ohms scale indicates the measured resistance directly.  The ohmmeter reads up scale regardless of the polarity of the leads because the polarity of the internal battery determines the direction of current through the meter movement.

Series Ohmmeter Circuit:

In below figures (a ) the circuit has 1500 ohm for ( R1 + rM). Then the 1.5 V cell produces.  1 mA, deflecting the moving coil full scale.  When these components are enclosed in a case as shown in figure (b), the series circuit forms an ohmmeter.  Note that M indicates the meter movement.

If the leads are short circuited together or connected across a short circuit, 1 mA flows.  he ohhmeter movement is deflected full scale to the right.  This ohmmeter reading is 0 ohm.

When the ohmmeter leads are open, not touching each other, the current is zero.  The ohmmeter indicates infinitely high resistance or an open circuit across its terminals.

Therefore, the meter face can be marked zero ohms at the right for full scale deflection and infinite ohms at the left for no deflection.  In between values of resistance result when less than 1 mA flows through the meter movement.  The corresponding deflection on the ohms scale indicates how much resistance is across the ohmmeter terminals.

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