Ohmmeter is an instrument used to measure the electrical resistance of a conductor. It is usually included in a single package with a voltmeter, and often an ammeter. In normal usage, the ohmmeter operates by using the voltmeter to measure a voltage drop, then converting this reading into a corresponding resistance reading through Ohm's law. If the current is known, the voltage drop across the unknown resistance may be read to give the resistance directly. If the current is not known, or if it is not possible to measure the voltage across the unknown directly, reading the voltage drop across a known resistance in the same circuit will give the current, and once this voltage is subtracted from the total drop for the circuit, the voltage drop across the unknown, and thus its resistance, may be found. Megohm meter is an ohmmeter for the measurements of very high resistances.
 

Figure 1: Megohm Meters Made by James G. Biddle Co.

 

Figure 2: Ohm meter (unknown model) Made by Hartmann & Braun.