Ohm meter (ohmmeter), also known as resistance table, is a specialized instrument for measuring electrical resistance. Electromechanical components impede the flow of current nature, called the resistance in ohms. Milliohm meter special measuring small resistance; and megohms meter, also known as Megaohmmeter, or high table, the special measure very great resistance.

Ohmmeter original design, with a small battery voltage is applied to the resistor, and, with a modified galvanometer (galvanometer) to measure the current flowing through the resistor. Galvanometer scale marked change in ohms. Due to the constant voltage applied to the battery to ensure that the current will be inversely proportional to the resistance. So, know the current, you can get resistance.

As shown right ohmmeter scale representation from zero to infinity. When in contact with the two probes, the resistance is zero; when separated, the resistance is infinite. The majority of the number of domains between the two numbers to a logarithmic scale mode of expression. So, assuming the battery of emf 6 \ volt \, \!, You want to set current 4 \ mA \, \!, The internal resistance must be adjusted to 1.5 \ k \ Omega \, \!. When two probe together, the total resistance is 1.5 \ k \ Omega \, \!, Current is 4 \ mA \, \!, Ohm meter display shows the resistance to needle 0 \ k \ Omega \, \! .

When displaying the needle in the middle of zero and infinity, the current is 2 \ mA \, \!. So, the total resistance is 3 \ k \ Omega \, \!, The measured resistance is 1.5 \ k \ Omega \, \!.

When displaying the needle in the middle between zero and 1.5 \ k \ Omega \, \! When current is 3 \ mA \, \!. So, the total resistance is 2 \ k \ Omega \, \!, The measured resistance is 500 \ \ Omega \, \!.

When the display needle 1.5 \ k \ Omega \, \! In the middle with the infinite, the current is 1 \ mA \, \!. So, the total resistance is 6 \ k \ Omega \, \!, The measured resistance is 4.5 \ k \ Omega \, \!.

This ohmmeter has an important drawback is that it requires a very stable battery voltage. After a period of use, the voltage will gradually decrease. This will make ohm meter lose accuracy. When in contact with the two probes, the display no longer needle will point 0 \ k \ Omega \, \!, And will point to the growing resistance.