ohmmeter, instrument for measuring electrical resistance, which is expressed in ohms. In the simplest ohmmeters, the resistance to be measured may be connected to the instrument in parallel or in series. If in parallel (parallel ohmmeter), the instrument will draw more current as resistance increases. If in series (series ohmmeter), current will decrease as resistance rises. Ratio meters measure the ratio of the voltage across the resistance to the current flowing through it. For high resistances, the scale is usually graduated in megohms (106 ohms), and the instrument is called a megohmmeter, or “megger.”