known current is applied through R (the unknown resistor), which is series con- known known reference meas A simple voltage divider is The un resistance can resistor R be found using: Wmeas (the voltage across the unknown) meas Vstd (the voltage across the known)
The exact value of the current is not important because the unknown resistance is de termined by the ratio of the voltages so long as the current is held constant. Many DMMs are battery-powered, portable are signed to enable them to withstand the rigors of field measurements. Others possess such features as autoranging operation (which means that the meter automatically ad justs its measuring circuits to the correct voltage, current, or resistance range), BCD or IEEE-448 output compatability, and measurement of conductance and even tempera- ture. The prices of DMMs vary from being very low (under $50) to quite high (thou- sands of dollars). But the least expensive models can now compete economically with their analog DMM counterparts.