The resistance can be measured with an ohmmeter. Normally, the test pen on the ohmmeter is connected to the resistor. A simple ohmmeter can apply the battery's voltage to an unknown resistance, generating a current that drives the meter's motion. According to Ohm's law, the current is inversely proportional to the sum of the internal resistance and the resistance under test, forming a nonlinear analog meter, calibrated from infinity to 0 ohms.
A digital multimeter using an active electronic component can change the current of the resistance under test. In this case, the voltage generated by the test resistance is linearly proportional to its resistance. In either case, the low resistance range of the ohmmeter passes more current through the test lead than the high resistance range so that the voltage exists at a reasonable level (usually less than 10 Volts) but can still be measured.
Measuring low resistance values, such as graded ohms, requires a four-terminal connection, depending on acceptable accuracy. A pair of terminals applies the known calibration current to the resistor, while the other pair detects the voltage drop at both ends of the resistor. Some laboratory ohmmeters, especially milliohms, and even some better digital multimeters can do this using four input terminals, which can be used for special test leads. Each of the two Kelvin clamps has a pair of grippers that are insulated from each other. The measured current is applied on one side of each clamp, while the other connection is only sensing the voltage drop. Ohm's law is used to calculate the resistance again. The measured voltage is divided by the applied current to obtain the resistance value.
