Karl measures the current through a resistor. What else will he need to measure in order to work out its resistance?

The voltage (V)

Explanation:

Ohms, in his law, has explained the relationship between current, resistance and voltage.

Ohm's law, which states that the voltage passing through a resistor is directly proportional to both the current and the resistance of the resistor, has the following equation:

V = IR

Where;

V = voltage (volts)

I = current (Amperes)

R = resistance (ohms)

According to this question, Karl measures the current, I, through a resistor. In order to calculate his resistance, R, he will need to measure the voltage, V.