Measuring Resistance
A battery is connected to a light bulb by wires. Current flows and bulb lights up. The light bulb gives resistance to the current. This resistance R is related to the current I by Ohm's law:
R = V / I,
where V is the potential difference across the bulb. We can calculate the resistance if we know the potential difference and the current.
In order to find the resistance, we must measure the current and potential difference. The potential difference across the light bulb is measured by connecting a voltmeter to two sides of the light bulb. The current is measured by inserting an ammeter into the circuit - cutting one wire and connecting the free ends to the ammeter.
In this arrangement, the voltmeter is said to be connected across the light bulb, or in parallel to the light bulb. The ammeter is said to be connected in series with the light bulb. Once these are connected, write down the readings on the voltmeter and ammeter, substitute into the Ohm's law:
R = V / I,
and calculate the resistance. Note that to use this formula, both V and I must be in SI units. If voltmeter reading is in mV, it must be converted to V first. If ammeter reading is in mA, it must be converted to A first. The conversions are:
1 mV = 0.001 V
1 mA = 0.001 A
Then R is in unit of ohms (Ω).
Exercise. A voltmeter is connected across a light bulb, and an ammeter is connected in series. The voltmeter reading is 0.9 V, and the ammeter reading is 90 mA. What is the resistance of the light bulb?
Answer. Use Ohm's law R = V / I. First, convert current to SI unit: 90 mA = 90 x 0.001 = 0.09 A. Then we find R = V / I = 0.9 / 0.09 = 10 Ω.