Video: Eg17S1-Physics-Q10A

Eg17S1-Physics-Q10A

03:00

Video Transcript

What is an ohmmeter used for?

The name of this device having to do with ohms gives us a clue about its purpose. George Ohm was an experimentalist who made measurements of electrical circuits. In particular, for different lengths and thicknesses and materials of conducting wire, he made measurements of current and potential difference across those wires. When he plotted his data points on a voltage-versus-current access, he noted that for many materials, those points lay along a straight line.

And Ohm found that the gradient or the slope of this line was equal to the electrical resistance ๐‘… of the circuit he was measuring. In other words, ๐‘… is equal to ๐‘‰ over ๐ผ. Or, in its more common form, ๐‘‰ is equal to ๐ผ๐‘…, Ohmโ€™s law. Based on this work, we may guess that the resistance value, ๐‘…, of a circuit has something to do with what an ohmmeter measures. And, indeed, weโ€™re on the right track.

Say we create a simple electrical circuit with three resistors ๐‘… one, ๐‘… two, and ๐‘… three arranged in series with a power supply and an ammeter to measure current. If we donโ€™t know the values of these three resistors, we canโ€™t make much progress in solving for them individually with the circuit as is. Knowing the potential difference ๐‘‰ and the circuit current ๐ผ measured by the ammeter, we could use Ohmโ€™s law to work back to solve for the overall circuit resistance. But that would only tell us about the combination of the resistances of these three resistors not their individual values.

To do that, to measure the resistance of a single one of these resistors, weโ€™d want to use an ohmmeter. These days, if one were to go out and purchase an ohmmeter, it might look something like this. There is a digital display for reading out the measured resistance. And there are two electrical leads which are part of conducting wires. An ohmmeter works by actually creating a complete electrical circuit including the element it wants to measure the resistance of.

So if we had a generic resistor ๐‘… whose resistance we wanted to measure with an ohmmeter, then we would connect one electrical lead to each end of this resistive element. And then, using voltage from a battery that helps power our ohmmeter, we would send a very small voltage through this circuit. That small voltage along with the resistor value would create a small current running through this circuit which itself is measured by our ohmmeter. And then, using what we could call the original form of Ohmโ€™s law, the resistance value of the resistor, ๐‘…, is calculated.

Because our ohmmeter uses a very sensitive circuit that it divides itself to measure the resistance of an element, then if we were to connector our ohmmeter to some element in this circuit to measure its resistance, it would be very important to open that circuit so that no current is flowing. That way the very carefully controlled potential difference, which creates a flow of current through the ohmmeterโ€™s circuit, is undisturbed. And an accurate measurement of the resistance value can be made.

Here then is what we could write about what an ohmmeter is used for. An ohmmeter is used to calculate the value of electrical resistance. Thatโ€™s the purpose of such a device.

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