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Potential dividers

A variable resistor uses two terminals

A rheostat connected in series uses only two terminals.

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The conducting path comes in at one terminal goes along a length of wire (which sets the value of the variable resistance) until it meets the jockey, then goes up into the thick conducting bar and out of the other terminal.  By moving the position of the jockey you change how much wire the conducting path includes.  The thick bar has a very low resistance so its length doesn't matter.

A potential divider uses three terminals.

The first thing to do is to connect the battery so the conducting path goes through the entire length of the wire.  The position of the jockey then doesn't matter and the resistance of the rheostat is just constant.

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But what happens to the voltage along the resistance wire?  It's 6 V, say, at one end and 0 V at the other end.  The voltage drops evenly from one end to the other.

By moving the position of the jockey you can 'tap off' this voltage.

You need a high load resistance

If you use a potential divider to control the brightness of a small bulb you may find it doesn't work as you hope.

If the resistance of the total length of wire wound onto the barrel is 10 ohms then with the jockey in the middle you'd hope to have 5 ohms either side.  This would divide the voltage in half, which is what you want.

However your load is connected in parallel with one of these halves.  If the load resistance is very big this doesn't matter because the effective resistance of a big resistance in parallel with a small resistance is a little less than the small resistance.

But if the resistance of the load is small, like a bulb, then the effective resistance of this parallel part becomes smaller than you'd expect.  So with the jockey in the middle there might be 5 ohms on one side but only 2 ohms on the other side.  This means the potential won't be divided in half but in the ratio of 5:2.

The rule of thumb is that the load needs to have about ten times higher resistance than the potential divider if you want to have a fairly linear response.

Fortunately potential dividers are often used to control logic gates or amplifiers, which have a very high input resistance.

back to Lesson 9: Series Circuits