- Suppose the bulb has a resistance of 20 Ohms and we use a 6 Volt battery.
- The variable resistor can go from zero up to a maximum of 20 Ohms.
- When the variable resistor has zero Ohms, then the bulb will take all of the energy and the voltmeter reading will be at a maximum of 6 Volts.
- When the variable resistor has 20 Ohms, then the bulb and the resistor will have the same resistance and they will have equal shares of the energy. Voltmeter will read 3 Volts.
- Hence by using a variable resistor like this, there is a limited range of voltages you can use. In this case it is between 3V and 6V. You can't get down to 0 V.
- I suppose that if you use a variable resistor with a maximum resistance massively bigger than the bulb, you'd get a better range.
It is much better to use this set up, called a potentiometer. All 3 connections on the variable resistor (rheostat) are used and you can have all the voltages from 0V up to 6V.
This is the set up we used in class. (see IV characteristics booklet). We used a version of a potentiometer when we had the wire along the meter stick in the observation room so that every 10 cm represented 1V.
