From galvanometer to voltmeter
Practical Activity for 14-16
Adding a multiplier in series with a galvanometer converts it to a voltmeter with a higher range.
Apparatus and Materials
For each student group
- Voltmeter, 0 to 5 V, DC
- Cells, 1.5 V in holder, 4
- Selection of high-value resistors, see technical note
Health & Safety and Technical Notes
You will need to select suitable resistors according to the resistance and full-scale-deflection (f.s.d.) of your galvanometers. If the galvanometer has f.s.d. 1 mV and resistance 10 Ω., and you want to increase this to f.s.d. 10 V, you will have to provide 100 kΩ. resistors.
- The ordinary commercial voltmeter that you use in the laboratory is really a
trickle meter. It measures the tiny trickle of current which the potential difference drives through a high resistance inside the voltmeter's case.
- To convert your galvanometer to a voltmeter reading, say, 5 V when the pointer is at the end of the scale, you must add a large resistance in series, as in the diagram. The galvanometer itself has only a small resistance, perhaps 10 Ω., and it gives a full scale reading when the voltage across it is 1 mV. So the p.d. across the extra resistance must be 4.999 V.
- You can discover the right resistance to add by trying it out. Choose the largest high resistance you are offered. Connect it in series with the galvanometer and connect to a 1.5 V cell, just for an instant. Does your voltmeter read 1.5 V, as you wished? If it reads more, you do not have enough resistance in it: add more resistance. If it reads too little, try less resistance in it.
- When you have adjusted the resistance, try your voltmeter on two 1.5 V cells in series. Also test those with a commercial voltmeter to make sure yours now reads as you wish.
- Convert your voltmeter to one with twice that range, 0-10 V; and try that on a 6 V battery.
- A voltmeter is always connected across the component whose potential difference is to be measured. It therefore taps off a trickle of charge from the main circuit. The higher the resistance of the voltmeter and its multiplier, the less the circuit is disturbed by this measurement. The resistance of the multiplier has to be high enough so that only a small current passes through the galvanometer.
- Students can either calculate the resistance needed or do it by trial and error, beginning with the largest resistance possible in series with the galvanometer.
- The commercial voltmeter enables students to adjust their home-made one to read as they wish, by trial instead of by calculation.
- NB. Digital voltmeters work quite differently.
This experiment was safety-tested in October 2006