# concept of voltmeter

by nil1996
Tags: concept, voltmeter
 P: 254 let Rg be the resistance of the galavanometer. A formula in my book states that V=Ig(Rg+R) my doubt is: here Ig(Rg+R) is the potential drop across the two resistors of the voltmeter. But how can this include the potential drop by the circuit element across which the voltmeter is connected.
 Sci Advisor P: 3,956 While you have the voltmeter in circuit, it is in parallel with whatever it is connected to and may affect that voltage slightly. There will be a voltage across the voltmeter which is the same as the voltage in the circuit. This voltage causes a current to flow in the galvanometer which is calibrated to give a correct reading for whatever voltage it is connected to. This calibration is done by adjusting the series resistor so that the combined voltmeter resistance causes exactly the right current to flow through the galvanometer for the applied voltage to match the voltage indicated on the voltmeter dial.
 P: 254 Is the formula correct?
P: 3,956

## concept of voltmeter

Yes, the formula is correct.

Why do you have a problem with it?

Assuming the resistance of the voltmeter is large compared with the resistance it is being measured across, it will draw sufficient current to make the meter needle deflect and give a voltage reading.

Did you read the explanation I gave above?
 P: 254 I am weak at concepts of current electricity. Here i am not understanding is ig(Rg+R) will give me voltage drop by the resistors of the voltmeter itself. so the voltage measured by this formula will give me the voltage drop by its own resistors.How can i get the voltage drop done by a circuit component by using this formula.
 Sci Advisor P: 3,956 The formula is correct, but maybe not appropriate in that form. If you rearrange it to read Ig = V / (Rg + R) it becomes more useful. The resistors are constant in value, so the current depends only on the voltage. This is a voltmeter so the starting point must be the voltage. As you change the voltage, the current changes and so does the deflection on the galvanometer needle.
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