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Postby TGTS0907129» Concept of a voltmeter

Explain the concept of a voltmeter


The voltage across the usual galvanometer is comparatively low. If higher voltages are to be measured, it is necessary to insert a resistor of high resistance in series with the moving coil of the instrument. Most of the potential drop will then occur across the multiplying resistor. By properly choosing this resistance any desired voltage may be measured.
Consider, for instance, the galvanometer mentioned in the last two examples above. The voltage that is required to produced full-scale deflection is 10 mV. in order to use this galvanometer as a voltmeter registering to 10 V, it is necessary only to increase the resistance until a potential difference 10 V is just sufficient to produce in the galvanometer and resistor a current of 0.0020 A, or enough for a full-scale deflection hence

So that the resistance of the meter (5.0 Ω) must be increased by the addition of a series resistance Rm of 4,995 Ω. The scale of the instrument should be labeled 0-10V, so that each division represent 0.1 V. if a potential difference of 5 V is applied to the terminals of this instruments, th e current is

Since 0.002 A is the full-scale current, the deflection will be just half scale. It should be noticed that the resistance of the voltmeter is R = Rm + Rg where Rm the series resistance is and Rg is that of the galvanometer.

By TGTT30071258 on 10/26/2015 11:17:27 PM
TGTS0907129 on 10/26/2015 4:49:29 AM

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