27.11:
Voltmeter
In an electric circuit, a voltmeter connected in parallel with the circuit element measures the potential difference between the two points of the circuit element.
An ideal voltmeter should have infinite resistance so that it does not draw current from the circuit. However, real voltmeters have finite resistance.
It is desirable to have a large value for resistance so that connecting the voltmeter to a circuit does not appreciably alter the measured potential difference.
Any galvanometer can be converted into a voltmeter by adding a current-limiting resistor in series.
The potential difference required for full-scale deflection for just the galvanometer is in millivolts.
The galvanometer's range can be increased by connecting a high-value resistor in series with the coil.
This configuration distributes the total potential difference between the two components in series. As a result, only a fraction of the total potential difference appears across the coil.
From this expression, the value of the series resistor required for designing a voltmeter with full scale reading can be determined.
27.11:
Voltmeter
A voltmeter is an electrical device that measures the potential difference or voltage between two points. It is connected in parallel with the circuit element it is measuring. A parallel connection is used because elements in parallel experience the same potential difference. The voltmeter is represented by the symbol "V ".
An ideal voltmeter would have infinite resistance, so connecting it between two points in a circuit would not alter any of the currents. Real voltmeters always have finite resistance, but a voltmeter should have enough resistance that connecting it to a circuit does not appreciably change the other currents. Inexpensive voltmeters have resistances on the order of 10 megaohms, whereas high-precision voltmeters have resistances on the order of 10 gigaohms.
A current-detecting galvanometer can be designed into a voltmeter by adding a high-value resistor in series. The voltage across the galvanometer coil at full-scale deflection is only in millivolts. This range can be extended by connecting a resistor in series with the coil. Then, only a fraction of the total potential difference appears across the coil, and the remainder appears across the series resistor.
A sensitive voltmeter is used in electromyography, a medical diagnostic technique. A fine needle containing two electrodes is inserted into a muscle in the patient's hand. A sensitive voltmeter is used to measure the potential difference between the electrodes. A physician can probe the muscle's electrical activity with these potential readings. Electromyography is an important technique for diagnosing neurological and neuromuscular diseases.
Suggested Reading
- Young, H.D. and Freedman, R.A. (2012). University Physics with Modern Physics. San Francisco, CA: Pearson. Pages 861–862.
- OpenStax. (2019). University Physics Vol. 2. [Web version]. Retrieved from https://openstax.org/details/books/university-physics-volume-2; section 10.4; pages 467–468.