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.
