Voltmeter
in electricity, any instrument for measuring the pressure, electromotive force, or difference of potentials at the ends of an electric current. The gold-leaf electroscope is a kind of voltmeter, but will only measure large differences of potential. If the terminals are connected with flat plates arranged parallel to each other, one of which is movable, the attractive force between the plates at a given small distance will be a voltmeter. This method is too coarse for ordinary currents, but a modification of it is employed in Thompson's quadrant electrometer. In Cardew's voltmeter the he heating effect of the current in a wire, which varies with the electromotive force, and is measured by. the expansion produced, is employed.
In the majority of instruments the electro-magnetic action is employed in some form of galvanometer. These are more usually wound to act as ammeters, and if wound with very thin wire the high resistance allows the electromotive force required to drive a certain current form of galvanometer. These are more usually wound to act as ammeters, and required to drive a certain current noted in volts. Such instruments are adif wound with very thin wire the high through them to be calibrated and dea "standard" voltaic cell or voltameter. We