(switchboards)

An instrument for determining the potential difference of any two points. In many cases it is a calibrated galvanometer wound with a coil of high resistance. The object to be attained is that it shall receive only an insignificant portion of current and that such portion shall suffice to actuate it. If connected in parallel with any portion of a circuit, it should not noticeably diminish its resistance. The divisions into which ammeters range themselves answer for voltmeters. In practice the same construction is adopted for both. The different definitions of ammeters in disclosing the general lines of these instruments are in general applicable to voltmeters, except that the wire winding of the coils must be of thin wire of great length. The definitions of ammeters may be consulted with the above understanding for voltmeters. In the use made of voltmeters there is a distinction from ammeters. An ammeter is a current measurer and all the current measured must be passed through it. But while a voltmeter is in fact a current measurer, it is so graduated and so used that it gives in its readings the difference of potential existing between two places on a circuit, and while measuring the current passing through its own coils, it is by calibration made to give not the current intensity, but the electro-motive force producing such current. In use it may be connected to two terminals of an open circuit, when as it only permits an inconsiderable current to pass, it indicates the potential difference existing between such points on open circuit. Or it may be connected to any two parts of a closed circuit. Owing to its high resistance, although it is in parallel with the intervening portion of the circuit, as it is often connected in practice, it is without any appreciable effect upon the current. It will then indicate the potential difference existing between the two points.