1911 Encyclopædia Britannica/Voltmeter

by John Ambrose Fleming

VOLTMETER, an instrument for measuring difference of electric potential (see Electrostatics) in terms of the unit called a volt. The volt (so called after A. Volta) is defined to be difference of potential which acting between the terminals of a resistance of one ohm sends through it a continuous current of one ampere. A voltmeter is therefore one form of electrometer (q.v.), but the term is generally employed to describe the instrument which indicates on a scale, not merely in arbitrary units but directly in volts, the potential difference of its terminals. Voltmeters may be divided into two classes, (a) electrostatic, (b) electrokinetic.

Electrostatic voltmeters are based on the principle that when two conductors are at different potentials they attract one another with a force which varies as the square of the potential difference (P. D.) between them. This mechanical stress may be made the measure of the P. D. between them, if one of the conductors is fixed while the other is movable, this last being subject to a constraint due to a spring or to gravity, means being also provided for measuring either the displacement of the movable conductor against the constraint or the force required to hold it in a fixed position relatively to the fixed conductor. One large class of electrostatic voltmeters consists of a fixed metal plate or plates and a movable plate or plates, the two sets of plates forming a condenser (see Leyden Jar). The movable system is suspended or pivoted, and when a P. D. is created between the fixed and movable plates, the latter are drawn into a new position which is resisted by the torque of a wire or by the force due to a weight. Utilizing this principle many inventors have devised forms of electrostatic voltmeter. One of the best known of these is Lord Kelvin's multicellular voltmeter. In this instrument (fig. 1) there are two sets of fixed metal plates, connected
Fig. 1.—Lord Kelvin's Multicellular Electrostatic Voltmeter. together and having a quadrantal shape, that is, approximately the shape of a quarter of a circular disk. In the space between them is suspended a "needle" which consists of a light aluminium axis, to which are affixed a number of paddle-shaped aluminium blades. This needle is suspended by a fine platinum silver wire, and its normal position is such that the aluminium paddle blades are just outside the quadrantal-shaped plates. If the needle is connected to one terminal of a circuit, and the fixed plates or cells to the other member of the circuit, and a difference of potential is created between them, then the movable needle is drawn in so that the aluminium blades are more included between the fixed plates. This movement is resisted by the torsional elasticity of the suspending wire, and hence a fixed indicating needle attached to the movable system can be made to indicate directly on a scale the difference of potential between the terminals of the instrument in volts. Instruments of this kind have been constructed not only by Lord Kelvin, but also by W. E. Ayrton and others, for measuring voltages from 10,000 volts down to 1 volt. In other types of electrostatic instruments the movable system rotates round a horizontal axis or rests upon knife edges like a scale beam; in others again the movable system is suspended by a wire. In the former case the control is generally due to gravity, the plates being so balanced on the knife edge that they tend to take up a certain fixed position from which they are constrained when the electric forces come into play, their displacement relatively to the fixed plates being shown on a scale and thus indicating the P. D. between them. In the case of high tension voltmeters, the movable plate takes the form of a single plate of paddle shape, and for extra high tensions it may simply be suspended from the end of a balanced arm; or the movable system may take the form of a cylinder which is suspended within, but not touching, another fixed cylinder, the relative position being such that the electric forces draw the suspended cylinder more into the fixed one. Electrostatic voltmeters are now almost entirely used for the measurement of high voltages from 2000 to 50,000 volts employed in electrotechnics. For such purposes the whole of the working parts are contained in a metal case, the indicating needle moving over a divided scale which is calibrated to show directly the potential difference in volts of the terminals of the instrument. One much-used electrostatic voltmeter of this type is the Kelvin multicellular vertical pattern voltmeter (fig. 2). For use at the switch-boards of electric supply Fig. 2.—Round Dial Kelvin Multicellular Electrostatic Voltmeter, 5-in. scale. For high pressure. stations the instrument takes another form known as the "edge-wise" pattern.

Another class of voltmeters comprises the electrokinetic voltmeters. In these instruments the potential difference between two points is measured by the electric current produced in a wire connecting to two points. In any case of potential difference measurement it is essential not to disturb the potential difference being measured; hence it follows that in electrokinetic voltmeters the wire connecting the two points of which the potential difference is to be measured must be of very high resistance. The instrument then simply becomes an ammeter of high resistance, and may take any of the forms of practically used ammeters (see Amperemeter). Electromagnetic voltmeters may therefore be thermal, electromagnetic or electrodynamic.

As a rule, electromagnetic voltmeters are only suitable for the measurement of relatively small potentials—0 to 200 or 300 volts. Numerous forms of hot-wire or thermal voltmeter have been devised. In that known as the Cardew voltmeter, a fine platinum-silver wire, having a resistance of about 300 ohms, is stretched in a tube or upon a frame contained in a tube. This frame or tube is so constructed of iron and brass (one-third iron and two-thirds brass) that its temperature coefficient of linear expansion is the same as that of the platinum silver alloy. The fine wire is fixed to one end of the tube or frame by an insulated support and the other end is attached to a motion multiplying gear. As the frame has the same linear expansion as the wire, external changes of the temperature will not affect their relative length, but if the fine wire is heated by the passage of an electric current, its expansion will move the indicating needle over the scale, the motion being multiplied by the gear. In the Hartmann and Braun form of hot-wire voltmeter, the fine wire is fixed between two supports, and the expansion produced when a current is passed through it causes the wire to sag down, the sag being multiplied by a gear and made to move an indicating needle over a scale. In this case, the actual working wire, being short, must be placed in series with an additional high resistance. Hot wire voltmeters, like electrostatic voltmeters, are suitable for use with alternating currents of any frequency as well as with continuous currents, since their indications depend upon the heating power of the current, which is proportional to the square of the current and therefore to the square of the difference of potential between the terminals.

Electromagnetic voltmeters consist of a coil of fine wire connected to the terminals of the instrument, and the current produced in that wire by a difference of potential between the terminals creates a magnetic field proportional at any point to the strength of the current. This magnetic field may be made to cause a displacement in a small piece of soft iron, as in the case of the corresponding ammeters, and this in turn may be made to displace an indicating needle over a scale so that corresponding to every given potential difference between the terminals of the instrument there is a corresponding fixed position of the needle on the scale. One of the most useful forms of electromagnetic voltmeter is that generally known as a movable coil voltmeter (fig. 3). In this instrument there is a Fig. 3.—Round Dial Voltmeter of Kelvin Siphon Recorder, dead beat moving coil type, with front removed. fixed permanent magnet, producing a constant magnetic field, and in the inter space between the poles is fixed a delicately pivoted coil of wire carried in jewelled bearings. The normal position of this coil is with its plane parallel to the lines of force of the field. The current is got in and out of the movable coil by means of fine flexible wires. The movable coil has attached to it an index needle moving over a scale, and a fixed coil of high-resistance wire is included in series with the movable coil between the terminals of the instrument. When a difference of potential is made between the terminals, a current passes through the movable coil, which then tends to place itself with its plane more at right angles to the lines of force of the field. This motion is resisted by the torsion of a spiral spring resembling the hair-spring of a watch having one end fixed to the coil axis, and there is therefore a definite position of the needle on the scale corresponding to each potential difference between the terminals, provided it is within the range of the control. These instruments are only adapted for the measurement of continuous potential difference, that is to say, unidirectional potential difference, but not for alternating voltages. Like the corresponding ammeters, they have the great advantage that the scales are equidivisional and that there is no dead part in the scale, whereas both the electrostatic and electrothermal voltmeters, above described, labour under the disadvantage that the scale divisions are not equal but increase with rise of voltages, hence there is generally a portion of the scale near the zero point where the divisions are so close as to be useless for reading purposes and are therefore omitted. For the measurement of voltages in continuous current generating stations,
Fig. 4.—Edgewise Voltmeter, Stanley D'Arsonval type. movable coil voltmeters are much employed, generally constructed then in the "edgewise" pattern (fig. 4).

Electrodynamic Voltmeters.—A high-resistance electrodynamometer may be employed as a voltmeter. In this case both the fixed and movable circuits consist of fine wires, and the instrument is constructed and used in a manner similar to the Siemens dynamometer employed for measuring continuous alternating current (see Amperemeter). Another much-used method of measuring continuous current voltages of unidirectional potential difference employs the principle of potentiometer (q.v.). In this case a high-resistance wire is connected between the points of which the potential difference is required, and from some known fraction of this resistance wires are brought to an electrostatic voltmeter, or to a movable coil electromagnetic voltmeter, according as the voltage to be measured is alternating or continuous. This measurement is applicable to the measurement of high potentials, either alternating or continuous, provided that in the case of alternating currents the high resistance employed is wound non-inductively and an electrostatic voltmeter is used. The high-resistance wire should, moreover, be one having a negligible change of resistance with temperature. For this purpose it must be an alloy such as manganin or constantan. It is always an advantage, if possible, to employ an electrostatic voltmeter for measuring potential difference if it is necessary to keep the voltmeter permanently connected to the two points. Any form of electrokinetic voltmeter which involves the passage of a current through the wire necessitates the expenditure of energy to maintain this current and therefore involves cost of production. This amount may not by any means be an insignificant quantity. Consider, for instance, a hot-wire instrument, such as a Cardew's voltmeter. If the wire has a resistance of 300 ohms and is connected to two points differing in potential by 100 volts, the instrument passes a current of one-third of an ampere and takes up 33 watts in power. Since there arc 8760 hours in a year, if such an instrument were connected continuously to the circuit it would take up energy equal to 263,000 watt-hours, or 260 Board of Trade units per annum. If the cost of production of this energy was only one penny per unit, the working expenses of keeping such a voltmeter in connexion with a circuit would therefore be more than £l per annum, representing a capitalized value of, say, £10. Electrostatic instruments, however, take up no power and hence cost nothing for maintenance other than wear and tear of the instrument.

The qualities required in a good voltmeter are:—(i.) It should be quick in action, that is to say, the needle should come quickly to a position giving immediately the P.D. of the terminals of the instrument. (ii.) The instrument should give the same reading for the same P.D. whether this has been arrived at by increasing from a lower value or decreasing from a larger value; in other words, there should be no instrumental hysteresis. (iii.) The instrument should have no temperature correction; this is a good quality of electrostatic instruments, but in all voltmeters of the electrokinetic type which are wound with copper wire an increase of one degree centigrade in the average temperature of that wire alters the resistance by 0.4%, and therefore to the same extent alters the correctness of the indications. (iv.) It should, if possible, be available both for alternating and continuous currents. (v.) It should be portable and work in any position. (vi.) It should not be disturbed easily by external electric or magnetic fields. This last point is important in connexion with voltmeters used on the switchboards of electric generating stations, where relatively strong electric or magnetic fields may be present, due to strong currents passing through conductors near or on the board. It is therefore always necessary to check the readings of such an instrument in situ. Electrostatic voltmeters are also liable to have their indications disturbed by electrification of the glass cover of the instrument; this can be avoided by varnishing the glass with a semi-conducting varnish so as to prevent the location of electrostatic charges on the glass.

See J. A. Fleming, Handbook or the Electrical Laboratory and Testing-Room (London, 1903); G. Aspinall Parr, Electrical Engineering Measuring Instruments (London, 1903); K. Edgecumbe and F. Punga, "On Direct Reading Measuring Instruments for Switchboard Use," Journ. Inst. Elec. Eng. (London, 1904), 33, 620.  (J. A. F.)