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VOLTMETER

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Originally appearing in Volume V28, Page 207 of the 1911 Encyclopedia Britannica.
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VOLTMETER , an See also:

instrument for measuring difference of electric potential (see See also:ELECTROSTATICS) in terms of the unit called a volt. The volt (so called after A. See also:Volta) is defined to be difference of potential which acting between the terminals of a resistance of one See also:ohm sends through it a continuous current of one See also:ampere. A voltmeter is therefore one See also:form of See also:electrometer (q.v.), but the See also:term is generally employed to describe the instrument which indicates on a See also:scale, not merely in arbitrary See also: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 See also: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 See also: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 See also:metal See also:plate or plates and a movable plate or plates, the two sets of plates forming a See also:condenser (see See also:LEYDEN See also:JAR). The movable See also:system is suspended or pivoted, and when a P. D. is created between the fixed and movable plates, the latter are See also:drawn into a new position which is resisted by the See also:torque of a See also:wire or by the force due to a See also:weight.

Utilizing this principle many inventors have devised forms of electrostatic voltmeter. One of the best known of these is See also:

Lord See also:Kelvin's multicellular voltmeter. In this instrument (fig. 1) there are two sets of fixed metal plates, connected together and having a quadrantal shape, that is, approximately the shape of a See also:quarter of a circular disk In the space between them is suspended a " See also:needle " which consists of a See also:light See also:aluminium See also:axis, to which are affixed a number of See also:paddle-shaped aluminium See also:blades. This needle is suspended by a See also:fine See also:platinum See also: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 See also: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 See also:movement is resisted by the torsional See also: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. See also:Instruments of this See also:kind have been constructed not only by Lord Kelvin, but also by W. E. See also:Ayrton and others, for measuring voltages from 10,000 volts down to i volt. In other types of electrostatic instruments the movable system rotates See also:round a See also:horizontal axis or rests upon See also:knife edges like a scale See also:beam; in others again the movable system is suspended by a wire. In the former See also:case the See also: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 See also: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 See also:

arm; or the movable system may take the form of a See also: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 volt-meters 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 See also:vertical See also:pattern voltmeter (fig. 2). For use at the switch-boards of electric See also:supply stations the instrument takes another form known as the " edge-See also:wise " pattern. Another class of voltmeters comprises the electrokinetic volt-meters. 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 am- See also:meter of high resistance, and may take any of the forms of practically used ammeters (see See also:AMPEREMETER). Electromagnetic voltmeters may therefore be thermal, electromagnetic or electrodynamic.

As a See also:

rule, electromagnetic voltmeters are only suitable for the measurement of relatively small potentials—o 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 See also:tube or upon a See also:frame contained in a tube. This frame or tube is so constructed of See also:iron and See also: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 See also:motion-multiplying See also:gear. As the frame has the same linear expansion as the wire, See also: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 See also: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 See also:short, must be placed in See also: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 See also:heating See also: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 See also: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 fixed permanent magnet, producing a See also:

constant magnetic field, and in the interspace between the poles is fixed a delicately pivoted coil of wire carried in jewelled See also:bearings. The normal position of this coil is with its See also: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 See also: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 See also: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 See also:spiral spring resembling the See also:hair-spring of a See also: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 See also:great See also:advantage that the scales are equidivisional and that there is no dead See also: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 See also:close as to be useless for See also:reading purposes and are therefore omitted. For the measurement of voltages in continuous current generating stations, 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 volt-meter.

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 See also:

Siemens See also:dynamo-meter employed for measuring continuous alternating current (see AMPEREMETER). Another much-used method of measuring See also:con- tinuous current voltages or unidirectional potential diference employs the principle of See also: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 See also:wound non-inductively and an electrostatic voltmeter is used. The high-resistance wire should, moreover, be one having a negligible See also: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 See also:expenditure of See also:energy to maintain this current and therefore involves cost of See also: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 See also:watts in power. Since there are 876o See also:hours in a See also:year, if such an instrument were connected continuously to the circuit it would take up energy equal to 263,000 See also:watt-hours, or 26o See also:Board of See also:Trade units per annum.

If the cost of production of this energy was only one See also:

penny per unit, the working expenses of keeping such a voltmeter in connexion with a circuit would therefore be more than L1 per annum, representing a capitalized value of, say, Do. Electrostatic instruments, however, take up no power and hence cost nothing for See also:maintenance other than See also:wear and See also:tear of the instrument. The qualities required in a See also:good voltmeter are:—(i.) It should be See also:quick in See also: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 See also:lower value or decreasing from a larger value; in other words, there should be no instrumental See also: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 See also:copper wire an increase of one degree centigrade in the See also: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 See also:work in any position. (vi.) It should not be disturbed easily by external electric or magnetic See also: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 See also: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 See also:glass See also:cover of the instrument; this can be avoided by varnishing the glass with a semi-conducting See also:varnish so as to prevent the location of electrostatic charges on the glass. See J.

A. See also:

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

End of Article: VOLTMETER

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