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AMPEREMETER). The fixed coil is called the current coil, and the movable coil is called the potential coil, and each of these coils has its ends brought to See also:

SEPARATE

separate terminals on the See also:

BASE

base of the instrument. The principle on which the instrument See also:

WORKS

works is as follows: Suppose any circuit, such as an electric motor, See also:

LAMP (from Gr. Xag ras, a torch, Xaµaav, to shine)

lamp or transformer, is receiving electric current; then the power given to that circuit reckoned 1 For the See also:

PROOF (in M. Eng. preove, proeve, preve, &°c., from O. Fr . prueve, proeve, &c., mod. preuve, Late. Lat. proba, probate, to prove, to test the goodness of anything, probus, good)

proof of this See also:

formula see J. A. See also:

Fleming, The Alternate Current Transformer in Theory and Practice, i. 168. power See also:

FACTOR (from Lat. facere, to make or do)

factor in those cases in which the circuit is inductive and the current alternating. Take first the simplest See also:

case of a .non-inductive power-absorbing circuit. If an electro-See also:

DYNAMOMETER (Gr. Suva u , strength, and OTpov, a measure)

dynamometer, made as above described, has its fixed circuit connected in See also:

SERIES (a Latin word from serere, to join)

series with the power-absorbing circuit and its movable coil (See also:

WOUND (O. Eng. wund,connected with a Teutonic verb, meaning to strive, fight, suffer, seen in O. Eng. winnan, whence Eng. " win ")

wound with See also:

FINE

fine wire) connected across the terminals of the power-absorbing circuit, then a current will flow through the fixed coil which is the same or nearly the same as that through the power-absorbing circuit, and a current will flow through the high resistance coil of the wattmeter proportional to the potential difference at the terminals of the power-absorbing circuit. The movable coil of the wattmeter is normally suspended so that its See also:

AXIS (Lat. for " axle ")

axis is at right angles to that of the fixed coil and is constrained by the torsion of a See also:

SPIRAL

spiral See also:

SPRING (from " to spring," " to leap or jump up," " burst out," O. Eng., springau, a common Teut. word, cf. Ger. springer, possibly allied to Gr. QnrFpxecOac, to move rapidly)

spring.

When the currents flow through the two coils, forces are brought into See also:

ACTION

action compelling the coils to set their axes in the same direction, and these forces can be opposed by another See also:

TORQUE, or TORO (Lat. torquis, torques, a twisted collar, torquere, to twist)

torque due to the See also:

CONTROL (Fr. contrdle, older form contre rolle, from Med. Lat. contra-rotulus, a counter roll or copy of a document used to check the original; there is no instance in English of the use of " control " in this, its literal, meaning)

control of a spiral spring regulated by moving a torsion See also:

head on the instrument. The torque required to hold the coils in their normal position is proportional to the mean value of the product of the currents flowing through two coils respectively, or to the mean value of the product of the current in the power-absorbing circuit and the potential difference at its ends, that is, to the power taken up by the circuit. Hence this power can be measured by the torsion which must be applied to the movable toil of the wattmeter to hold it in the normal position against the action of the forces tending ' to displace it. The wattmeter can therefore be calibrated so as to give See also:

DIRECT

direct readings of the power reckoned in See also:

watts, taken up in the circuit; hence its name, wattmeter. In those cases in which the power-absorbing circuit is inductive, the coil of the wattmeter connected across the terminals of the power-absorbing circuit must have an exceedingly small inductance, else a considerable correction may become necessary. This correcting factor has the following value: If Ts stands for. the See also:

time-See also:

CONSTANT, BENJAMIN (1845-1902)

constant of the movable circuit of the wattmeter, commonly called the potential coil, the time constant being defined as the ratio of the inductance to the resistance of that circuit, and if T. is the time-constant similarly defined of the power-absorbing circuit, and if F is the correcting factor, and p =2r times the frequency n, then,' +. p2Ts2 F = +p2TsTx• Hence an electrodynamic wattmeter, applied to measure the See also:

electrical power taken up in a circuit when employing alternating currents. gives absolutely correct readings only in two cases—(i.) when the potential circuit of the wattmeter and the power-absorbing circuit have negligible inductances, and (ii.) when the same two circuits have equal time-constants. If these conditions are not fulfilled, the wattmeter readings, assuming the wattmeter to have been calibrated with continuous currents, may be either too high or too See also:

low when alternating currents are being used. In See also:

order that a wattmeter shall be suitable for the measurement of power taken up in an inductive circuit certain conditions of construction must be fulfilled. The framework and case of the instrument must be completely non-metallic, else eddy currents induced in the supports will cause disturbing forces to See also:

ACT (Lat. actus, actum)

act upon the movable coil. Again the shunt circuit must have practically zero inductance and the series or current coil must be wound or constructed with stranded See also:

COPPER (symbol Cu, atomic weight 63.1, H=1, or 63.6, O = 16)

copper wire, each strand being See also:

SILK

silk covered, to prevent the See also:

PRODUCTION (Lat. productionem, from producere, to pro-duce)

production of eddy currents in the See also:

mass of the conductor. Wattmeters of this See also:

KIND (O. E. ge-cynde, from the same root as is seen in " kin," supra)

kind have been devised by J. A.

Fleming, See also:

Lord See also:

KELVIN, WILLIAM THOMSON, BARON (1824-1907)

Kelvin and W. DuddelI and See also:

Mather. W. E. Sumpner, however, has devised forms of wattmeter of the dynamometer type in which See also:

IRON [symbol Fe, atomic weight 55.85 (0=16)]

iron cores are employed, and has defined the conditions under which these See also:

INSTRUMENTS

instruments are available for accurate measurements. See " New Alternate Current Instruments," Jour. Inst. Elec. Eng., 41, 227 (1908). There are methods of measuring electrical power by means of electrostatic voltmeters, or of quadrant electrometers adapted ,for the purpose, which when so employed may be called electrostatic wattmeters. If the quadrants of an See also:

ELECTROMETER

electrometer '(q.v.) are connected to the ends of a non-inductive circuit in series with the power-absorbing circuit, and if the See also:

NEEDLE (O. Eng. ncedl; the word appears in various forms in Teutonic languages, Ger. Nadel, Dutch naal, the root being ne-, to sew, cf. Ger. nahen, and probably Lat. nere, to spin, Gr. vi)ats, spinning)

needle is connected to the end of this last circuit opposite to that at which the inductionless resistance is connected, then the deflexion of the electrometer will be proportional to the power taken up in the circuit, since it is proportional to the mean value of (A—B) {C—2 (A +B)}, where A and B are the potentials of the quadrants and C is that of the needle. This expression, however, See also:

MEASURES

measures the power taken up in the power-absorbing circuit.

In the case of the See also:

VOLTMETER

voltmeter method of measuring power devised by W. E. See also:

AYRTON, WILLIAM EDWARD (1847-1908)

Ayrton and W. E. Sumpner in 1891, an electrostatic voltmeter is employed to measure the fall of potential Vi down any inductive circuit in which it is desired to measure the power absorption, and also the volt-drop V2 down an inductionless resistance R in series with it, and also the volt-drop V3 down the two together. The power absorption is then given by the expression (V32-V12-V22)/2R. For methods of employing the See also:

HEATING

heating power of a current to construct a wattmeter see a See also:

paper by J. T. Irwin on " Hot-wire Wattmeters," Jour. Inst. Elec. Eng.

(1907), 39, 617. For the details of these and many other methods of employing wattmeters to measure the power absorption in single and polyphase circuits the reader is referred to the following works: J. A. Fleming, Handbook for the Electrical Laboratory and Testing See also:

ROOM

Room (1903); Id., The Alternate Current Transformer in Theory and Practice (1905) ; G. Aspinall See also:

Parr, Electrical See also:

ENGINEERING

Engineering Measuring Instruments (1903); A. See also:

Gray, See also:

ABSOLUTE (Lat. absolvere, to loose, set free)

Absolute Measurements in See also:

ELECTRICITY

Electricity and See also:

Magnetism (19o0); E. See also:

Wilson, " The Kelvin Quadrant Electrometer as a Wattmeter," Proc. See also:

ROY, WILLIAM (1726-1790)

Roy. See also:

Soc. (1898), 62, 356; J. See also:

SWINBURNE, ALGERNON CHARLES (1837–1909)

Swinburne, " The Electrometer as a Wattmeter," Phil. Mag.

(See also:

JUNE

June 1891) ; W. E. Ayrton and W. E. Sumpner, " The Measurement of the Power given by an Electric Current to any Circuit," Proc. Roy. Soc. (1891), 49, 424; Id., " Alternate Current and Potential Difference Analogies in the Method of Measuring Power," Phil. Mag. (See also:

AUGUST (originally Sextilis)

August 1891); W. E. Ayrton, " Electrometer Methods of Measuring Alternating Current Power," Journ.

Inst. Elec. Eng. (1888), 17, 164; T. H. See also:

BLAKESLEY, JOSEPH WILLIAMS (18o8–1885)

Blakesley, " Further Contributions to Dynamometry or the Measurement of Power," Phil. Mag. (See also:

APRIL

April 1891) ; G. L. Addenbrooke, " The Electrostatic Wattmeter and its See also:

CALIBRATION

Calibration and See also:

ADAPTATION (from Lat. adaptare. to fit to)

Adaptation for Polyphase Measurements," Electrician (1903), 51, 811; W. E. Sumpner, " New Iron-cored Instruments for Alternate Current Working," Jour.

Inst. Elec. Eng., 36, 421 (1906). (J. A.

End of Article: WATTMETER

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