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[[Image:Wattmeter.jpg|thumb|Wattmeter]]
The '''wattmeter''' is an instrument for measuring the [[electric power]] (or the supply rate of [[electrical energy]]) in [[watt]]s of any given [[electrical network|circuit]].

An instrument which measures electrical [[energy]] in [[watt hour]]s ([[electricity meter]] or [[energy analyser]]) is essentially a wattmeter which accumulates or averages readings; many such instruments measure and can display many parameters and can be used where a wattmeter is needed: volts, current ,in amperes, apparent instantaneous power, actual power, power factor, energy in [k]Wh over a period of time, and cost of electricity consumed.

==Electrodynamic==
[[Image:WattmeterAcostaMexico.JPG|thumb|right| Early wattmeter on display at the Historic Archive and Museum of Mining in [[Pachuca]], [[Mexico]].]]
The traditional analog wattmeter is an [[electrodynamic]] instrument. The device consists of a pair of fixed [[coil]]s, known as ''current coils'', and a movable coil known as the ''potential coil''.



The current coils connected in [[Series and parallel circuits|series]] with the circuit, while the potential coil is connected in [[Series and parallel circuits|parallel]]. Also, on [[analog signal|analog]] wattmeters, the potential coil carries a needle that moves over a scale to indicate the measurement. A current flowing through the current coil generates an [[electromagnetic field]] around the coil. The strength of this field is proportional to the line current and in phase with it. The potential coil has, as a general rule, a high-value [[resistor]] connected in series with it to reduce the current that flows through it.

The result of this arrangement is that on a [[direct current|dc]] circuit, the deflection of the needle is proportional to ''both'' the [[electric current|current]] ''and'' the [[voltage]], thus conforming to the equation W=VA or ''P''=''VI''. 

For [[AC power]], current and voltage may not be in step, owing to the delaying effects of circuit [[inductance]] or [[capacitance]]. On an [[alternating current|ac]] circuit the deflection is proportional to the average instantaneous product of voltage and current, thus measuring [[AC_power#Real.2C_reactive.2C_and_apparent_powers|true power]], ''P''=''VI'' cos ''φ''. Here, cos''φ'' represents the [[power factor]] which shows that the power transmitted may be less than the apparent power obtained by multiplying the readings of a [[voltmeter]] and  [[ammeter]] in the same circuit.

The two circuits of a wattmeter can be damaged by excessive current. The [[ammeter]] and [[voltmeter]] are both vulnerable to overheating&nbsp;— in case of an overload, their pointers will be driven off scale&nbsp;— but in the wattmeter, either or even both the current and potential circuits can overheat ''without'' the pointer approaching the end of the scale! This is because the position of the pointer depends on the [[power factor]], [[voltage]] and [[electric current|current]]. Thus, a circuit with a low [[power factor]] will give a low reading on the wattmeter, even when both of its circuits are loaded to the maximum safety limit. Therefore, a wattmeter is rated not only in [[watt]]s, but also in [[volt]]s and [[ampere]]s.

A typical wattmeter in educational labs has two voltage coils (pressure coils) and a current coil. We can connect the two pressure coils in series or parallel to each other to change the ranges of the wattmeter. Another feature is that the pressure coil can also be tapped to change the meter's range. If the pressure coil has range of 300 volts, the half of it can be used so that the range becomes 150 Volts.

==Electrodynamometer==
[[Image:Siemens electrodynamometer.png|thumb|right|250px|Siemens electrodynamometer, circa 1910.
F = Fixed coil, D = Movable coil, S = Spiral spring, T = Torsion head, MM = Mercury cups, I = Index needle]]
An early current meter was the electrodynamometer. Used in the early 20th century, the Siemens electrodynamometer, for example, is a form of an electrodynamic ammeter, that has a fixed [[coil]] which is surrounded by another having its axis at right angles to that of the fixed coil. This second coil is suspended by a number of silk fibres, and to the coil is also attached a spiral spring the other end of which is fastened to a [[Torsion (mechanics)|torsion]] head. If then the torsion head is twisted, the suspended coil experiences a torque and is displaced through an angle equal to that of the torsion head. The current can be passed into and out of the movable coil by permitting the ends of the coil to dip into two mercury cups.

If a current is passed through the fixed coil and movable coil in series with one another, the movable coil tends to displace itself so as to bring the axes of the coils, which are normally at right angles, more into the same direction. This tendency can be resisted by giving a twist to the torsion head and so applying to the movable coil through the spring a restoring torque, which opposes the torque due to the dynamic action of the currents. If then the torsion head is provided with an index needle, and also if the movable coil is provided with an indicating point, it is possible to measure the [[Dihedral angle|torsional angle]] through which the head must be twisted to bring the movable coil back to its zero position. In these circumstances, the torsional angle becomes a measure of the torque and therefore of the product of the strengths of the currents in the two coils, that is to say, of the square of the strength of the current passing through the two coils if they are joined up in series. The instrument can therefore be calibrated by passing through it known continuous currents, and it then becomes available for use with either continuous or alternating currents. The instrument can be provided with a [[calibration curve]] or table showing the current corresponding to each angular displacement of the torsion head.

==Electronic wattmeter==
[[Image:Plug-in Power & Energy Monitor in UK Domestic Mains Socket.jpeg|thumb|right|Prodigit Model 2000MU (UK version), shown in use and displaying a reading of 10 Watts being consumed by the appliance.]]
Electronic wattmeters are used for direct, small power measurements or for power measurements at frequencies beyond the range of electrodynamometer-type instruments.

===Digital===
A modern digital electronic wattmeter/energy meter samples the voltage and current thousands of times a second. For each sample, the voltage is multiplied by the current at the same instant; the average over at least one cycle is the real power. The real power divided by the apparent [[volt-ampere]]s (VA) is the power factor. A computer circuit uses the sampled values to calculate RMS voltage, RMS current, VA, power (watts), power factor, and kilowatt-hours. The readings may be displayed on the device, retained to provide a log and calculate averages, or transmitted to other equipment for further use. Wattmeters vary considerably in correctly calculating energy consumption, especially when real power is much lower than VA (highly [[reactance|reactive]]loads, e.g. [[electric motors]]). Simple meters may be calibrated to meet specified accuracy only for [[Sine wave|sinusoidal]] waveforms. Waveforms for [[Switching power supply|switching power supplies]] as used for much [[electronic equipment]] may be very far from sinusoidal, leading to unknown and possibly large errors at any power. This may not be specified in the meter's manual.

There are limitations to measuring power with inexpensive wattmeters, or indeed with any meters not designed for low-power measurements. This particularly affects low power (e.g. under 10 watts), as used in standby; readings may be so inaccurate as to be useless (although they do confirm that standby power is low, rather than high)<ref name=ll>[http://standby.lbl.gov/measure.html US Lawrence Livermore laboratory, Standby Power, measuring standby]</ref>. The difficulty is largely due to difficulty in accurate measurement of the alternating current, rather than voltage, and the relatively little need for low-power measurements. The specification for the meter should specify the reading error for different situations. For a typical plug-in meter the error in wattage is stated as ±5% of measured value ±10W (e.g., a measured value of 100W may we wrong by 5% of 100W plus 10W, i.e., ±15W, or 85-115W); and the error in kWh is stated as ±5% of measured value ±0.1kWh.<ref>Data listed in text from manual for inexpensive plug-in electricity meter Brennenstuhl PM230.  The lowest measurable current is given as 0.02A, which corresponds to about 5w at 230VAC</ref> If a laptop computer in sleep mode consumes 5W, the meter may read anything from 0 to 15.25W, without taking into account errors due to non-sinusoidal waveform.<!--Typical example: I have found power measured by an accurate wattmeter as 5W to show as 9W on a simple plugin meter. pol098-->  In practice accuracy can be improved by connecting a fixed load such as an incandescent light bulb, adding the device in standby, and using the difference in power consumption<ref name=ll/>. This moves the measurement out of the problematical low-power zone.

==Radio frequency==
Instruments with moving coils can be calibrated for direct current or [[utility frequency|power frequency]] currents up to a few hundred Hz. At radio frequencies a common method is a rectifier circuit arranged to respond to current in a [[transmission line]]; the system is calibrated for the known circuit impedance. Alternatively the source of power to be measured can be used to heat a sensor whose temperature, which depends upon power dissipated, is measured ([[bolometer]]).

==See also==
{{Portal|Electronics}}
{{Commons category|Wattmeters}}
* [[Ammeter]]
* [[Voltmeter]]
* [[Power factor]]
* [[Electricity meter]]
* [[Microwave power meter]]

==References==
* {{1911}}
{{Reflist}}
{{Electrical and electronic measuring equipment}}

[[Category:Measuring instruments]]
[[Category:Electronic test equipment]]
[[Category:Electric power]]

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