Electrical Measurement

Summary

Electrical measurement has three primary aspects—the definition of units to describe the electrical properties being measured; the modeling, design, and construction of instrumentation by which those units may be applied in the measurement process; and the use of measurement data to analyze the functioning of electrical circuits. The measurement of any electrical property depends on the flow of electric current through a circuit. A circuit can exist under any conditions that permit the movement of electric charge, normally as electrons, from one point to another. In a controlled or constructed circuit, electrons move only in specific paths, and their movement serves useful functions.

Definition and Basic Principles

Electrical measurement refers to the quantification of electrical properties. As with all forms of measurement, these procedures provide values relative to defined standards. The basic electrical measurements are voltage, resistance, current, capacitance, and waveform analysis. Other electrical quantities such as inductance and power are generally not measured directly but are determined from the mathematical relationships that exist among actual measured properties of an electric circuit.

An electric circuit exists whenever physical conditions permit the movement of electrons from one location to another. It is important to note that the formation of a viable electric circuit can be entirely accidental and unexpected. For example, a bolt of lightning follows a viable electric circuit in the same way that electricity powering the lights in one's home follows a viable electric circuit.

Electrons flow in an electric circuit because of differences in electrical potential between one end of the circuit and the other. The flow of electrons in the circuit is called the current. When the current flows continuously in just one direction, it is called direct current, or DC. In direct current flow, the potential difference between ends of the circuit remains the same, in that one end is always relatively positive and the other is relatively negative. A second type of electric current is called alternating current (AC). In alternating current, the potential difference between ends of the circuit alternates signs, switching back and forth from negative to positive and from positive to negative. The electrons in alternating current do not flow from one end of the circuit to the other but instead oscillate back and forth between the ends at a specific frequency.

The movement of electrons through an electric circuit is subject to friction at the atomic level and to other effects that make it more or less difficult for the electrons to move about. These effects combine to restrict the flow of electrons in an overall effect called the resistance of the circuit. The current, or the rate at which electrons can flow through a circuit, is directly proportional to the potential difference (or applied voltage) and inversely proportional to the resistance. This basic relationship is the foundation of all electrical measurements and is known as Ohm's law.

Another basic and equally important principle is Kirchoff's current law, which states that the electric current entering any point in a circuit must always be equal to the current leaving that point in the circuit. In light of the definition of electric current as the movement of electrons from point to point through a conductive pathway, this law seems concrete and obvious. It is interesting to note, however, that it was devised in 1845, well before the identification of electrons as discrete particles and the discovery of their role in electric current.

Electrical measurement, like all measurements, is a comparative process. The unit of potential difference—called the volt—defines the electrical force required to move a current of one ampere through a resistance of one ohm. Devices that measure voltage are calibrated against this standard definition. This definition similarly defines the ohm but not the ampere. The ampere is defined in terms of electron flow, such that a current of one ampere represents the movement of one coulomb of charge (equivalent to 6.24 × 1018 electrons) past a point in a period of one second.

The capacitance of a device in an electric circuit is defined as the amount of charge stored in the device relative to the voltage applied across the device. The inductance of a device in an electric circuit is more difficult to define but may be thought of as the amount of current stored in the device relative to the voltage applied across the device. In both cases, the current flow is restricted as an accumulation of charge within the device but through different methods. Whereas a capacitor restricts the current flow by presenting a physical barrier to the movement of electrons, an inductor restricts current flow by effectively trapping a certain amount of flowing current within the device.

Background and History

Wild electricity—lightning and other natural phenomena that result from differences in the oxidation potentials of different materials—has been observed and known for ages. Artificially produced electricity may have been known thousands of years ago, although this has not been proven conclusively. For example, artifacts recovered from some ancient Parthian or Sasanian tombs near Baghdad, Iraq, bear intriguing similarities in construction to those of more modern electrochemical cells or batteries. Reconstructions of the ancient device have produced an electric current at about 0.87 volts, and other observations indicate that the devices may have been used to electroplate metal objects with gold or silver.

The modern battery, or voltaic pile, began with the work of Alessandro Volta in 1800. During the nineteenth century, a number of other scientists investigated electricity and electrical properties. Many of the internationally accepted units of electrical measurement were named in their honor.

How It Works

Ohm's Law. The basis of electrical measurement is found in Ohm's law, derived by Georg Simon Ohm in the nineteenth century. According to Ohm's law, the current flowing in an electric circuit is directly proportional to the applied voltage and inversely proportional to the resistance of the circuit. In other words, the greater the voltage applied to the circuit, the more current will flow. Conversely, the greater the resistance of the circuit, the less current will flow. This relationship can be stated mathematically as E = I × R (where E = voltage, I = current, and R = resistance), in which voltage is represented as the product of current and resistance.

Given this relationship, it is a relatively simple matter to design a device that uses two specific properties to determine the third. By constructing a device that employs set values of voltage and resistance, one can measure current. Similarly, by constructing a device that describes a system in which current and resistance are constant, one can measure voltage, and by devising a system in which current and voltage are regulated, one can measure resistance.

If the three primary properties of a circuit are known, then all other properties can be determined by arithmetic calculations. The capacitance of a circuit or circuit component, for example, is the amount of charge stored in the device at a given applied voltage, and the amount of charge is proportional to the current in the device. Similarly, the inductance in a circuit or circuit component depends on the current passing through the device at a given voltage.

Units of Measurement. All electrical properties must have an associated defined standard unit to be measurable. To that end, current is measured in amperes, named after André-Marie Ampère. The potential difference, sometimes called electromotive force, is measured in volts, named after Volta. Resistance is measured in ohms, named after Ohm. Power is measured in watts, named after James Watt. Capacitance is measured in farads, named after Michael Faraday. Inductance is measured in henrys, named after Joseph Henry. Conductance, the reciprocal of resistance, is measured in Siemens, named after Ernst W. von Siemens. Frequencies are measured in hertz, or cycles per second, named after Gerhard Hertz.

Basic Electricity Concepts. Electricity can be produced in a continuous stream known as direct current (DC), in which electrons flow continuously from a negative source to a positive sink. The potential difference between the source and the sink is the applied voltage of the circuit, and it does not change. Electricity can also be produced in a varying manner called alternating current (AC), in which electron flow oscillates back and forth within the circuit. The applied voltage in such a system varies periodically between positive and negative values that are equal in magnitude. It is important to understand that circuits designed to operate with one type of applied voltage do not function when the other type of voltage is applied. In other words, a circuit designed to perform certain functions when supplied with a constant voltage and direct current will not perform those functions when supplied with a varying voltage and alternating current. The fundamental concept of Ohm's law applies equally to both cases, but other characteristics such as phase and frequency differences and voltage waveform make the relationships more complex in alternating current applications. Electrical measurement devices are designed to accommodate these characteristics and are capable of extremely fine differentiation and precision.

Applications and Products

The easiest electrical properties to measure accurately are voltage and resistance. Thus, the primary application tool of electrical measurement is the common volt-ohm meter (VOM), either as an analog device or as its digital counterpart, the digital volt-ohm meter (DVOM).

Basic Analog Measuring Devices. Two systems are required for any measuring device. One system is the structure by which the unknown value of a property is measured, and the other is the method of indicating that value to the user of the device. This latter feature was satisfied through the application of electromagnetic induction in the moving coil to produce a D'Arsonval movement. The strength of a magnetic field produced by current flowing through a coil is directly proportional to the magnitude of that current. In a basic analog measuring device, the small coil is allowed to pivot freely within a permanent magnetic field. The amount by which the coil pivots is determined by the amount of current flowing through it. A needle attached to the coil indicates the appropriate value being measured on a calibrated scale. Analog meters used to measure most electrical properties employ the moving-coil system to indicate the value of the property being measured.

Basic Digital Measuring Devices. The advent of digital electronics brought about a revolution in electrical measurement devices. Digital meters use several systems to produce the desired information. The basic operation of digital devices is controlled by a central clock cycle so that the value of inputs is effectively measured thousands of separate times per second rather than continuously. This is known as sampling. Because the flow of electricity is a continuous or analog process, sampling converts the analog input value to a digital data stream. The data values can then be manipulated to be displayed directly as a numerical readout, eliminating the guesswork factor involved in reading a needle scale indicator. Another advantage of digital measurement devices is their inherent sensitivity. Using an operational amplifier, or op-amp integrated circuits (IC), input signals can be amplified by factors of hundreds of thousands. This allows extremely small electrical values to be measured with great accuracy.

Other Measuring Devices. One of the most valuable devices in the arsenal of electrical measurement is the oscilloscope, which is available in both analog and digital models. Like a typical meter, the oscilloscope measures electrical inputs, but it also has the capability to display the reading as a dynamic trace on a display screen. The screen was typically a cathode-ray tube (CRT) display, but later versions use liquid crystal display (LCD) screens and even can be used with desktop and laptop computers.

Another highly useful device for electrical measurement is the simple logic probe used with digital circuitry. In digital electronics, the application of voltages, and therefore the flow of current, is not continuous but appears in discrete bits as either an applied voltage or no applied voltage. These states are known as logic high and logic low, corresponding to on and off. The manipulation of these bits of data is governed by strict Boolean logic (a system for logical operations on which digital electronics is based). Accordingly, when a digital device is operating properly, certain pins (or leads) of an integrated-circuit connection must be in the logic high state while others are in the logic low state. The logic probe indicates which state any particular pin is in, generally by whether an indicator light in the device is on or off. Unlike a meter, the logic probe does not provide quantitative data or measurements, but it is no less invaluable as a diagnostic tool for troubleshooting digital circuitry.

Ancillary Devices. A more specific analytical device is the logic analyzer. Designed to read the input or pin signals of specific central processing unit (CPU) chips in operation, the logic analyzer can provide a running record of the actual programming used in the function of a digital electronic circuit. Another device used in electrical measurement, more typically as an electrical source than a measuring device, is the waveform generator. This device is used to provide a specific shape of input voltage for an electric circuit to verify or test the function of the circuit.

Indirect Applications. Because of the high sensitivity that can be achieved in electrical measurement, particularly in the application of digital electronic devices, the measurement of certain electrical properties is widely used in analytical devices. The most significant electrical property employed in this way is resistance measurement. Often, this is measured as its converse property—conductivity. Gas-phase detectors on analytical devices such as gas chromatographs, high-performance liquid chromatographs, and combination devices with mass spectrometers are designed to measure the resistance, or conductivity, of the output stream from the device. For example, in gas chromatography, a carrier gas is passed through a heated column packed with a chromatography medium. When a mixture of compounds is injected into the gas stream, the various components become separated as they pass through the column. As the materials exit the column, they pass through a detector that measures changes in conductivity that occur with each different material. The changes are recorded either as a strip chart or as a collection of data.

The sensitive measurement of electrical resistance is made possible by the use of specific electric circuits, most notably the Wheatstone bridge circuit. In a Wheatstone bridge circuit, four resistance values are used, connected in a specific order. Two sets of resistance values are connected parallel to each other, with each set containing two resistance values connected in series with each other. One of the resistance values is the unknown quantity to be measured, while the other three are precisely known values. The voltage between the two midpoints of the series circuits of the bridge changes very precisely with any change in one of the resistance values. In addition, the output voltage signal can be amplified by several orders of magnitude, making even very small changes in output voltage meaningful. The role of digital electronics and op-amps cannot be overstated in this area.

Careers and Course Work

Studies related to the use of electrical devices provide a good, basic knowledge of electrical measurement. A sound basis in physics and mathematics will also be required. College and university-level coursework will depend largely on the chosen area of specialization. Options at this level range from basic electrical service technician to fundamental physics. At a minimum, students will pursue studies in mathematics, physical sciences, industrial technologies (chemical, electrical, and mechanical), and business at the undergraduate level or as trade students. More advanced studies usually consist of specialized courses in a chosen field. The study of applied mathematics will be particularly appropriate in advanced studies, as this branch of mathematics provides the mathematical basis for phase relationships, quantum boundaries, and electron behavior, which are the central electrical measurement features of advanced practices.

As electrical measurement technology, applications, and regulations governing the distribution of electric energy change, those working in the field can expect to be required to upgrade their working knowledge on an almost continual basis to keep abreast of changes.

Social Context and Future Prospects

Economics drives the production of electricity for consumption. A considerable amount of research and planning has been devoted to the smart grid, in which an electric grid is capable of controlling electricity distribution according to demand. Effective electrical measurement is necessary for this concept to become a widespread reality.

At a very basic level, the service industry for the maintenance of electrically powered devices will continue to employ many, particularly as the green movement tends to shift society toward recycling and recovering materials rather than merely disposing of them. Accordingly, repair and refurbishment of existing units and the maintenance of residential wiring systems would focus more heavily on effective troubleshooting methods to determine the nature of flaws and correct them before any untoward incident should occur.

The increased focus on alternative energy sources, particularly on the development of solar energy and fuel cells, will also place higher demands on the effectiveness of electrical measurement technology. This will be required to ensure that the maximum amount of usable electric energy is being produced and that the electric energy produced is used effectively.

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