Beatrice Lang
The development of electrical measurement systems is closely tied to the advancements in electricity and the need for accurate billing. The invention of the battery by Alessandro Volta in 1800 sparked interest in electricity, but it wasn't until the introduction of the dynamo by Anyos Jedlik in 1861 and Werner von Siemens in 1867 that electricity could be generated in large amounts. As electricity became a commodity, the need for measurement arose. Early attempts at electrical measurement include Samuel Gardiner's lamphour meter in 1872 and Thomas Edison's chemical meter in 1879. However, these had their limitations. The development of the first electrical distribution systems by Edison and Nikola Tesla, and the invention of the watt-hour meter by Oliver Shallenberger in 1888, marked significant advancements. The work of Lucian Gaulard, John Dixon Gibbs, Károly Zipernowsky, Ottó Titusz Bláthy, Miksa Déri, and others, contributed to the AC electricity system, which gradually replaced DC systems. Today, electrical measurements are essential for various applications, from voltage and current meters to advanced wireless multimeters, with ever-increasing accuracy and safety.
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What You'll Learn
Early standardisation
Standardisation of measurement systems began with the French Revolution and the introduction of the metric system in 1799. This provided a uniform basis for modern measurement technology.
The invention of the battery by Alessandro Volta in 1800 meant electricity became widely accessible for the first time. This led to the development of the first ammeters, based on galvanometers, which, however, were confined mostly to laboratories due to their size and susceptibility.
In 1861, William Thomson, Lord Kelvin, invented the Kelvin bridge, a four-terminal sensing technique for measuring very low resistances. This was followed by the invention of the first portable DC voltmeter by Edward Weston in 1866, which had an accuracy of 0.5%.
The next significant development in electrical measurement technology came in 1872 with the invention of the first electricity meter by Samuel Gardiner of the USA. This lamphour meter measured the duration of energy supply to the load. However, it became obsolete with the introduction of Edison's light bulb, which allowed for the subdivision of lighting circuits.
In 1881, the Americans William Edward Ayrton and John Perry described the principle of the pendulum meter, which was later constructed independently by Hermann Aron in Germany in 1884. Aron's meter had two pendulums with a coil on each, connected to the voltage, with current coils winding in opposite directions, causing one pendulum to run slower and the other faster than without a load.
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The first electrical distribution systems
The California Electric Company used two direct current generators in 1879 to supply power to multiple residents in San Francisco. This was the first instance of a utility selling electricity from a central plant to multiple customers via transmission lines. The success of this system led to the construction of a second plant with four additional generators to serve more customers.
In the 1890s, further advancements were made, and alternating-current (AC) high-voltage distribution transmission systems were developed. These new AC lines allowed for the transmission of electric power over much longer distances than the previous inefficient DC systems. In 1896, George Westinghouse constructed an 11,000-volt AC line connecting Niagara Falls to Buffalo, New York, spanning a distance of 20 miles.
The evolution of electrical distribution systems continued, and by the late 1800s, the electric transmission and distribution system in the United States had transformed into a vast grid, bringing electricity to almost every part of the country. This grid comprises a complex network of independently owned transmission lines, spanning over 150,000 miles of high-voltage transmission lines that link generating facilities to load centres through interconnected transmission systems across states and territories.
Today, there is a growing trend towards distributed power systems as centralised power plants struggle to meet the high demand for AC electrical current. Distributed energy storage systems, utilising advanced grid energy storage technologies, offer the potential for cleaner and more reliable energy sources.
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The development of the multimeter
The galvanometer, invented in 1820, was the first moving-pointer current-detecting device. While it was useful in laboratories, it had limited practicality in the field due to its bulkiness and delicate nature. In 1866, Edward Weston addressed the need for a portable measuring device by publishing a portable DC voltmeter with an impressive accuracy of 0.5%.
The invention of the first multimeter is credited to British Post Office engineer Donald Macadie in 1920. Macadie created a single tool that could measure amps, volts, and ohms, inspired by the galvanometer. This multifunctional meter was named the Avometer. The first multimeters measured volts, milliamps, and ohms, and France introduced these innovations in 1924.
Over time, multimeters became more compact and portable, with smaller and more feature-rich versions becoming available in the 1930s. Handheld multimeters gained popularity in the 1960s, and the first handheld digital multimeter was developed by Frank Bishop of Intron Electronics in 1977.
The 1980s saw the emergence of digital multimeters, offering electricians more precision and special features that improved safety and accuracy. Today, wireless multimeters allow electricians to log and graph results on a computer, and advancements in smartphone technology have enabled seamless data transfer and storage in the cloud.
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Measuring AC electrical energy
The measurement of AC electrical energy is a complex process due to the alternating current's ever-changing direction. The voltage and current in an AC circuit periodically change direction, making power measurements more intricate than in direct current systems. To measure power, one must determine the rate at which energy is generated or used, which is calculated by measuring the RMS (root mean square) current and voltage and applying the formula P = VrmsArms.
The RMS value is the most common way to specify the value of AC voltage and current. It indicates the level of power available from a waveform, an essential attribute of any AC source. The average value of an AC waveform only has meaning over half of its cycle, as the average over a complete cycle is zero for a symmetrical waveform.
To measure real power, one must measure voltage and current at the same points on the waveform, multiply them, and then take the average value. This method is effective regardless of phase shift or wave shape. However, in the case of a reactive load, the real power may not be equal to the product of RMS voltage and current.
In higher energy circuits or when the meter is far from the power circuit, a Current Transformer (CT) and Potential Transformer (PT) are used. A single-phase three-wire system, for example, requires two single-phase wattmeters or one polyphase instrument with two measuring elements.
The development of electrical measurement technology has come a long way. Early galvanometers in the 1830s paved the way for the first multimeters in 1924. Handheld multimeters gained popularity in the 1960s, and digital multimeters emerged in the 1980s, offering greater precision to electricians.
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Four-terminal sensing
The technique involves using two-wire connections, or Kelvin connections, with a force-and-sense pair connected to a single terminal or lead. The current is supplied via the force connections (or current leads), which generate a voltage drop across the impedance, according to Ohm's law (V=IR). The sense connections (voltage leads) are placed immediately adjacent to the target impedance, excluding the voltage drop in the force leads. This arrangement ensures that only the voltage drop across the impedance is measured, without any interference from voltage drops in the current-carrying wires.
The four-terminal measurement is activated through the identification marking of pins in a pin table. During the measurement, the generator delivers a constant current, and the voltage drop across the resistor (RMEAS) is measured by a high impedance voltage measurement unit (VMU). This allows for accurate resistance measurements down to the milli-ohm range.
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Frequently asked questions
Four-terminal sensing, also known as Kelvin sensing, was invented by William Thomson, Lord Kelvin, in 1861. This system is used to measure very low resistances.
Four-terminal sensing is commonly used in low-voltage power supplies and resistance thermometers. It is used to measure the voltage delivered to a load independently of the voltage drop in the supply wires.
The first electrical measurement devices were early galvanometers in the 1830s, which led to the development of the first multimeters in 1924. The first mass application of electricity was lighting, and the earliest meter was Samuel Gardiner's lamphour meter, patented in 1872. The invention of the dynamo by Anyos Jedlik in 1861 and Werner von Siemens in 1867 allowed for the generation of large amounts of electrical energy.
