Wiktor Wise
Electrical resistance is a fundamental concept in electronics, and accurate measurement is crucial for designing circuits and troubleshooting equipment. The instrument used to measure electrical resistance in a conductor is called an ohmmeter. It is a type of ammeter that measures the current flowing in a circuit branch and the potential difference between any two points in a circuit. Multimeters are also used to measure resistance and can be used to measure voltage, check continuity, and perform other functions.
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What You'll Learn
Ohmmeter
An ohmmeter is an electrical instrument used to measure electrical resistance. It measures the opposition offered by a circuit or component to the flow of electric current.
To use an ohmmeter, it must be set to the lowest scale on the resistance setting, usually denoted by the Greek letter omega Ω. The ohmmeter is then connected to the circuit or component, and a small electrical current is applied to check the continuity of the circuit. The ohmmeter then measures the resulting voltage and calculates the resistance using Ohm's law.
It is important to note that an ohmmeter should not be connected to a circuit or component that is carrying a current or is connected to a power source. The power should be disconnected before connecting the ohmmeter. Ohmmeters can be connected in series or parallel, depending on whether the resistance being measured is part of a circuit or is a shunt resistance.
More advanced ohmmeters have four terminals, called Kelvin contacts, which improve the precision of the measurements. Two terminals carry the current from and to the meter, while the other two measure the voltage across the resistor. This technique, called Kelvin sensing, was invented by William Thomson, Lord Kelvin, in 1861, to measure very low resistances.
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Multimeter
A multimeter is a device that measures electrical properties such as voltage, current, and resistance in circuits and electrical components. It is a versatile tool that can also check continuity and perform other functions, making it invaluable for technicians and researchers in diagnosing problems and understanding circuit behaviour.
When choosing a multimeter, factors such as precision requirements and environmental conditions should be considered. While multimeters offer quick and convenient assessments, more detailed insights may require the use of specialised ohmmeters, which provide enhanced sensitivity and are better suited for fine adjustments and sensitive electronic parts.
In conclusion, multimeters are essential tools for measuring electrical resistance and other properties of circuits and electrical components. They offer versatility, portability, and accuracy, making them widely used in electrical engineering and troubleshooting applications.
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Wheatstone Bridge
The Wheatstone Bridge is an electrical circuit used to measure unknown electrical resistance. It was invented by Samuel Hunter Christie in 1833 and improved and popularised by Sir Charles Wheatstone in 1843. The Wheatstone Bridge works by balancing two legs of a bridge circuit, one of which includes the unknown component. The resistance in the known leg is adjusted until the bridge is "balanced" and no current flows through the galvanometer.
The Wheatstone Bridge is particularly useful when high accuracy is required. It can be used to compare an unknown resistance to a known resistance to determine its value with a high degree of precision. This allows for the accurate measurement of very low resistance values, down to the milli-Ohms (mΩ) range. The Wheatstone Bridge is also used to measure physical parameters such as temperature, light, and strain.
The Wheatstone Bridge has four resistances, which are connected to give a null centre value. The formula used for the Wheatstone Bridge is:
> {\displaystyle {\begin{aligned}V_{G}&=\left({R_{2} \over {R_{1}+R_{2}}}-{R_{x} \over {R_{x}+R_{3}}}\right)V_{s}\\ [6pt]R_{x}&={{R_{2}\cdot V_{s}-(R_{1}+R_{2})\cdot V_{G}} \over {R_{1}\cdot V_{s}+(R_{1}+R_{2})\cdot V_{G}}}R_{3}\end{aligned}}}
In this formula, VG is the voltage of node D relative to node B. The Wheatstone Bridge illustrates the concept of difference measurement, which can be extremely accurate.
The Wheatstone Bridge has many applications in sensing a range of mechanical and electrical quantities. One simple application is in the measurement of light using a photoresistive device. One of the resistors within the bridge network is replaced by a light-dependent resistor (LDR), which converts changes in visible light levels into a change in resistance and hence a voltage.
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Kelvin Bridge
The Kelvin Bridge, also known as the Kelvin double bridge or Thomson bridge, is a measuring instrument used to measure unknown electrical resistance in conductors. It was invented by William Thomson, Lord Kelvin, in 1861 to measure very low resistances. It is an advancement of the Wheatstone Bridge, which is used to measure higher resistances.
The Kelvin Bridge is specifically designed to measure resistors that are constructed as four-terminal resistors. These resistors have a pair of current terminals and a pair of potential or voltage terminals. In use, a current is passed between the current terminals, and the voltage drop across the resistor is measured at the potential terminals. This allows for the accurate measurement of the resistor itself, as the parasitic resistance of the leads carrying the current is not included in the potential circuit.
The Kelvin Bridge is particularly useful for measuring resistors with a value of less than 1 ohm. For resistors above 1 ohm, other techniques such as an ohmmeter or Wheatstone Bridge can be used. The Kelvin Bridge is often paired with a Null Detector to achieve the highest precision.
The operation of the Kelvin Bridge is similar to the Wheatstone Bridge but uses two additional resistors. The Kelvin Bridge is a precise instrument for measuring low-resistance values, typically in the range of 1mΩ to 1kΩ, with a smallest resolution of 1µΩ. It is an important tool for electrical engineering and circuit design, ensuring product quality and optimal performance.
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Megohmmeter
A megohmmeter, also known as an insulation resistance tester, is a type of ohmmeter used to measure the electrical resistance of insulators. Insulators are components used to impede electric current, and their resistance must be tested to ensure the safety and functionality of electrical equipment and installations.
To use a megohmmeter, the resistance to be measured is connected across the terminals, which are connected in series with the deflecting coil and across the generator. When the current is supplied to the coils, they experience torque in opposite directions. If the resistance being measured is high, no current will flow through the deflecting coil, and the controlling coil will set itself perpendicular to the magnetic field.
In addition to megohmmeters, there are other instruments available for measuring electrical resistance, such as multimeters and specialised ohmmeters. Each instrument has its advantages and is chosen based on the specific requirements of the testing environment and the level of precision needed.
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Frequently asked questions
An ohmmeter is an electrical instrument used to measure electrical resistance. It measures the opposition offered by a circuit or component to the flow of electric current.
There are two main types of ohmmeters: simple ohmmeters and precision ohmmeters. Simple ohmmeters are based on a meter movement known as a 'ratiometer' and are similar to galvanometers. Precision ohmmeters, on the other hand, have four terminals, or Kelvin contacts, which allow for more accurate measurements of very small resistances.
An ohmmeter applies a current to the circuit or component whose resistance is to be measured. It then measures the resulting voltage and calculates the resistance using Ohm's law.
Yes, in addition to ohmmeters, multimeters can also be used to measure electrical resistance. Multimeters are versatile tools that can measure voltage, check continuity, and perform various other functions. They are widely used for diagnosing problems and verifying circuit behaviour.
