Copyright for the above image is held by MAAS and may be subject to third-party copyright restrictions. This includes artworks, artifacts, images and recordings of people who may have passed away, and other objects which may be culturally sensitive. Next Continue. A Wheatstone bridge can be used to measure resistance by comparing an unknown resistor against precision resistors of known value. A Kelvin double bridge is a variant of the Wheatstone bridge and used for measuring very low resistances.

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No text on electrical metering could be called complete without a section on bridge circuits. The standard bridge circuit, often called a Wheatstone bridge , looks something like this:. To measure resistance with a Wheatstone bridge, an unknown resistance is connected in the place of R a or R b , while the other three resistors are precision devices of known value. Either of the other three resistors can be replaced or adjusted until the bridge is balanced, and when balance has been reached the unknown resistor value can be determined from the ratios of the known resistances.

A requirement for this to be a measurement system is to have a set of variable resistors available whose resistances are precisely known, to serve as reference standards. For example, if we connect a bridge circuit to measure an unknown resistance R x , we will have to know the exact values of the other three resistors at balance to determine the value of R x :.

Each of the four resistances in a bridge circuit are referred to as arms. The resistor in series with the unknown resistance R x this would be R a in the above schematic is commonly called the rheostat of the bridge, while the other two resistors are called the ratio arms of the bridge. Accurate and stable resistance standards, thankfully, are not that difficult to construct. Here is a photograph of an antique resistance standard unit:.

This resistance standard shown here is variable in discrete steps: the amount of resistance between the connection terminals could be varied with the number and pattern of removable copper plugs inserted into sockets.

Wheatstone bridges are considered a superior means of resistance measurement to the series battery-movement-resistor meter circuit discussed in the last section.

Unlike that circuit, with all its nonlinearities nonlinear scale and associated inaccuracies, the bridge circuit is linear the mathematics describing its operation are based on simple ratios and proportions and quite accurate. It is the preferred method of resistance measurement in calibration laboratories due to its high accuracy.

There are many variations of the basic Wheatstone bridge circuit. Most DC bridges are used to measure resistance, while bridges powered by alternating current AC may be used to measure different electrical quantities like inductance, capacitance, and frequency. Its schematic diagram is as such:.

The low-value resistors are represented by thick-line symbols, and the wires connecting them to the voltage source carrying high current are likewise drawn thickly in the schematic. This oddly-configured bridge is perhaps best understood by beginning with a standard Wheatstone bridge set up for measuring low resistance, and evolving it step-by-step into its final form in an effort to overcome certain problems encountered in the standard Wheatstone configuration.

If we were to use a standard Wheatstone bridge to measure low resistance, it would look something like this:. We have a problem, in that the connections and connecting wires between R a and R x possess resistance as well, and this stray resistance may be substantial compared to the low resistances of R a and R x. However, the two remaining E wire voltage drops will cause problems, as the wire connecting the lower end of R a with the top end of R x is now shunting across those two voltage drops, and will conduct substantial current, introducing stray voltage drops along its own length as well.

We can manage the stray voltage drops between R a and R x by sizing the two new resistors so that their ratio from upper to lower is the same ratio as the two ratio arms on the other side of the null detector. This is why these resistors were labeled R m and R n in the original Kelvin Double bridge schematic: to signify their proportionality with R M and R N. The actual balance equation of the Kelvin Double bridge is as follows R wire is the resistance of the thick, connecting wire between the low-resistance standard R a and the test resistance R x :.

However, the lower the resistances of R m and R n , the more sensitive the null detector will be, because there is less resistance in series with it. Increased detector sensitivity is good, because it allows smaller imbalances to be detected, and thus a finer degree of bridge balance to be attained.

Unfortunately, though, the lower the values of R m and R n , the more current they will carry, which will increase the effect of any junction resistances present where R m and R n connect to the ends of R a and R x. As you can see, high instrument accuracy demands that all error-producing factors be taken into account, and often the best that can be achieved is a compromise minimizing two or more different kinds of errors.

Sergio Franco. Don't have an AAC account? Create one now. Forgot your password? Click here. Latest Projects Education. Textbook Bridge Circuits. Home Textbook Vol. Pages in Chapter 8 What is a Meter? For example, if we connect a bridge circuit to measure an unknown resistance R x , we will have to know the exact values of the other three resistors at balance to determine the value of R x : Each of the four resistances in a bridge circuit are referred to as arms.

Here is a photograph of an antique resistance standard unit: This resistance standard shown here is variable in discrete steps: the amount of resistance between the connection terminals could be varied with the number and pattern of removable copper plugs inserted into sockets. Its schematic diagram is as such: The low-value resistors are represented by thick-line symbols, and the wires connecting them to the voltage source carrying high current are likewise drawn thickly in the schematic.

A Wheatstone bridge can be used to measure resistance by comparing the unknown resistor against precision resistors of known value, much like a laboratory scale measures an unknown weight by comparing it against known standard weights.

A Kelvin Double bridge is a variant of the Wheatstone bridge used for measuring very low resistances. Its additional complexity over the basic Wheatstone design is necessary for avoiding errors otherwise incurred by stray resistances along the current path between the low-resistance standard and the resistance being measured. Published under the terms and conditions of the Design Science License. You May Also Like.

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No text on electrical metering could be called complete without a section on bridge circuits. The standard bridge circuit, often called a Wheatstone bridge , looks something like this:. To measure resistance with a Wheatstone bridge, an unknown resistance is connected in the place of R a or R b , while the other three resistors are precision devices of known value. Either of the other three resistors can be replaced or adjusted until the bridge is balanced, and when balance has been reached the unknown resistor value can be determined from the ratios of the known resistances.

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## Kelvin Bridges

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## Kelvin and Wheatstone bridge

The Kelvin Bridge is a variation of the Wheatstone bridge which enables low resistances to be measured. When measuring the resistance of a component Rx a test current is forced through the component and the test meter measures the voltage at its terminals. The meter then calculates and displays the resulting resistance and is known as a twowire measurement. It should be noted that the meter measures the voltage at its terminals and not across the component.