# How does a Wheatstone Bridge work?

Ravyan Asro
I have never understood the concept of the Wheatstone Bridge and how it works. Even the following equations and numericals on this topic of platinum resistance thermometer confuses me a lot... I need help.

Gold Member
Your 'following equations' confuses me to since there are none.

gjonesy
Wheatstone Bridge works by measuring unknown electrical resistance in an circuit .

Gold Member
I have never understood the concept of the Wheatstone Bridge and how it works. Even the following equations and numericals on this topic of platinum resistance thermometer confuses me a lot... I need help.
You can think of it as a battery with two potential dividers connected across it. When they are both set to give the same voltage, if we connect a meter between the tapping points, it will read zero.
It does not matter what the actual resistances of the potential dividers are, it is just the ratios that are important, so that the tapping points give the same voltage.

PietKuip
Gold Member
I have never understood the concept of the Wheatstone Bridge and how it works. Even the following equations and numericals on this topic of platinum resistance thermometer confuses me a lot... I need help.
Any reactions?

Ravyan Asro
if the bridge is balanced, does it mean that the objective of resistance is fulfilled? thank you for making this doubt a whole lot clear.

Gold Member
if the bridge is balanced, does it mean that the objective of resistance is fulfilled? thank you for making this doubt a whole lot clear.
When it is balanced, the ratios of the two potential dividers are the same, and if three of the resistor values are also known, the third can be calculated using the formulas given previously.

Ravyan Asro
Ravyan Asro
electric resistance of a metal wire increases monotonically with temperature and may be used to define a temperature scale. the platinum resistance thermometers are used to measure resistance, can they also be used to measure temperature?

Gold Member
electric resistance of a metal wire increases monotonically with temperature and may be used to define a temperature scale. the platinum resistance thermometers are used to measure resistance, can they also be used to measure temperature?
I think the clue is in the name Thermo (as in heat / temperature) and Meter (as in measuring device). That is their prime purpose. A standard resistance will be specified at a given temperature and can be used in a wheatstone bridge for measuring other unknown resistances. But the reason for choosing platinum resistance thermometers - as opposed to iron or aluminium resistance thermometers is that platinum doesn't corrode as easily and it maintains its characteristics over a wide temperature range.

Don Jusko and gjonesy
Let'sthink
I would like to point out an interesting fact here with regard to balanced wheatstone bridge. Normally if we connect any two points of a complicated circuit by a resistance the effect produced by it depends on the value of the connected resistance. But in the case of a balanced wheatstone bridge you can connect any resistance from zero to infinity between the points across which the bridge is balanced no effect will be produced in any part of the circuit. Zero potential difference has created this incredible thing!

jim hardy
Gold Member
I would like to point out an interesting fact here with regard to balanced wheatstone bridge. Normally if we connect any two points of a complicated circuit by a resistance the effect produced by it depends on the value of the connected resistance. But in the case of a balanced wheatstone bridge you can connect any resistance from zero to infinity between the points across which the bridge is balanced no effect will be produced in any part of the circuit. Zero potential difference has created this incredible thing!
That it is the whole point of a wheatstone bridge isn't it? It is a method of the measuring equipment not loading the circuit under test at the most important time. Not sure if that was the reason it was developed but it is certainly a perk.

Gold Member
That it is the whole point of a wheatstone bridge isn't it? It is a method of the measuring equipment not loading the circuit under test at the most important time. Not sure if that was the reason it was developed but it is certainly a perk.
I think the Charles Wheatstone invented the bridge to locate the position of an Earth fault on a telegraph cable. I believe the principle is that when the wires are made into a bridge, the actual resistance of the Earth fault does not influence the distance measurement.

tim9000
I think the Charles Wheatstone invented the bridge to locate the position of an Earth fault on a telegraph cable. I believe the principle is that when the wires are made into a bridge, the actual resistance of the Earth fault does not influence the distance measurement.
What do you mean? Could you please draw a diagram of the cable (fault) and bridge?

Gold Member
What do you mean? Could you please draw a diagram of the cable (fault) and bridge?
If you look at the Murray Loop Test at this link it will give the diagram you need.
http://www.electrical4u.com/blavier-test-murray-loop-test-varley-loop-test/

Gold Member
The cable out and Earth return loop was connected as one of the resistors in the bridge . Test resistors were then inserted systematically into the balancing arm of the bridge until a value was found which balanced the bridge . The value of resistor found when used with tables or a formula gave distance from base station to fault position on line .

Actually done with a purpose made test box called a Post Office Box . Early types had plug in keys to set test values of balancing resistance . Later ones had dial up settings .

An important later use of the Wheatstone bridge was with sensitive instrumentation . Best known examples are use with strain gauges and sensitive resistance thermometers . Often the bridge is used with two identical instruments in the balance arms of the bridge - one in a controlled environment and one as the active detector .

Bridges are not always used by bringing them to balance . A sensitive voltmeter across the bridge can sometimes give useful direct readings .

You can also insert an amplifier to get a differential output directly:

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Nidum
Gold Member
Dearly Missed
I would like to point out an interesting fact here with regard to balanced wheatstone bridge. Normally if we connect any two points of a complicated circuit by a resistance the effect produced by it depends on the value of the connected resistance. But in the case of a balanced wheatstone bridge you can connect any resistance from zero to infinity between the points across which the bridge is balanced no effect will be produced in any part of the circuit. Zero potential difference has created this incredible thing!

Aha ! The significance of zero once again !

Take yourself back a hundred years to the days before precision electronic amplifiers and voltmeters.
How could you measure something accurately?

This link has pictures of some 1920's lab equipment.
http://physics.kenyon.edu/EarlyApparatus/Electrical_Measurements/Kenyon/Kenyon.html

Small currents were measured with galvanometers that instead of a needle had a small mirror - a beam of light reflected onto a wall several yards away gave tremendous movement for small current. That's amplification without electronics...

They had good enough wire to make resistors that matched well. Manganin was popular because of its near zero temperature coefficient.
So they could make accurate voltage dividers.

A popular local voltage reference was the weston Standard Cell, a small battery sealed in glass. It produced a constant voltage so long as you didn't ask any current of it.

Small voltages were measured by a Wheatstone bridge.
When balanced , the bridge produces zero voltage as Let'sthink observed.

With primitive equipment it's easy to accurately measure zero voltage because there's no deflection on the galvanometer. But it's difficult to measure the value of any other voltage.
That's the significance of zero.

With the simple tools of just an accurate slidewire, a standard cell and a sensitive zero detecting galvanometer , one can make a Wheatstone bridge that'll measure accurately the millivolts from a thermocouple. We used them well into the 1970's. The user balances the bridge manually by turning a big knob that rotates the slidewire .

electric resistance of a metal wire increases monotonically with temperature and may be used to define a temperature scale. the platinum resistance thermometers are used to measure resistance, can they also be used to measure temperature?

Sure ! I personally prefer thermocouples
but resistance thermometers are common now, platinum and copper are both used.

old jim