## kelvin's bridge

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hi everyone.

I need some help in understanding some details of kelvin's bridge.

What many sites have told me is that potential difference between a and b is the same as that between ac. How is that possible since the voltage between ab is actually a fraction of the voltage ac(since it is a voltage divider.)

One of the sites offered a more logical explanation which said that voltage ab is equal to voltage ad and used math to prove it.

Also why do we need the r wire. Cant we just make do by connecting m and n up close to the resistors to decrease the effect of wire resistence.

thanks for help

The Kelvin Bridge idea is intended to compensate for stray voltage drops due to currents through lead or contact resistances.

That diagram is a very odd way to show it. As I read it, "r" is the resistance being measured. At each end, it has stray resistances "R" and "S". It appears to me that "P", "Q", "p", and "q" represent the resistance of the wires connecting to the galvanometer. But, the circuit makes no sense - why would wires be shunting the resistance being measured and why is the galvo where it is. Frankly, I do not understand the circuit as drawn. In fact, I am very suspicious of it.

Understand that this is designed for REALLY high precision measurements. An example would be to measure the temperature coefficient of resistivity of the material used for "r". So, yes you can reduce the end resistances, but you can never eliminate them. When you are trying to measure parts per million, or smaller, "short leads" are not enough.

What you find more commonly in modern circuits are called "Kelvin Contacts". You find these particularly in PT100 (platinum 100 ohm) temperature sensors. It is a way of measuring the voltage drop across the sensor without the voltage drops of the connecting leads. You can also find these on some load cells. This is NOT the same as the Kelvin Bridge.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

ka7ehk wrote:
Kelvin Contacts

aka "4-wire connection"

Also commonly found on shunt resistors for current measurement

https://en.wikipedia.org/wiki/Four-terminal_sensing

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Indeed. I was trying to think of some of the other names (for Kelvin Contacts) but just could not think of any.

Thanks

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

This might give you some good equations to chew through.  In the old days, they came up with great ways of making very precise & accurate measurements, using poor tolerance components and a good standard ref.

http://www.eeeguide.com/kelvins-bridge-theory/

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Now, that diagram is more like what I have seen. The one, above, probably resolves into the referenced one, but, wow, do they make it hard.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

ka7ehk wrote:
Indeed. I was trying to think of some of the other names (for Kelvin Contacts) but just could not think of any.

I lost track of which forum I was in, and thought the topic was on the historic Kelvin Bridge...

Kelvin is an unincorporated community in Pinal County, Arizona, United States. Kelvin is located near the Gila River, 24.6 miles (39.6 km) east-northeast of Florence. The community was originally named Riverside; its name was changed to Kelvin, after Kelvin Grove in Scotland, in 1900.[1]

The Kelvin Bridge and the Mineral Creek Bridge, which are listed on the National Register of Historic Places, are located near Kelvin.[

https://bridgehunter.com/az/pina...

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