Meter bridge experiment

1. Jun 19, 2014

Hardik Batra

why the meter bridge experiment not useful for the measurement of small resistance?

2. Jun 19, 2014

Staff: Mentor

I assume you mean what people (in the US at least) usually call a "Wheatstone bridge", rather than the kind of meter bridge that is used in an audio recording studio:

http://www.sweetwater.com/c434--Mixer_Meter_Bridges

Before reading your post, I had never seen a Wheatstone bridge referred to as a "meter bridge", but a Google search revealed that in some places it does indeed have that name. I've learned something new today!

Small compared to what?

3. Jun 19, 2014

Hardik Batra

4. Jun 19, 2014

UltrafastPED

5. Jun 19, 2014

sophiecentaur

On the senior side of the pond, we spell it Metre when we mean the length and Meter when we mean the measuring instrument. There's no confusion that way.
Most Bridges don't use anything as crude as a length of resistance wire, these days, of course.
Btw, actually how long is the wire that you use in the bridge that consists of a wooden board, brass terminals and an old fashioned galvanometer?

6. Jun 19, 2014

Staff: Mentor

The one that we use in our introductory lab apparatus is in fact one meter (metre) long, mounted on top of a meter stick! (which is in turn mounted along with electrical terminals on top of a board)

7. Jun 20, 2014

sophiecentaur

I was visualising a stick with 36 major (inch) divisions on it, with 16 small ones between each pair. :big grin: Good for training in Arithmetic!!

8. Jul 29, 2014

Hardik Batra

In meter bridge experiment,
I have resistor values R1 = 0.5 ohm, R2 = unknown(that we want to find. consider R2 has also very small value.)
R3 and R4 decided by moving jokey key on meter wire.

Some one says me you can't use this method because the resistance value is too small.

So this is true or false.

True then which method is used?

9. Jul 29, 2014

sophiecentaur

IF you want 0.5Ω to drop a significant voltage across it (which is what a bridge needs, when you get down to it) then you need to pass significant current through it. Isquared R implies that the resistor will be dissipating significant power (more than it is designed for) and it could be roasted.

You can't use a high value of reference resistor or, again, the voltage ratio will be small and difficult to find accurately as the ratio of two lengths of wire. There is also the problem that contact resistance can add to the measured value. (More voltage drop to confuse things)

There has been a lot of work in the past on measuring low resistances - particularly in the context of Measuring Contact Resistance and there have been many improvements on the straight metre bride arrangement. The Kelvin Bridge was one.
Also, if you use AC for the test, you can use a transformer to 'step-up' the value of your test resistor - by a known ratio, and measure this transformed resistance with a straightforward bridge. AC is a good way of seeing (on a scope) any hints of non-linearity, which can occur with low value components - which may have contact resistance problems inside the box.