Re: Understanding the (point of) the Wheatstone Bridge

On 9/20/07 7:18 PM, in article
1190341085.272074.293050@xxxxxxxxxxxxxxxxxxxxxxxxxxx, "Jamie Jackson"
<mySpamB8@xxxxxxxxx> wrote:

My son and I are working through some circuits from a Forrest M.
Mims / Radio Shack learning lab. We got to a Wheatstone Bridge
circuit, but I'm trying to understand the usefulness of it.

Let's use this diagram, for reference:
http://en.wikipedia.org/wiki/Wheatstone_bridge

If you need to adjust R2 in order to get the value of Rx, how do you
even know what R2 is anymore (since you've adjusted it)? Is R2 some
fancy high-accuracy *graduated* variable resistor that I've never
heard of?

In expensive bridges, such as the ones the telcos use to measure cable
faults, the R2 in the diagram is not a variable resistor. It is a large
group of fixed, precision, resistors on rotary switches than can be switched
in, in a sequence, to balance the bridge. The Rx resistance is then read
from the positions of the switches.



After seeing the diagram, I'm thinking: Well, if I have to measure my
R2 with my multimeter, I might as well measure Rx while I'm at it,
which blows the point.

Is the point just that they're perfectly balanced, and the point is
not what the actual values are?

To be balanced R2 must equal Rx. The ones I used in the Toll Testroom could
measure to within .01 Ohm.



Thanks,
Jamie


.

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