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Old Apr 28th 2017, 11:59 PM   #1
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End correction in meter bridge practical

Could end correction be minus ? In meter bridge practical , I got minus end correction values .. is it possible ? Can anyone explain me what that means ?
Thanks in advance .
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Old Apr 29th 2017, 05:52 AM   #2
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Hello Madhu and welcome.

It is really good to see some practical work being done, this is just so important.



The answer to your question lies in understanding where the end corrections come from.

I have drawn a greatly exaggerated error to show the issue.

When you read the bridge you measure from the balance point to what you hope are the ends of the slidewire.
The ends of the slidewire are the points where the heavier copper bus bars are attached (eg soldered) on. They may not be exactly 1 metre apart. I have labelled these points M and N. Also the ends of the ruler may not be exactly aligned with these points.

So I have shown two different errors, alpha and beta, one where the ruler sticks out and the other where the wire sticks out further.

Measurements L1 and L2 are shown and the true lengths for the measurements are L1+a and L2+b.
Of course a is positive and b is negative in this example.

Does this help?
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End correction in meter bridge practical-endcorr1.jpg  
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Old Apr 29th 2017, 12:45 PM   #3
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Thumbs up Thank you.

Thank you very much .. it was really helpful . I couldn't understand the reason for the minus end correction . Because of your explanation , now I understand .
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Old Apr 29th 2017, 01:18 PM   #4
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Originally Posted by MadhuNissanka View Post
Thank you very much .. it was really helpful . I couldn't understand the reason for the minus end correction . Because of your explanation , now I understand .
There is another possible error source, called connection or contact resistance.

Although the bus bars are heavy enough to consider them as having nelgigible (zero) resistance, the solder joint where they conect to the wire may not.

If each joint has a resistance r, and the test resistances are P and Q then measuring from the balance point each arm now sees a resistance of

P + r and Q + r

If we write Q/p = n or Q = nP then we have the ratio of the resistances as

(P + r) / (nP + r) instead of P/Q or P/nP

You might like to think about this as r becomes large compared to P.
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