Measurement Of Unknown Resistance Using A Post Office Box

MEASUREMENT OF UNKNOWN RESISTANCE USING A POST OFFICE BOX

Now, the resistances in P and Q are made 100 Ω and 1000 Ω, respectively, and the process is repeated.

Equation (1) is used to compute S.  The ratio P/Q is progressively made 1:10, and then 1:100. Thus, the resistance S can be accurately measured.

Errors

The major sources of error are the connecting wires, unclear resistance plugs, change in resistance due to Joule heating, and the insensitivity of the Wheatstone bridge.

These errors may be removed by using thick connecting wires, clean plugs, keeping the circuit on for very brief periods (to avoid Joule heating) and calculating the sensitivity.

In order that the sensitivity is maximum, the resistance in the arm P is kept close to the value of the resistance S.

Illustration14. In an experiment with a post−office box, the ratio arms are 1000 : 10.  If the value of the third resistance is 999 Ω,. find the unknown resistance.

Solution: The ratio arms are 1000 : 10

∴ P/Q = 1000/10 = 100

Third resistance R = 999 Ω

Let x be the unknown resistance.

We know that,

P/Q = R/X

∴ X = Q/P x R = 1/100 x 999 = 9.99 Ω

Illustration15.When two resistances X and Y are put in the left hand and right hand gaps in a meter bridge,  the null point is at 60 cm. If X is shunted by a resistance equal to half of itself then find the shift in the null point.

Now,

⇒I = 33.3 ∴ Shift  = 60 – 33.3 = 26.7 cm

Related Topics

• Ohm's Law
• Kirchhoff Law
• RC-Circuit