Metre Bridge-Resistance of a wire

 Materials required:

  •     Metre bridge (slide wire bridge)
  •     Leclanche cell or Battery eliminator
  •     Galvanometer
  •     Resistance box 
  •     Jockey  
  •     One way key
  •     A resistance wire
  •     Screw gauge
  •     Metre scale
  •     Connecting wires

Real Lab Procedure:

  • Arrange the required materials on a table and make the connections as per the connections diagram.
  • Connect the resistance wire in the left gap (between c & d) and resistance box in the right gap.
  • Introduce some resistance in the circuit by taking out some resistance from the resistance box.
  • Plug the key. Bring the jockey in contact with the end A first, and then with C.  Note the deflection on the galvanometer.
  • If the galvanometer deflects in the opposite direction, the connections are right and the null point is in between A and C.
  • If not so, change the resistance in the resistance box and repeat the process so that the null point is somewhere between A and C.
  • If the galvanometer deflected towards a single side, then check the connection.
  • Now, slide the jockey slowly over the wire starting from one and (say, A) and note the galvanometer deflection. Continue the process till the balancing point is reached.
  • Balancing point is the point at which the galvanometer shows zero deflection. Now, note the position of the jockey from end A. Take it as the balancing length (l) using the metre scale.
  • Repeat the process for different values of R. The balancing length is measured  each time.
  • Now, interchange the position of resistance wire and resistance box in gaps AB and CD.
  • Repeat the above steps to find the balancing length, for the same values of R.
  • We can calculate the unknown resistance of the resistance wire by using the relation,

X=Rfrac{l}{left ( 100-l ight )}

  • Measure the diameter of the given resistance wire using a screw gauge. Hence, its radius(r) can be found.
  • Also measure the length (L) of the wire using a metre scale.
  • From the measured values, the specific resistance (resistivity) of the given resistance wire can be calculated using the relation, 

ho =frac{pi r^{2}X}{L}

  • Simulator Procedure (as performed through the Online Labs):

  • Your simulator will consist of a metre bridge kept on a table, battery, resistance box and wires on the side bar menu.
  • Click on the battery and the resistance box shown on the side bar menu to place them near to the metre bridge.
  • Drag one of the wires to the right gap of the metre bridge.
  • Now the button, “Start experiment” will be enabled.
  • Now you can select your desired resistance from the resistance box just by clicking on the box and then choosing the resistance from the pop-window, “Select Resistance”. Now close the pop-window.
  • Click on the enabled button and "Insert Key”.
  • Now you can move the jockey from one left end to right either by moving the jockey with your mouse or by moving the slider, “Jockey Position”.
  • Simultaneously check the readings of the galvanometer, once the needle reaches the zero reading, stop moving the jockey and note down the length of the wire from the balanced position on the left side, let say “AB” which is l cm.
  • Repeat the same by moving the jockey from the right end to the left and note down the length of the wire from the balanced position on the right side, let take it as “BC” which is (100-l) cm.
  • Repeat the same procedure with second wire and note down the lengths.
  • For each wire take three readings and calculate its mean readings/resistance.
  • You can calculate the unknown resistance of the resistance wire by using the relation,

                                                       X=R\frac{l}{( 100-l )}

  • If L is the length and r is the readius of the wire, the specific resistance (resistivity) of the given resistance wire can be calculated using the relation,  

                                                     \rho =\frac{\pi r^{2}X}{L}

Observations:

To find the resistance of the given wire:

No. Resistance, R (Ω) Reistance wire in left gap Resistance wire in the rigth gap Mean, \dpi{80} X= \frac{X_{1}+X_{2}}{2} (Ω)
Balancing length, AB =l (cm) Length, BC =(100-l) (cm) X_{1}= frac{Rl}{left ( 100-l 
ight )} (Ω) Balancing length, A'B' = l' (cm) Length, B'C' =(100-l') (cm) X_{2}= frac{Rl'}{left ( 100-l' 
ight )} (Ω)
1                
2                
3                
4                
5                

                                                                                                         Mean resistance, X = .................. Ω

To find the diameter of the given wire:

Least count of the screw gauge (LC)       =…….. mm

Zero correction of the screw gauge (Z)   = ……. mm

 

No. PSR (mm) Observed HSR, (a) (mm) Corrected HSR, (a+Z) (mm) Corrected HSRx LC (mm) Total Reading, d = PSR+ (Corrected HSRx L C ) (mm)
1          
2          
3          
4          
5          

                                                                                                                      Mean, d =………mm

Calculations

Diameter of the wire, d         =………...mm

Radius of the wire, r             = d/2

                                          =……......mm

                                          = ………..10-3 m

Length of the wire, L             = ............cm

                                          = ............10-2 m

Resistance of the wire, X      = ............. Ω

Resistivity ( specific resistance) of the wire,      

                                       
ho =frac{pi r^2X}{L}

                                          = ............Ω m

Result

The unknown resistance of the given resistance wire, X                 = ............ Ω
The specific resistance (resistivity) of the given resistance wire, ρ  = ……...... Ω m

 

 

 

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