Aim:
To verify the laws of combination (parallel) of resistances using a metre bridge
Materials Required:
1. 2 different resistances (carbon or wirewound resistors),
2. metre bridge,
3. galvanometer,
4. a cell or battery eliminator,
5. a jockey,
6. a rheostat,
7. a resistance box,
8. a plug key,
9. sandpaper and
10.thick connecting wires.
Theory:
Consider two resistances r1 and r2 are connected in series.
The series combination resistance RS is given by RS = r1 + r2
When connected in parallel, the resistance of the combination is given by Rp
Diagram
Resistance connected in Parallel
Resistances r1 and r2 connected in parallel to one arm of a metre bridge
Procedure
 Set up the circuit as shown in the figure above.
 Connect R1 and R2 as shown in the figure.
 Tighten all plugs in the resistance box by pressing and rotating each plug to assure that all plugs make good electrical connections. Using sandpaper clean the ends of connecting wires before making the connections.
 Remove some plug(s) from the resistance box to get the suitable value of resistance R
 Get a null point D on the metre bridge wire by sliding the jockey between ends A and C.
 Note the value of the resistance R and lengths AD and DC.
 Calculate the experimental value of the equivalent parallel resistance
 Repeat the experiment for four more values of resistances R. Obtain the mean value of unknown resistance.
Observations And Calculations
sl.no 
Resistance R (ohm) 
Resistance from the resistance box R (ohm) 
Length AD=l 
Length DC=1001 
Rp=1001/l 
Mean resistance (ohm) 

R1 only 
1 2 3 4 5 
r1 = 

r2 only 
1 2 3 4 5 
r2 = 

r1 and r2 in parallel 
1 2 3 4 5 
Rp = 
Precautions
 Ensure the connection is neat, clean and tight
 Insert key only while taking an observation
 Move jockey gently over the metre bridge wire.
Sources Of Error
 The instrument screw may be loose
 Unavailability of thick connecting wires
Result
 The experimental value of Xp = ohm
 The theoretical value of Xp = ohm
 The experimental and theoretical value of Xp is found to be the same. Hence, the law of resistance in parallel is verified.
Viva Questions
1. List the factors affecting the resistance?
ANS: The factors affecting the resistance are,
 Length
 material
 Area of cross section
 The temperature of the conductors
2. What is the mathematical form of Ohm’s law?
ANS: V=IR
3. What is resistance?
ANS: The ratio of potential difference V across the ends of a conductor to the current I flowing through it. It is represented by R and is given by R=V/I
4. What is ohmic resistance?
ANS: The resistance which obeys ohms law is known as Ohmic resistance.
5. Give examples of nonohmic resistance?
ANS: Transistors, vacuum tube diodes and semiconductor diodes.