Q1. Why are the resistances used in a resistance box is like 1,2,2,5,10,20,20,50,100,200,
Ans: In this arrangement all natural numbers upto 10000 ohms (and above) can be realized.
1, 2, 2+1=3,2+2=4,5,5+1=6,5+2=7,5+2+1=8,5+2+2=9 etc
Try it yourself to get all numbers from 1 to 10000
Q2. Why wire is double folded in resistance box?
Ans: The resistance wires in resistance box and in standard resistances are double folded and then coiled so as to nullify the effect inductance. When double folded and coiled, in one half the current will be clockwise and in next half the current will be anticlockwise. Therefore, the magnetic induction created by one half will be cancelled by the other half.
Q3. we make coil of wire when finding resistance of wire through metre bridge ?
Ans: This is not correct. If it is to be coiled, it should be done as in Q1.
Normally the wire is stretched on a half meter scale and fixed with terminals (or arranged in some other manner)
If you feel that the wire need to be coiled, then take care to avoid the magnetic effect.