A wire has a resistance R. It is stretched uniformly so that its length becomes 2L. The new resistance is

1. Stretching keeps volume V = A × L constant; if L doubles, A halves. 2. Original R = ρ L / A. New length L' = 2L, new area A' = A/2. 3. New R' = ρ (2L) / (A/2) = 4 ρ L / A = 4R. 4. So on doubling the length by stretching, resistance becomes four times. _Source: syllabus Physics, Ch 3 "Current Electricity", §3.4_