The alternate discs of iron and carbon, having same area of cross-section are cemented together to make a cylinder whose temperature coefficient of resistivity is zero. The change in temperature in two alternate discs is same. The ratio of their thickness and ratio of heat produced in them is found out. The resistivity of iron and carbon at 20^@C" are "1 xx 10^(-7) and 3 xx 10^(-5)Omegam and their temperature coefficient of resistance are 5 xx 10^(-3)""^(@)C and -7.5 xx 10^(-4)""^(@)C, respectively, Thermal expansion is neglected. Here, triangleR_1 +triangleR_2= 0 (where triangleR_1, and triangleR_2, are the increase in resistances of iron and carbon, respectively, with the rise in temperature) because combined temperature coefficient of resistivity is given as zero. A copper wire is stretched to make it 1% longer. The percentage change in its resistance is Electrical resistance of certain materials, known as super conductors changes abruptly from a non zero value to zero as their temperture is lowered below a critical temperature T_c(0) An interesting property of superconductors is that their critical temperature becomes smaller than T_c(0) if they are placed in a magnetic field, that is, the critical temperature T_c(B) is a function of the magnetic field strength B. The dependence of T_c(P) on magnetic field is shown in the below figure.