4.3.1 Operating principle A Where r= the resistance (2), L= the length of the conductor ) a-the cross-sectional area of conductor(cm p= the resistivity of material&2 cmA L R  = (4.1) where R = the resistance ( ), L = the length of the conductor (c m ), A = the cross-sectional area of conductor ( 2 cm )  = the resistivity of material ( cm ) 4.3.1 Operating principle