Let us take an electrical conductor in which the electrical energy supplied is entirely converted into heat . If , for the conductor , the terminal potential difference= V, the curent through it = I and its resistance = R , then the electrical energy cnsumed in its t is ,W=I^(2)t(from Ohm'slaw R=(V)/(I)) . So , if thr electrical and the heat energies both are expressed in joule , the heat developed in tiem t isH=I^(2)Rt . However , If H is expressed in the conventional unit calorie , then from the law , W = JH , we may write H=(I^(2)Rt)/(J) , where , J = mechanical equivalent of heat=4.2J."cal"^(-1) . The resistance R of a conducting wire depends on its material , its lengthl and its area of cross section a .The resistivity of the meterial of the conductor is , rho=(RA)/(l) . When more than one heat -producing conductors are kept in series in a circuit , the same current passes through each of them , but as their resistance are f=different in general , the terminal potential differences are also unequal . On the other hand , each conductor has the same terminal potential difference in a parallel combination , however , the curents through them are different .The first one of two wires ,m of the same meterial and of equal cross section , is longer than the second . A current through their series combination produces heat in them at the rates h_(1)andh_(2),respectively .