Now consider two closed loops. C1 and C2 carrying currents i1 and i2, respectively. The currents are initially zero and are to be increased to I1 and I2, respectively. To find the amount of work required, we first keep i2=0 and increase i1 from zero to i1. This requires a work W1 in loop C1, as given in Eq.(6-157) or (6-158); no work is done in loop C2, since i2=0. Next we keep i1 at I1 and increase i2 from zero to I2. Because of mutual coupling, some of the magnetic flux due to i2 will link with loop C1, giving rise to an induced emf that must be overcome by a voltage V21=L21 di/dt in order to keep i1 constant at is value I1. The work involved is