<Dd> L = inductance measured in henries </Dd> <Dd> I = current measured in amperes </Dd> <P> Now let's consider a cylindrical coil with conductors of a rectangular cross section . The mean radius of coil is R. a and b are width and depth of the conductor . f is called form function which is different for different shapes of coil . ξ (xi) and δ (delta) are two parameters to characterize the dimensions of the coil . We can therefore write the magnetic energy stored in such a cylindrical coil as shown below . This energy is a function of coil dimensions, number of turns and carrying current . </P> <Dl> <Dd> E = 1 2 R N 2 I 2 f (ξ, δ) (\ displaystyle E = (\ frac (1) (2)) RN ^ (2) I ^ (2) f \ left (\ xi, \ delta \ right)) </Dd> </Dl>

Ac power is inferior in quality to dc power