How to generate a voltage in a coil?

The voltage induced into a coil is proportional to the rate of change of current (dI/dt) through that coil. If the current is a constant value, then no voltage is induced. The equation is as follows: V = - L (dI/dt)

where L is the inductance of the coil, measured in henrys, and dI/dt means 'change of current divided by change of time'. The minus sign indicates that the induced voltage opposes the change in current.