of the signal passing through secondary inductor 20 the a c components being present because the d c voltage derived from diodes 22 is a half wave rectified signal and because the a c current passing through primary inductor 18 induces an a c current in secondary inductor 20 by filtering out the a c components of the signal passing through secondary inductor 20 filter 24 prevents these a c components from inducing an undesired a c current in primary inductor 18 and prevents the a c components from damaging any of the circuit elements in the circuit that includes secondary inductor 20 the current passing through secondary inductor 20 may alternatively be provided by a small battery such as a watch battery or any other suitable source for example there may be ways to derive a voltage from a transformer on main power a remote source such as a computer the power supply or the receiver shown in fig4 below it is also possible to use the voltage across the filament on one or the other side of the fluorescent lamp either in the low frequency embodiment of fig1 or the high frequency embodiment of fig3 below this current may be an alternating current in which case no filter is needed if the alternating current has the same frequency as the alternating current passing through primary inductor 18 if an a c current is used through secondary inductor 20 a greater current is needed to vary the degree of saturation of the magnetic core than would be required if a d c current were used the d c or a c voltage through secondary inductor 20 is preferably relatively low as is the current control device.