This is equivalent to driving the primary coil at high voltage. Opening the points causes the current through the primary coil to stop. This causes the electromagnetic field in the primary coil to collapse rapidly. The bogies are of welded steel construction with primary coil springs and secondary air suspension. The resistance of the pot, as viewed from the primary coil, appears larger. A primary coil was wound on the outside of the glass bulb. There are two coils in this invention, the primary coil and the secondary coil. A single primary coil can provide power to multiple secondary coils at the same time.