Autotransformers and Inductors

As usual in this course, there is no real switch, and a clip lead is simply touched for a short time ( momentarily ) to a wire or terminal, to put a short burst of current through half of the 12 volt coil (ordinarily the secondary, but acting as a primary here). When the current is suddenly increased, a downward spike appears on the scope, showing that the top half of the center-tapped coil has 

The next step is to suddenly open the switch, and an upward spike appears, meaning that a voltage has been generated in the top half-coil that adds to the bottom halfs voltage. The overall action therefore tends to prevent the current from stopping quickly. 

The action of the top and bottom halves of the coil is similar to the action of the primary and secondary of an ordinary transformer, except that in the above experiment they are parts of the same coil, and they have the same numbers of turns. In some cases it is desirable to use just one continuous coil with a tap, but not necessarily at the center, and in fact it can be movable, to provide varying voltages. This is called an autotransformer, and it is often used to convert 120 volts to either lower or higher voltages, all ac, for operating small heaters or 

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