Whenever an electric current does work of any kind, it does it at the expense of a part of its own energy. Anything that increases the resistance of a circuit, decreases the strength of the current. But such a diminution may be caused by a counter electromotive force set up somewhere in the circuit. The E. M. F. of polarization is an example of the truth under consideration. Whenever a current is used to drive an electric motor, the action of the motor generates a back current that diminishes the current of the battery or dynamo. All of the current that is not expended in some such way, in external work, is dissipated as heat. The dissipation may be in the battery (or dynamo), in the external circuit or in both. The heat will appear wherever there is resistance. If the poles of a battery or dynamo be short circuited, most of the heat will be developed in the battery or dynamo. If the external circuit be a thin wire of high resistance, it will grow hot while the generator will remain comparatively