energy represented by the product of current and drop of e.m.f. on passing the lamp into heat energy, some of which appears as light waves. As far as this conversion is concerned, the direction in which the current flows is quite immaterial. If the current always flows the same way, we call it a continuous or direct current (abbreviation D.C.). If the current changes its direction periodically, we call it an alternating current (abbreviation A.C.).
When a current changes its direction or sense of flow, there must necessarily be a moment when the flow is neither in one nor in the other direction; in other words, for a moment there is no current at all. We say the current strength passes from a positive value through zero to a negative value. Alternating currents are produced by machines of the type shown in Fig. 15. As the poles sweep by the active wires the induced e.m.f., and therefore also the resultant current, changes gradually, and if we represent this change graphically by plotting time on the horizontal and either e.m.f. or current on the vertical, we get a wavy line as shown in Fig. 19. The distance a to b is called the periodic time of the current, and the greatest amplitude of the wave is called the "crest value" of the
