Sub-varieties of these can be distinguished as “direct” or “reversed” currents according as the electrical impulses are all of one kind or are alternately positive and negative. “Direct” currents may still further be distinguished as “positive” or “negative,” according as the impulses are of one kind or of the other. An _intermittent current_ is characterised by the alternate presence and absence of electricity upon the circuit; A _pulsatory current_ results from sudden or instantaneous changes in the intensity of a continuous current; and An _undulatory current_ is a current of electricity, the intensity of which varies in a manner proportional to the velocity of the motion of a particle of air during the production of a sound: thus the curve representing graphically the undulatory current for a simple musical tone is the curve expressive of a simple pendulous vibration—that is, a sinusoidal curve. Telephonic currents of electricity may be: {Direct {Positive 1 Positive intermittent current. Intermittent { {Negative 2 Negative ” ” { —— Reversed 3 Reversed ” ” {Direct {Positive 4 Positive pulsatory current. Pulsatory { {Negative 5 Negative ” ” { —— Reversed 6 Reversed ” ” {Direct {Positive 7 Positive undulatory current. Undulatory { {Positive 8 Negative ” ” { —— Reversed 9 Reversed ” ”