174 ALEXANDER GRAHAM BELL 



various forms of telephonic apparatus, and it has led me to the 

 conception of that peculiar species of telephonic current, 

 designated as undulatory, which has rendered feasible the 

 artificial production of articulate speech by electrical means. 



A horizontal line is taken as the zero of current, and im- 

 pulses of positive electricity are represented above the zero 

 line, and negative impulses below it, or vice versa. 



The vertical thickness of any electrical impulse, measured 

 from the zero line, indicates the intensity of the electrical 

 current at the point observed, and the horizontal extension 

 of the electric line indicates the duration of the impulse. 



Nine varieties of telephonic currents may be distinguished, 

 but it will only be necessary to show six of these. The three 

 primary varieties are designated as intermittent, pulsatory, 

 and undulatory. 



Subvarieties 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 nega- 

 tive, according as the impulses are of one kind or of the other. 



An intermittent current is characterized 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 in- 

 tensity 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 note is the curve expressive of a 

 simple pendulous vibration — that is, a sinusoidal curve. 



And here I may state, that, although the conception of 

 the undulatory current of electricity is entirely original with 

 myself, methods of producing sound by means of intermittent 

 and pulsatory currents have long been known. For instance, 

 it was long since discovered that an electromagnet gives 

 forth a decided sound when it is suddenly magnetized or 

 demagnetized. When the circuit upon which it is placed is 

 rapidly made and broken, a succession of explosive noises 

 proceeds from the magnet. These sounds produce upon the 



