Bell's Telephonic Researches 



nately 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 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 con- 

 tinuous current; and 



An undulatory current is a current of electric- 

 ity, the intensity of which varies in a manner pro- 

 portional to the velocity of the motion of a par- 

 ticle of air during the production of a sound: 

 thus the curve representing graphically the un- 

 dulatory current for a simple musical note is the 

 curve expressive of a simple pendulous vibra- 

 tion that is, a sinusoidal curve. 



And here I may remark, that, although the 

 conception of the undulatory current of electri- 

 city 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 

 electro-magnet 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 ear the effect of a musical note 

 when the current is interrupted a sufficient num- 

 ber of times per second. 

 63 



