Nine varieties of telephonic currents may be distinguished, but it will only be necessary to show you six of these. The three primary varieties designated as “intermittent,” “pulsatory,” and “undulatory,” are represented in [lines 1, 2, and 3].
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: (written vertically in first column) | ||||||||
| ‡ |  | Intermittent |  | Direct |  | Positive 1 | Positive | intermittent current |
| Negative 2 | Negative | ” ” | ||||||
| —— | Reversed 3 | Reversed | ” ” | |||||
Pulsatory | Direct | | Positive 4 | Positive | pulsatory current | |||
| Negative 5 | Negative | ” ” | ||||||
| —— | Reversed 6 | Reversed | ” ” | |||||
Undulatory | Direct | | Positive 7 | Positive | undulatory current | |||
| Negative 8 | Negative | ” ” | ||||||
| —— | Reversed 9 | Reversed | ” ” | |||||
And here I may remark, 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 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 number of times per second. The discovery of “Galvanic Music,” by Page,[2] in 1837, led inquirers in different parts of the world almost simultaneously to enter into the field of telephonic research; and the acoustical effects produced by magnetization were carefully studied by Marrian,[3] Beatson,[4] Gassiot,[5] De la Rive,[6] Matteucci,[7] Guillemin,[8] Wertheim,[9] Wartmann,[10] Janniar,[11] Joule,[12] Laborde,[13] Legat,[14] Reis,[15] Poggendorff,[16] Du Moncel,[17] Delezenne,[18] and others.[19] It should also be mentioned that Gore[20] obtained loud musical notes from mercury, accompanied by singularly beautiful crispations of the surface during the course of experiments in electrolysis; Page[21] produced musical tones from Trevelyan’s bars by the action of the galvanic current; and further it was discovered by Sullivan[22] that a current of electricity is generated by the vibration of a wire composed partly of one metal and partly of another. The current was produced so long as the wire emitted a musical note, but stopped immediately upon the cessation of the sound.
For several years my attention was almost exclusively directed to the production of an instrument for making and breaking a voltaic circuit with extreme rapidity, to take the place of the transmitting tuning-fork used in Helmholtz’ researches. I will not trouble you with the description of all the various forms of apparatus that were devised, but will merely direct your attention to one of the best of them, shown in [fig. 2]. In the transmitting instrument T, a steel reed a is employed, which is kept in continuous vibration by the action of an electro-magnet e and local battery. In the course of its vibration the reed strikes alternately against two fixed points m, l, and thus completes alternately a local and a main circuit. When the key K is depressed an intermittent current from the main battery B is directed to the line-wire W, and passes through the electro-magnet E of a receiving instrument R at the distant end of the circuit, and thence to the ground G. The steel reed A is placed in front of the receiving magnet, and when its normal rate of vibration is the same as the reed of the transmitting instrument it is thrown into powerful vibration, emitting a musical tone of a similar pitch to that produced by the reed of the transmitting instrument, but if it is normally of a different pitch it remains silent.
