Academy of Science. 27 



and type, soon gained popular attention, not only by their interest but by tiieir 

 practical applications also. Bell's researches were begun and carried forward in 

 the true spirit of the philosopher, and though the goal reached was not at first 

 aimed at, he exhibited true genius in seizing the clue offered, and following it out 

 so faithfuli5^ Following in the footsteps of a father somewhat famous as the 

 author of " Visible Speech," Bell was seeking some way to make the sound-wave 

 give a permanent, visible and legible record of itself. He found the manometric 

 capsule of Konig, and the phomautograph of Leon Scott, unsuited to his purpose, 

 and sought to improve the latter by making it after the model of the human edr. 

 A friend suggested the use of the ear itself, and the success of their experiments 

 with this novel apparatus gave the clue that Bell needed. If, thought he, a mem- 

 brane like the tj^mpanuin of the ear can move a series of bones, relatively of large 

 size, why will not a larger membrane move a magnet that will vary the intensity 

 of the electric current in waves corresponding to the sound waves? He had before 

 sought in vain to reach a similar result by the means of sympathetic vibrations, 

 but his new effort with the vibrating plate was crowned with triumphant success. 



I have indulged in this sketch of the genesis of the vibrating plate since it is to 

 be regarded as the fundamental idea of the second class of telephones, and one of 

 the most suggestive and fruitful discoveries of the age. Three forms of instru- 

 ments were exhibited at the Centennial, Philadelphia. The principle in each was 

 the same, and in construction there was a battery sending a continuous current 

 between the receiving and sending stations. At each of these places it passed 

 through coils of fine insulated wire, having as a core a piece of soft iron. This of 

 course became a magnet, and directly in front of it, distant say one- thirty-second 

 of an inch, was fixed a thin plate of iron. A suitable mouth piece before the plate 

 converged the sound upon it and gave tlie vibrations. It must be remembered that 

 the iron plate is in the magnetic field and has become a magnet. Its vibration, 

 therefore, according to well-known electrical principles, caused electric pulsations 

 to pass through the coil of wire, the intensity and direction of the currents being 

 proportional to the velocity and direction of the moving plate. A vibrating plate, 

 according to well-known acoustic laws, has property of dividing into nodes and 

 ventres so that it can harmonize with any sound-wave, however complex. Thus it 

 results that the plate copies the sound-wave and the electric pulsations copy the 

 plate. Now these pulsations passing through the helix at the receiving station, 

 change the magnetic state of its iron core, and consequently the attractive force of 

 the latter on the plate in front, just in proportion to the frequency, intensity 

 and direction of those pulsations. A vibration thereupon ensues which copies in 

 all its minute detail the original vibration of the first plate and sound-wave. This 

 is transmitted to the air and thence to the ear. Thus we have a sound-wave which 

 by instrumentality of a magnetic plate is copied by an electric wave, and this in 

 turn is retranslated into a similar sound-wave. The original sound dies like any 

 other sound, but its photograph, as it were, has been copied in a more subtile me- 

 dium than air, and so it lives and moves and is born again. 



In a few months after its exhibition at Philadelphia, this invention had received 

 such development at the hands of Mr. Bell that subsequent progress has been slow, 

 and the most recent telephones show little improvement over those made a year ago. 

 Bell found that the size and thickness of the vibrating plate, likewise the strength 

 of the battery, could be altered in pretty wide limits without materially changing 

 the loudness of the tones at the receiving station. In his experiments he varied 

 the plate from one inch to two feet in diameter, and from oue-sixty-fourth to three- 

 eighths of an inch thick. Between these limits he found the articulation perfect, 



