232 PROCEEDINGS OP THE AMERICAN ACADEMY 



stances of many of our experiments, in which the duration of the 

 current was from less than one half of a vibration up to one vibra- 

 tion, it certainly is not possible that the vibration should continue 

 without entire change of rate and form for from four to eight times 

 the duration of the forced vibration, which would have to be the 

 case in order to have two complete sound-waves produced. 



Furthermore, the damping effect of the magnet upon a telephone 

 diaphragm is such as to prevent more than a very slight amount of 

 persistence of vibration of any kind whatever. This is shown by 

 the facts that the natural note of the diaphragm of a magneto-re- 

 ceiver is never perceived in the ordinary use of the instrument, and 

 that the quality of the sound heard at the receiver is substantially 

 the same, although wide variations may be made in the dimensions 

 of its diaphragm. And if the after-tone of the diaphragm persisted 

 after each pause sufficiently long to produce a recognizable sound 

 of definite pitch, say for the period of two complete vibrations, 

 this would necessarily be noticeable in the actual operation of the 

 instrument, — an effect which is not observed in practice. 



Still further, in none of our experiments was there any note ob- 

 served which corresponded in pitch to the natural note of the 

 diaphragm, — a note whose pitch was so high that it could not have 

 failed to impress itself upon our attention had it been present to 

 any material extent. 



These observations seem to show that with ordinary telephone 

 currents there is no material vibration of the diaphragm of the re- 

 ceiver after the forced vibration has ceased. With a stronger 

 current we might expect a certain continuance of the free vibration. 

 What undoubtedly occurs is, that as soon as the electrical undula- 

 tions cease to act on the receiver, its diaphragm begins to move at 

 a different rate, passing to its free rate of vibration with great 

 rapidity, and assuming this rate unless sooner brought to rest by 

 acoustic and magnetic damping. This action would probably pro- 

 duce a noise of constant pitch, and distinct from the sound of the 

 transmitting tuning-fork. 



But although for these reasons it appears extremely unlikely 

 that the diaphragm could continue to vibrate with its rate substan- 

 tially unchanged for more than a very minute fraction of a vibra- 

 tion, it nevertheless seemed desirable to ascertain by actual and 

 direct experiment whether this is the case. 



For this purpose the following apparatus was devised. A Lissa- 

 jous comparator, with its vibration maintained electrically, was 



