84 PROCEEDINGS OF THE AMERICAN ACADEMY 



Nevertheless, it would appear to be theoretically true that such inter- 

 ference should take place under appropriate conditions. 



Very many other changes were rung upon the conditions for getting 

 articulate sounds, such as enlarging the bobbins, placing them on 

 both sides of the plate : in one case one was made for a core three inches 

 in diameter, and was used as a tube through which to speak. In 

 another case the bobbin was wound about a pint cup, and a large com- 

 pound magnet weighing about twenty pounds was placed with one 

 pole close to the bottom of the cup. This made a fair speaking tele- 

 phone, both as sender and receiver. 



Telephones with oval diaphragms, square diaphragms very small 

 and very large, a foot or more in diameter, were made ; diaphragm 

 fastened to large resonant surfaces, fastened to the edge, to the middle, 

 to both edge and middle, with magnet fastened to one edge of dia- 

 phragm, and the free end of the magnet opposite the middle of it, and 

 so on, — all of these except the largest making good speaking tele- 

 phones ; and these largest when mounted concentrically, leaving a free 

 edge five or six inches all round, make excellent calls, as they may be 

 struck with a billet of wood, which starts a current that can be felt in 

 an ordinary receiving telephone, and heard plainly thirty or forty feet 

 away from one. A good many forms of calls were invented, one of 

 them being the so-called " Devil's Fiddle : " a catgut string fastened to 

 the middle of the disk is pulled through a bit of leather with resin on 

 it. The sound of such a device is familiar to every one. I have heard 

 it from a receiving telephone in another room, with the door closed 

 between. Also a tuning-fork call, in which a rather large tuning-fork 

 is made to vibrate and then held so that the vibrations of one leg 

 strike against the telephone plate. This was used as early as January, 

 1877. Another call consists of a hammer resting lightly upon the 

 plate of the telephone, which will be thrown over by a strong call, and 

 thus ring a bell, or set off an alarm. This can be done with the voice. 

 To measure the amplitude of vibration of such a telephone plate, I 

 had one mounted with a system of levers, one of them carrying a small 

 mirror that reflected a beam of light across the room, when I got a 

 displacement of as much as two feet for some sounds. 



So far the experiments have been with the magneto-telephone, where 

 the principle depends upon the varying electro-motive force originat- 

 ing in a magnetic field. But I turned my attention to the other 

 method of varying the current, — namely, by varying the resistance, — 

 and accomplished it in several ways. 



First, by making a single battery-cell a sender. If a cell be coupled 



