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PROCEEDINGS OF THE AMERICAN ACADEMY 



XVI. 



A NEW RECEIVING TELEPHONE. 



By Professor A. E. Dolbear. 



Presented May 14, 1879. 



There are two forms of receiving telephones in use, the more com- 

 mon being the magneto-telephone, the other a modification of the 

 motograph invented several years ago by Edison. In the latter a dia- 

 phragm is made to vibrate by the varying friction between a metallic 

 strip and a rotating cylinder of chalk that is saturated with some elec- 

 trolytic substance. When a current of electricity is passed between 

 these while the cylinder is rotated, the friction is found to be less as 

 the current is greater, and hence a current varying with the phase of 

 vibration of a sound-wave will give a like movement to a diaphragm 

 properly connected to the strip pressing upon the cylinder. 



It has happened to me to discover another method entirely different 

 from either of these. Imagine a straight bar electro-magnet, two or 

 three inches long, so mounted that the core may be rotated on its axis, 

 the core to project half an inch beyond the bobbin on each end. Let 

 an armature be made of a U form, the ends of the U to lie upon the 

 projecting ends of the magnet. At the bend of the armature a short 

 rod connects it to the vibrating plate of any sort, mica, or paper, or 

 thin iron, mounted as in the ordinary way. Now, when a current of 

 electricity is made to traverse the coil, the core becomes a magnet and 

 the armature is attracted to it ; if the core be rotated, the adhesion of 

 the armature will carry the middle of the diaphragm towards the mag- 

 net, and if the current be stopped the elasticity of the plate will cause 

 it to return to its original place. A vibrating current will thus set up 

 corresponding vibrations in the diaphragm so long as the magnet 

 turns. 



The principle involved here may be readily put to the test by taking 

 almost any form of an electro-magnet and coupling it in circuit with a 



