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POPULAR SCIENCE MONTHLY 



electrically. There were two membrane-diaphragm magneto-tele- 

 phones, which could be used either as transmitters or as receivers. The 

 only difference was that in one a single-pole electromagnet was used 

 and a double-pole electromagnet in the other. The armature of each 

 electromagnet consisted of a small piece of steel spring glued to the 

 center of the membrane which was three inches in diameter. The 

 castings were of brass, mounted on black-walnut base-boards, and the 

 cones were of japanned tin (Fig. I). 1 A third form of speaking tele- 



Bcll's Centennial Single Pole Magneto Telephone. 



Section of Same. 



Bell's Centennial Double Pole Magneto Telephone. 

 Fig. l. 



phone was shown, intended to be used only as a receiving instrument. 

 The electromagnet was enclosed in a hollow box of iron, and a lid of 

 iron was used as an armature. This lid formed a thin circular metallic 

 diaphragm, resting by its edge upon the rim of the iron box, its central 

 portion not quite touching the pole of the electromagnet underneath. 

 This receiver could be placed in circuit with either one of the mem- 



1 The illustrations shown (Figs. 1 to 5 ) are reproduced with permission 

 from the general brief of the American Bell Telephone Company presented in 

 ' The Telephone Appeals,' before the Supreme Court of the United States, Oc- 

 tober term, 1886. 



