Bell's Telephonic Researches 



should not a larger and thicker membrane be 

 able to vibrate a piece of iron in front of an 

 electro-magnet, in which case the complication 

 of steel rods shown in my first form of telephone, 

 Fig. 3, could be done away with, and a simple 

 piece of iron attached to a membrane be placed 

 at either end of the telegraphic circuit. 



Figure 6 shows the form of apparatus that I 

 was then employing for producing undulatory 

 currents of electricity for the purpose of multiple 

 telegraphy. A steel reed, A, was clamped firmly 

 by one extremity to the uncovered leg h of an 

 electro-magnet E, and the free end of the reed 

 projected above the covered leg. When the 

 reed A was vibrated in any mechanical way the 

 battery current was thrown into waves, and 

 electrical undulations traversed the circuit 

 B E W E', throwing into vibration the corre- 

 sponding reed A' at the other end of the circuit. 

 I immediately proceeded to put my new idea to 

 the test of practical experiment, and for this 

 purpose I attached the reed A (Fig. 7) loosely 

 by one extremity to the uncovered pole \ of the 

 magnet, and fastened the other extremity to the 

 centre of a stretched membrane of goldbeaters' 

 skin n. I presumed that upon speaking in the 

 neighbourhood of the membrane n it would be 

 thrown into vibration and cause the steel reed A 

 to move in a similar manner, occasioning undula- 

 tions in the electrical current that would corre- 

 spond to the changes in the density of the air 

 during the production of the sound ; and I further 

 71 



