i8o ALEXANDER GRAHAM BELL 



through which the apparatus passed, but shall merely say 

 that after a time I produced a different form of instrument 

 which served very well as a receiving telephone. In this 

 condition my invention was, in 1876, exhibited at the Cen- 

 tennial exhibition in Philadelphia. The lecture room tele- 

 phone was used as a transmitting instrument, and the other 

 as a receiver, so that vocal communication was only estab- 

 lished in one direction. 



The articulation produced from the instrument was 

 remarkably distinct, but its great defect consisted in the fact 

 that it could not be used as a transmitting instrument, and 

 thus two telephones were required at each station, one for 

 transmitting and one for receiving spoken messages. 



It was determined to vary the construction of the lecture 

 room telephone, and I sought, by changing the size and 

 tension of the membrane, the diameter and thickness of the 

 steel spring, the size and power of the magnet, and the coils 

 of insulated wire around their poles, to discover empirically 

 the exact effect of each element of the combination, and thus 

 to deduce a more perfect form of apparatus. It was found 

 that a marked increase in the loudness of the sounds resulted 

 from shortening the length of the coils of wire, and by en- 

 larging the iron diaphragm which was glued to the membrane. 

 In the latter case, also, the distinctness of the articulation 

 was improved. Finally, the membrane of gold beaters' skin 

 was discarded entirely, and a simple iron plate was used 

 instead, and at once intelligible articulation was obtained. 

 The new form of instrument was made, and, as had been 

 long anticipated, it was proved that the only use of the 

 battery was to magnetize the iron core, for the effects were 

 equally audible when the battery was omitted and a rod of 

 magnetized steel substituted for the iron core of the magnet. 



It was my original intention and it was always claimed 

 by me, that the final form of telephone would be operated by 

 permanent magnets in place of batteries, and numerous 

 experiments had been carried on by Mr. Watson and myself 

 privately for the purpose of producing this effect. 



At the time the instruments were first exhibited in pubHc 

 the results obtained with permanent magnets were not nearly 



