DEVELOPMENTS IN TELEPHONY AND TELEGRAPHY JEWETT. 497 



Two problems are involved in the application of amplifiers to 

 ordinary two-way telephone circuits. One is the problem of the 

 amplifier instrument itself and involves fundamentally the con- 

 struction of an instrument which will add energy from a local source 

 to the telephone circuit under the control of the attenuated tele- 

 phone current from the distant transmitting station. To be suc- 

 cessful the instrument must have such characteristics that no ap- 

 preciable distortion of the speech is produced in its operation. 



The other is the problem of so inserting the amplifying device in 

 the telephone line that it will work equally well in both directions 

 and without any tendency to " sing," i. e., to operate in a local closed 

 cycle. 



Both of these problems were for many years formidable and un- 

 surmounted obstacles. Recently, however, both have been solved, 

 and the engineers of the Bell telephone system have developed a 

 number of types of amplifying devices capable of application to 

 either nonloaded or loaded circuits in cable or open wire. All of 

 these different types are in successful operation to-day. The appli- 

 cation of amplifiers to loaded circuits is particularly difficult on 

 account of the peculiar characteristics of such circuits, and the solu- 

 tion has involved many radical changes in the previously existing art 

 of loading. 



TRANSMITTERS AND RECEIVERS, 



Although the fundamental principles employed in the manufacture 

 of commercial transniitters and receivers have not undergone radical 

 change within the past 10 years, there have been marked advances 

 in the construction of the apparatus. In particular, transmitters 

 have been adapted for use interchangeably on local and common 

 battery systems. They have been made much more rugged, with 

 better " quality " and louder volume than the instruments standard 

 10 years ago. 



An even more marked improvement has been made in connection 

 with receivers due to the employment of pole pieces which are elec- 

 trically welded to the permanent magnet portion of the receiver. 



Special transmitters and receivers for a variety of services have 

 been developed. In particular, throat and chest transmitters, which 

 operate directly from the muscular movements caused by the opera- 

 tion of speaking, have been constructed for aeroplane and mine- 

 rescue work, where ordinary type instruments can not be employed. 

 Special transmitters have also been developed for employment in 

 theaters, churches, and other auditoriums for the purpose of enabling 

 partially deaf persons to understand better w-hat is being said. 

 Special telephones have also been developed for stethoscopic and 

 cardiographic observations. 

 18618°— SM 1915 32 



