498 ANNUAL EEPOET SMITHSONIAN" INSTITUTION, 1915. 



LOUD-SPEAKING TELEPHONE APPARATUS. 



While the successful completion of the loading and amplifier de- 

 velopments which have made transcontinental telephony possible 

 have completely eliminated the necessity and desirability for loud- 

 speaking telephone apparatus for general telephone service, the last 

 three or four years have seen material progress in the development 

 of such equipment for specialized services. In particular, there has 

 arisen a demand for loud-speaking telephonic equipment for an- 

 nouncing purposes, for passing signals and orders to switch towers 

 along railway lines, and for use in connection with moving pictures. 

 AVhile active development work on this kind of apparatus is still in 

 progress, much has already been done to meet the demands which 

 have arisen. For many of the services amplifjdng devices are used 

 in conjunction with special transmitters and receivers. 



TELEPHONE CIRCUITS WITH SUPERIMPOSED TELEGRAPH. 



While the original application of ordinary ground-line telegraph 

 operation to wires normally employed for telephone service was made 

 a good many years ago, notable improvements in the equipment have 

 been made during the last 10 or 12 years. The development of phan- 

 toming and loading, and particularly the combination of the two, 

 introduced many difficulties into the successful operation of super- 

 imposed telegraph circuits. Within the last few years it has been 

 necessary to redesign completely the terminal apparatus used at the 

 central office. In addition, the presence of the telegraph current 

 necessitated a special design of loading coil for use on very long cir- 

 cuits, in order to avoid impairment of speech quality. 



All of these difficulties have been overcome and the situation to-day 

 is one in which every wire in a modern telephone toll plant is avail- 

 able for operation as a grounded telegraph circuit, irrespective of 

 whether the wire forms part of a loaded circuit or a loaded phantom 

 circuit and irrespective of whether it is in cable or open wire. Fur- 

 ther, the telegraph circuit so produced is suitable for the highest class 

 of duplex operation. In the United States there are thousands of 

 miles of such superimposed telegi-aph circuits in daily operation. 



INTERFERENCE FROM HIGH-TENSION LIGHTING, POWER, AND RAILWAY 



CIRCUITS. 



Within the last 10 years the art of electric transmission of power 

 has undergone radical changes and the whole art of single-phase 

 alternating-current railway operation has come into existence. De- 

 velopments of a protective nature for telephone and telegraph cir- 

 cuits have kept pace with the developments in the disturbing circuits. 



