SIGNAL CORPS SCHOOL FOR TELEPHONE ELECTRICIANS, UNIVERSITY OF MICHIGAN, INSTRUMENT AND SWITCHBOARD REPAIR CLASS.
FIELD WORK WITH UNDAMPED WAVE TRANSMITTER AND TRENCH ANTENNA.
SIGNAL CORPS EQUIPMENT INSTALLED IN A DUGOUT.
Left to right—S. C. field telephone, type "EE-4;" field service buzzer; signal lamp (projector), type "EE-6"; S. C. radio set, SCR-76 and S. C. radio set, SCR-54A.
The special telephones adopted for use in the field were different from any in commercial use in America. The Signal Corps had developed certain special instruments combining both telephone and telegraphic principles. The field telephone, model 1917, for instance, was a telephone which included a telegraph buzzer on its telephone circuit. This instrument was used when great secrecy in communication was required. The messages were sent in telegraphic code, the buzzers being heard by the receiver. Another instrument was known simply as the buzzer. This was an instrument which utilized the telephone receiver for telegraphic messages. It was a supreme development for use over defective lines. An instrument closely related to the buzzer, but which gave even greater secrecy, was known as the buzzerphone. The buzzerphone was put into production just before the close of hostilities.
The mobile switchboard in most general use by our troops at the front was developed originally by the French and was known as the monotype. It was designed in units and could be extended to accommodate up to 12 trunk lines leading away from the board. This apparatus was the "central" of the front-line dugouts. It could be put into operation in a few minutes and was easily carried by a soldier.
The switchboard of the dugouts was the only telephone equipment not of American design used by the American Expeditionary Forces. It was put into production in the autumn of 1917 in three American plants, under the general policy of the Signal Corps to contract with more than one factory for the production of any important device.