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March, 1931] THE CALL ANNOUNCER 367
four strips are mounted symmetrically around the circumference of the drum. The actual speaking time required for a single digit is about 0.27 second, and the silent interval allowed is 0.08 second. These values are for a film running at the standard speed of 90 feet a minute.
The three-stage amplifier associated with each of the sixteen (including two spares) sound reproducing systems is located just beneath and in front of the rotating drums. By means of these amplifiers the current from the photo-electric cell is brought up from an acoustic level of about —70 or —75 db. to a level of about —25 db. From these amplifiers, the current is fed into telephone repeaters mounted on an associated relay rack, which act as final power amplifiers and bring the acoustic level up to approximately the zero reference point, 6 milliwatts. When a number is dialed, it is recorded on relays in the sending mechanism, where it remains stored until the operator who is to complete the call is free to do so. She then pushes a button and the relays, in which the number is stored, actuate other relays to cut in the proper films in the correct sequence in order to announce to the operator at the manual board the number which has been dialed.
These call announcers, at least for the present, will not be placed in dial exchange offices, but only in dial tandem offices, which are used to connect widely separated telephones. Three call announcers will be placed in each of these offices, one of which is held in reserve, the load being automatically distributed between the two in service. The call announcers in service are monitored automatically by relays in each reproducing circuit. These relays are held operated as long as the reproduced speech in each circuit is maintained at a predetermined level. If, through failure of the photo-electric cell, the lamp, or from any cause, the volume falls appreciably in any reproducing circuit, the particular call announcer is switched out of service and the spare one connected in its place and a signal is given to the maintenance force.
Development work on the call announcer was completed and a public demonstration was made in 1929. It was demonstrated to members of the Society on the occasion of their visit to Bell Telephone Laboratories during the Fall Meeting, October 20-23, 1930. This apparatus has given entire satisfaction in the numerous tests made with it and the first commercial installation was put into service January 3, 1931.
*Due to the lower illumination level of the lamp, this current is much less than in sound picture practice.