540 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1952 



by the two operating speeds and the three cam ratios. Approximately 

 90 per cent of any cycle time is available for recording or reproduction, 

 and the remaining 10 per cent is occupied by the return of the head to 

 the beginning of the helix. 



The recorded track is 0.100 inches wide, and when an eight-turn helix 

 is used, there is a separation of 0.025 inches ))etween tracks. The previ- 

 ously described low-freciuency alternating current erase is used. 



The 3A announcement system employs three channels of this recording 

 eciuipment in a complex control circuit which provides facilities for 

 erasing, recording, monitoring and automatic switchover to stand-by 

 channels in event of failure. Figs. 5 and G show the recording equipment 

 in the Cleveland installation. 



Other ecjuipments using this recording medium have been designed to 

 furnish transcribed message service for intercept of calls to vacant, 

 changed and unassigned numbers, to quote delays on long distance calls, 

 and to furnish stock cjuotation service. Some of these ecj[uipments are now 

 undergoing service trials preparatory to standardization for Bell Sys- 

 tem use. 



This new recording medium has been developed to provide the maxi- 

 minn attainable life and reliability in applications requiring an enormous 

 number of repetitions of voice messages. Ecjuipment for such applica- 

 tions is usually located in central offices where the temperature range 

 and other operating conditions are fairly w^ell stabilized. These favorable 

 conditions have facilitated the development of a recording medium 

 which has made it possible to design simple and economical magnetic 

 recorders which are sufficiently versatile and reliable to stimulate the 

 use of transcribed message services to an extent hitherto unrealizable. 



There are a number of potential Bell System applications for tran- 

 scribed message services which do not recjuire an extreme numbei- of 

 message repetitions, but put a premium on low initial cost and trouble- 

 free operation in intermittent service under wide extremes of en\'iron- 

 ment. It may prove desirable to meet the life requirements for applica- 

 tions of this type with a different approach to the lubrication problem, 

 with an unlubi'icated compound, or with a coated medium which would 

 have some transmission advantages. It is expected that finiher work in 

 these fields will produce improved recording media for applications of 

 this nature, to expand the field of use in the telephone plant. 



