204 BELL SYSTEM TECHNICAL JOURNAL 



three units, the switching equipment itself which is wholly mechanical, 

 together with the so-called # 4 and ;i^ 5 switchboards. The # 4 board is a 

 cordless, key-typed call distributing board which is used in conjunction 

 with the new switching system for such calls as must be given to an operator 

 by offices not equipped for intertoU dialing. The operators at this board 

 function as combined inward, through and tandem operators, thus eliminat- 

 ing the provision of separate units to provide these particular services. In 

 brief, there is no basic difference between the essential operation of the # 5 

 board and the conventional through board where delayed traffic is handled; 

 however, operators handling calls at this board must make use of the new 

 switching system to obtain both the calhng and called offices by dialing. 



Prior to the cutover the first trainees were given experience by handling 

 some 300,000 test calls of every conceivable traffic characteristic. These 

 were routed through the new system to break in the equipment and to 

 shake down potential troubles. Two weeks prior to cutover a dress re- 

 hearsal was held, at which time about ten per cent of the circuits were put 

 through their paces. 



To provide information of value for future installations, arrangements 

 were made for liberal provision of registers and meters to measure any and 

 all phases of the various steps performed by the equipment. Some of these 

 aids are not entirely new to telephone work but their application to toll, 

 inward and through service is a departure. 



The ^4 System is running satisfactorily and both the equipment and 

 the operators who use it deliver a high grade of service. Daily some 80,030 

 tandem, inward and through connections formerly handled by operators 

 are routed through the equipment. 



In connection with postwar planning, studies are now being made to 

 determine future installations in order to take advantage of the possibilities 

 of the new system. It is confidently expected that this will provide faster 

 service on outward, inward and through calls and that transmission will be 

 improved. These advantages should result in overall economies in outside 

 plant and operating. 



Theoretical Limitation to Transconductance in Certain Types of Vacuum 

 Tubes.^ J. R. Pierce. The thermal-velocity distribution of thermioni- 

 cally emitted electrons limits the low-frequency transconductance which 

 can be attained in tubes in whose operation space charge is not important. 

 A relation is developed by means of which this dependence may be evaluated 

 for tubes employing electric and magnetic control. This relation is applied 

 to deflection tubes with electric and magnectic control and to stopping- 



*Proc. I. R. E., December 1943. 



