208 BELL SYSTEM TECHNICAL JOURNAL 



ation on alternating currents, ranging from 16 cycles ringing fre- 

 quency to voice frequencies of 2,000 c>'cles per second. The load or 

 work required of these relays and electromagnets \aries from a fraction 

 of a gram controlled through a few thousandths of an inch to 25 pounds 

 controlled through a distance of ^4 of an inch. Some rela>s are 

 operated where the annual power charges are negligible while in other 

 designs annual power charges may be controlling. The technical 

 considerations which determine the design features, therefore vary 

 throughout a wide range as to the proportioning of the magnetic 

 parts and the design of the windings. Other general design character- 

 istics that must be carefully considered are as follows: 



1. Operating capability of the structure — 



(a) Switching conditions or circuit control required of the relay. 



(b) Design of contacts required to safely carry the energ\' re- 

 quired by condition (a) throughout the estimated "life" 

 requirements of the switchboard. 



(c) Capability of the structure with respect to the input power 

 to satisfy condition (a). 



2. Determination of winding test suited for the circuit. 



3. Temperature limitation of the wintling under extreme contiilions. 



4. Ease of adjustment. 



5. Permanence of adjustment — 



(a) For a period of service operations representing the "life" 

 of the relay in the switchboard. 



(b) Under extreme weather conditions. 



6. Size and mounting facilities. 



(a) When used for additions to old eciuiiJUienl where it should 

 mount in the same space as the apjiaratus it replaces. 



(b) Economy of space for new equipments. 



(c) Stability of mounting. 



7. Terminals — arrangement and distribution lor most ad\antageous 

 electrical connections. 



8. Insulating materials. 



(a) Windings. 



(b) Switch control of contacts. 



9. Cover design. 



(a) Protection from dust. 



(b) Effect of cover on operation and protection from stra\' Ihix. 



10. Speed of operation and release. 



11. Transmission efficiency with respect to voice frequencies. 



