A)/rs7(7.v cit.iR.UTi-Kisrics OF r.i.r.cTRinf.icxiirs 2m 



12. Mochanical ilosij^n fo.it iiros with special reference to manu- 

 facture. 



13. Electro-nicdianical erticii.'ncy. 



14. First cost and annual charges. 



.■\s it is not within the scope of the present paper to discuss in 

 detail all of the above characteristics the following have lx;en selected 

 as f)erhaps the more important and the most interesting: 



1. The design of the magnetic parts for various telephone switch- 

 board requirements. 



2. Methods of calculating windings and the determination of 

 temperature characteristics. 



3. Considerations which determine the spool dimensions. 



4. Discussion of designs used extensively in the telephone plant. 



Design of M.xc.netic P.vrts 



The fundamental requirements of an electromagnet or relay are 

 generally the load or pull, the distance through which the load must 

 be moved and the time limits of operation. The last requirement, 

 of course, is reflected in the load or pull requirement as an added pull 

 or force of acceleration. 



The fundamental constants of design are the flux leakage coefficient, 

 the core flux density and the flux density in the pole face or area where 

 the pull is e.xerted. If the designer is given data which fix these 

 constants the remainder of the work is usually a comparati\ely simple 

 matter of calculation. 



The leakage coefficient has been determined experimentally through- 

 out a range of designs where the load to be controlled varied from 

 1 gram to 5,000 grams. The results show that the leakage depends 

 almost entirely upon the armature air-gap reluctance and the ratio of 

 the core length to the core diameter. The leakage flux is defined 

 as that percentage of the total core flux which does not cross the 

 armature air-gap, and consequently can not be utilized for producing 

 traction. The per cent useful flux is then the ratio of the flux crossing 

 the armature air-gap to the total flux in the relay core. The curves 

 in Fig. 2 for single spool electromagnets and Fig. 3 for double sf>ooI 

 electromagnets give the per cent useful flux for various air-gaps and 

 core lengths which are expressed in terms of the core diameter. In 

 cases where the core is round and the pole face area is equal to the 



