224 BELL SYSTEM lECHNlCAL JOURNAL 



a corresponding change in the length of the coil. Fig. 11 shows the 

 relative dimensions of three designs of spools taken from Figs. 9 and 10. 



Some very interesting information can be obtained from Fig. 11 

 in regard to the relation between the volume of wire, as reflecting 

 the first cost, and the ampere turn operating efficiency. Design "A" 

 contains a volume of copper of 3.65 cubic inches, while in design 

 "B" the volume of copper has been reduced to 2.00 cubic inches 

 although the loss in eff^ective ampere-turns is only 10 per cent. In 

 design "C" the volume has been reduced to 1.10 cubic inches with a 

 loss in efficiency of 30 per cent. 



Obviously the design "C" is the cheapest in first cost because of 

 the small copper volume and will also give the lowest annual charge 

 where the time of operation is very short and the charge for power 

 relativ'ely low. Where the magnet is required to operate very often 

 and the price of power is high the design "A" will prove the most 

 economical. Design "B" may be considered as intermediate between 

 designs "A" and "C". 



In the above considerations of sjkioI dimensions the examples 

 given should not be taken as an accurate generalization but simply 

 as a method which, with a given set of requirements, should enable 

 reasonable first approximations to be made. Thus, if annual power 

 charges are controlling, a relatively short and deep spool will give 

 the best results, although there may be exceptions where for instance, 

 the operating current is reduced to a holding value and where the 

 leakage is relati\ely small due to the fact that the armature is oper- 

 ated. In such a case and unless operating efficiency is also of prime 

 importance the design "A" would be more expensive than necessary 

 in first cost. Other cases often arise where the input wattage is 

 very small but the operating requirements are very exacting so that 

 the most efficient winding is required and the first cost is relativeh' 

 unimportant. In this case a larger volume than "A" can be used to 

 advantage. These examples may be used as a guide therefore, in 

 determining spool dimensions which are later refined as the design 

 is completely worked out. The illustrations of designs gi\-en in the 

 latter part of this paper show how accurately certain final design 

 dimensions can be worked out to give the minimum annual charge. 



Discussion of Di;si(;ns Used E.xtknsivei.v 

 IN Tin: Tki.fphone Pl.\.nt 



To an\' one familiar with telephone systems it is ob\ious that it is 

 impracticable to design all the relays required at maximum elficiency 

 and economy for each particular condition that arises. Sucli a pro- 



