etic Circuit of Dynamo Machines. 499 



S / , spires on the coils of the field-magnet part of one 

 magnetic circuit. In the Edison-Hopkinson and 

 Kapp machines there is only one magnetic circuit, and 

 S' is here the total number of spires on the field-mag- 

 nets ; but in the Manchester, Crompton, A-Gramme, and 

 other forms there are two magnetic circuits, and S x is 

 here the number of spires on only one of the magnetic 

 circuits. 

 A', amperes in each spire of the field magnet-coils ; or 

 a' } amperes per square centimetre of cross section of the 

 winding. 



Any person who has engaged in making measuring-instru- 

 ments or dynamo machines is aware that for a given volume 

 of winding, whether the wires are small or large, for the 

 same distribution of temperature there will be the same 

 number of ampere-turns and there will be the same rate of 

 loss of energy by heating, if the volume of insulating material 

 is always in the same ratio to the volume of the copper. 

 With fine wire the volume of insulating material becomes 

 greater, but not to such an extent as to make useless this 

 very important roughly correct practical rule for the makers 

 of instruments and dynamo machines. This rule we have 

 regularly used since 1881 in our measuring-instruments. 



This has led us to speak of a. the current in amperes per 

 square centimetre of cross section of a coil, rather than the 

 density of current in the copper alone, and we have been led 

 to some general rules of considerable practical interest in 

 consequence. 



To make our rules more complete, we begin with one which 

 is well known. It will be observed that we use Mr. Kapp's 

 method of counting wires on the armature, so as to make the 

 rule suitable both for the Hefner- Alteneck and the Gramme 

 armature. 



Total E.M.F. of armature =^i (1) 



10 8 v ' 



E.M.F. developed in unit length \ _ vkfa m 



of wire passing through the field J — 77-IO 8 ' ' 



Observe that it is only the wire on the convex outer surface 



of the armature which is here considered. 



W=^SA (3) 



W= 2 -f* (4) 



