1 64 ELEMENTS OF ELECTRICAL ENGINEERING. 



action (a). Motors which are repeatedly reversed would not 

 operate satisfactorily with low resistance brushes of metal. 



A current in a circuit of large inductance has great electrical 

 momentum and the quick reversal of a current in such a circuit 

 requires a very considerable electromotive force. Therefore the 

 reversal of current in the short-circuited sections of an armature 

 during the brief time that the short-circuit continues, is vitally 

 dependent upon a small value of the inductance of the sections. 

 Small inductance is especially important inasmuch as the action 

 (b) above described is chiefly relied upon, and this action is equiv- 

 alent to a very small electromotive force. The inductance of 

 an armature section, wound in slots of a given size, is proportional 

 to the square of the number of turns of wire in the section. 

 Therefore a small inductance necessitates few turns of wire in 

 each armature section. 



79. Standard rules for ratings and guarantees. In the early 

 days of electrical engineering, different manufacturers followed 

 widely divergent rules for fixing the ratings of their machines 

 with the result that ratings were almost meaningless. In addi- 

 tion to this, the terminology of electrical engineering, and espe- 

 cially the mathematical notation employed in electrical engineer- 

 ing literature was badly confused. To remedy this state of affairs, 

 a Committee on Standardization was appointed by the American 

 Institute of Electrical Engineers in 1898 and their report,* some 

 of the details of which have since been revised, was laid before 

 the Institute in June, 1899. The revised report is dated May, 

 1902. 



The recommendations of this committee on rise of temperature, 

 insulation, and rating of dynamo machines are given below. The 

 remainder of the report of the committee is devoted chiefly to 

 recommendations concerning notation, to definitions, and to the 

 enunciation of principles. This part of the report is adequately 

 represented in the general mode of treatment adopted in this text. 



* Trans. A. I. E. ., Vol. XVL, pp. 255-268. This report has been issued by 

 the Institute as a separate publication. 



