OF ARTS AND SCIENCES. 



439 



TABLE I. 



It thus appears that the position of the commutator has but little 

 influence upon the strength of the current ; but, as the maximum was 

 obtained in each case from position No. G, it was kept in that place in 

 all further experiments. 



Kext, to determine the relation between the four variables, speed 

 of revolution, resistance in circuit, current, and electro-motive force. 

 An attempt was also made to measure the work required to run the 

 machine, and the coefficient of efficiency ; but, from lack of proper 

 dynamometric facilities, the attempt was necessarily abandoned after 

 the first series of experiments. 



The results of these experiments are given in the following tables, 

 in which R is the resistance of the circuit, expressed in ohms ; S is 

 the speed, or number of revolutions of the armature per minute ; O is 

 the current in vebers per second ; E is the electro-motive force in volts ; 

 E^ is the coin[)ute(l electro-motive force in volts, which would have 

 been obtained with a speed of 1,000 revolutions per minute; IF is the 

 work expended, in foot-pounds, including friction. IF. C'is the work 

 the current is capable of doing, in foot-pounds ; and C E is the coeffi- 

 cient of efficiency of the machine, obtained by subtracting the work 

 required to drive the machine on an open circuit from the actual work 

 JF, and dividing the computed work W. G by the remainder. 



From an examination of these tables, several important conclusions 

 may be drawn. For large resistances, over 38 ohms for instance, the 

 electro-motive force is nearly proportional to the speed, and is given 

 by the equation ^'=:.007*S'. The advantage of placing the magnet in 

 the main circuit is here in a great measure lost, since the large outside 

 resistance so far reduces the current that its effiict on the magnet is 

 slight. The constant .007 affords a good means of comparing various 

 machines of this form, since its magnitude depends directly on the 

 arrangement of the magnet and armature. For resistances less than 



