Governing of Electromotors. 69 



armature circuit, as in fig. 3, the accumulators being joined up 

 so as to help the main current C. In that case the resistance 

 a of the armature must be increased by a', the resistance of the 

 accumulators ; and the two equations we are finally led to for 

 the speed at which the motor governs, and the constant 

 current for which it governs, are 



«= z -±^, (13) 



C=^ (14) 



This solution is a practical one, and consists of a simple- 

 shunt-motor, with some accumulators in the armature circuit. 

 Further, it is one not requiring many accumulators, for if np, 

 the back E.M.F. of the motor due to the residual magnetism 

 alone be small, as it w T ill be if the field-magnet iron core be 

 soft, and further, if a + a' be also not large, e need not be large 

 in order that the value of C, the current for which the motor 

 governs, may be as large as we like. The essence of this 

 new method for making a motor supplied with a constant 

 current revolve at a constant speed independently of the load 

 does not consist in combining a shunt-motor with a series 

 brake-dynamo, which was the essence of our original method, 

 but in combining a shunt-motor w T ith a store of electric 

 energy in the armature-circuit, which store of energy, as the 

 equations show, is automatically dealt out to help the motor 

 exactly in proportion as the demand for power is required to 

 keep the speed constant when the load on the motor is varied. 

 And this store can be easily maintained by reversing the 

 connexions of the accumulators when they are disconnected 

 from the motor, and the motor is at rest, and leaving the 

 accumulators without supervision to be charged by the con- 

 stant current that is continuously supplied to the station. 



While on the subject of electromotors it may be well to 

 point out a fact which we thought was well known, viz. that 

 when motors are being tested one against another for 

 efficiency and for power developed per pound weight, it is 

 most important that the motors should each be supplied with 

 the P.D. they are intended to work at and should be run at 

 their normal speed. But we have been much astonished to 

 find in the ' Technology Quarterly ' for September last, 

 published by the Massachusetts Institute of Technology, a 

 paper by Mr. H. Clifford on u The Efficiency of Small Electro- 

 Motors," in which the remits of experiments on the relative 

 power and efficiency of a number of motors are given without 

 any attention having been paid to the particular P.D. and 



