448 BEPORT— 1882. 



an important problem to ascertain what modification of tte present arrangements 

 will minimise if not entirely prevent it. 



6. Demands of a System of Electrical Distribtition. 

 By F. J. Spragde, JI.S. Navy. 



The multiple arc system of distribution, having the advantages of simplicity, 

 independence of circuits, and reliability, is the only one which can have any wide- 

 spread application for domestic purposes where the current is taken directly from 

 the generators. Considering a single machine, the external resistauce diminishes 

 with the increase in the number of circuits, the current varies inversely with the 

 external resistance, and the potential at the terminal remains constant,' while the 

 electromotive force of the machine does not. If r is the ratio of external to 

 internal resistance, IS the electromotive force developed when these resistances are 

 equal, and E' the electromotive force when ;■ has some other value than one, then 



The curve being traced shows : — 



1st. In a single generator the electromotive force is not proportional to the 

 number of lights or motors or the current developed. 



2nd. No matter how liigh the external resistance, or how limited the number 

 of derived circuits, the electromotive force of a machine required to maintain a 

 fixed diflerence of potential at the terminals, can never be less than one-half that 

 developed when the external and internal resistances are equal. 



3rd. The increase of electromotive force required as the external resistance 

 decreases, or the number of derived circuits is increased, is very gradual up to the 

 point where the external resistance is three or four times the" internal, beginning 

 then to rise more rapidly. 



4th. When the external resistance falls below the internal, the rise is very 

 rapid, and if this diflference is marked, then it becomes impossible to maintain a 

 fixed diflference of potential, because of the great increase necessary in the strength 

 of the magnetic field and the velocity of the armature. A great amount of heat, 

 and consequently wasted energy, would be developed in the armature. 



In a general system there would be a low external resistance and large currents, 

 the energies of which are proportional to the squares. 



Consequently the resistance of lamps should be high; but there is a limit, 

 determined principally by the illumination required, and the potential. Since for 

 a given illuminating surface small section carbons give the highest resistance and 

 least mass, we should diminish the diameter and increase the length and resistance 

 of carbon, meeting this by an increase of potential. Several reasons, one of the 

 principal being_ safety, indicate about 150 volta as the limit at the terminal of 

 the lamp circuits. With a given standard, it is necessary to have also lamps of 

 higher power. _ For this we must increase the width of carbons, or their length, 

 while the specific resistance is reduced. Flat carbons are preferable to circular 

 ones, because the resistance of the last diminishes as the square of the increase of 

 surface. 



The energy required for a system is expended in overcoming friction and 

 inertia, magnetising useless parts, and generating currents which do work in the 

 armature, field coils, conductors, and lamps. Of these, the work required for 

 friction and inertia, and for the field magnets, is about the same whatever the 

 niunber of lights on a single machine. Various considerations lead me to the 

 following conclusions : — 



1. The resistance of the armature of the generator should be as low as possible 

 compatible with the power to furnish the necessary electromotive force. 



2. The generator should be able to produce this electromotive force with the 

 field magnet at less than the maximum economic saturation, and at a velocity lesa 

 than that at which the armature can be safely run, in order that there may be a 

 margin for increased efficiency in case of an unexpected demand. 



