CVNAMO-fiLECTRIC MACHINES, 95 



and twelve coils respectively, which gave rise to considerable spark- 

 ling. They are now wound with a greatly increased number. It 

 has been found the greater the number of coils on a ring, and conse- 

 quently of sections, in a commutator, the more even is the flow of 

 the current ; because, by increasing the number of coils the outlets 

 are correspondingly increased, the current is not bottled up, and is 

 therefore more uniform and regular. 



Machines of both classes vary in constructive detail, dependent 

 upon their uses. One that is suitable for giving a single large light, 

 is not suitable Tor giving several smaller ones, and a machine, on the 

 other hand, constructed for several lights is not suitable for the pro- 

 duction of a single large light. 



The writer when first attempting to build a machine used wire 

 on the armature altogether too fine for the purpose intended. The 

 armature ring, too, was of solid iron. The two causes combined, 

 produced so much heat that it would very soon have destroyed 

 itself. Several alterations have since been made, the results of 

 which are highly satisfactory. The machine, although small, will gen- 

 erate ample current with sufficient electro-motive force for three arc 

 lamps of 2000 candle power. Its weight is about 400 lbs. 



Much coarser wire is used both for armature and magnet coils, 

 in a machine for a single light, than in one for several, the require- 

 ments being largely intensity, or quantity of current, with little electro 

 motive force. The first thing to be considered, in designing a 

 machine, is the purpose to which it will be applied. If the work to 

 be done is only of moderate resistance, comparatively coarse wire can 

 be used. If the work opposes great resistance, much finer wire will 

 be necessary. If the machine is designed for lighting, the number 

 of lamps to be fed by it should be previously known. It is to be 

 understood that in all cases the resistances to be overcome, whether of 

 a number of lamps, of conductor, or of feeding another machine at 

 a distance, can only be overcome by a machine constructed with 

 special reference thereto. Some machines have a very small internal 

 resistance, as notably those used for electro-chemical purposes, and 

 for a single arc light. The resistance to be overcome outside, in the 

 circuit, in both cases, are small, and machines are made for those 

 purposes opposing resistances as low as one quarter ohm. Others 

 again, have a very high internal resistance, such as those of Brush, 



