CHAP, x.] ELECTRICITY AND MAGNETISM. 373 



electric machines or generators, to distinguish them 

 from the generators in which permanent steel magnets 

 are employed. In either case the current is due to 

 magneto-electric induction ; and in either case also the 

 energy of the currents so induced is derived from the 

 dynamical power of the steam-engine or other motor 

 which performs the work of moving the rotating coils 

 of wire in the magnetic field. Of the many modern 

 machines constructed on this principle the most famous 

 are those of Siemens, Gramme, and Brush. They differ 

 chiefly in the means adopted for obtaining practical con- 

 tinuity in the current. In all of them the electromotive- 

 force generated is proportional to the number of turns 

 of wire in the rotating armature, and (within certain 

 limits) to the speed of revolution. When currents of 

 small electromotive-force, but of considerable strength, 

 are required, as for electroplating, the rotating armatures 

 of a generator must be made with small internal resist- 

 ance, and therefore of a few turns of stout wire or ribbon 

 of sheet copper. For producing currents of high electro- 

 motive-force for the purpose of electric lighting, the 

 armature must be driven very fast, and must consist of 

 many turns of wire, which may be thin, as its resistance 

 is not of great moment in a long circuit where there 

 already are considerable resistances. 



4O9. Siemens' Machine. The dynamo -electric 

 generator, invented by Siemens and Von Hefner Alteneck, 

 usually called the Siemens' machine, is shown in Fig. 

 151. Upon a stout frame are fixed four powerful flat 

 electromagnets, the upper pair having their N.- poles 

 facing one another and united by arched pieces or 

 cheeks of iron. The two S. -poles of the under pair are 

 similarly united. In the space between the upper and 

 lower cheeks, which is, therefore, a very intense magnetic 

 field, lies a horizontal axis, upon which rotates an armature 

 consisting of eight longitudinal coils, each end of each 

 coil being connected with a commutator at the anterior 



