and Transmission of Power. 27 



In magneto machines, the magnets are permanent steel ones, 

 but in dynamos the magnets are of iron, with, coils of wire 

 wrapped upon them, and the magnetism is produced by currents 

 of electricity passing along the wire : such currents may either 

 be produced by the machine itself, or by a separate "exciting" 

 machine or battery. Again, in direct current dynamo machines 

 in which the magnets are excited by their own currents, the 

 magnets may be coiled with comparatively thick wire, and made 

 to receive all the current generated, which, after passing along 

 the coils surrounding the magnets, proceeds to the lamps or 

 external circuit, thence back to the machine. These are termed 

 "series" machines. Instead of the magnets having a com- 

 paratively short length of thick wire, thus producing but few 

 turns, they may have an immense length of fine wire coiled 

 upon them, and returning direct to the revolving armature 

 (which is the name applied to the rotating coils of wire in which 

 the currents are generated), with a separate set of conductors 

 leading to the lamps ; thus only a very small proportion of the 

 current generated in the armature passes round the magnets, in 

 consequence of the fineness of the wire and its extreme length. 

 These are termed "shunt wound" machines. It is noteworthy 

 that the small amount of current which passes round the 

 magnets in a "shunt" machine is quite as effective as the large 

 or total amount of current which flows round the magnets of 

 a "series" machine, in consequence of the greater number of 

 turns in the case of a "shunt" arrangement, as one ampere 

 (the unit applied to quantity) passing along one hundred turns 

 of wire on a magnet is as effective, practically, as one hundred 

 amperes passing once round the iron. Frequently machines 

 have their magnets coiled both with a fine "shunt'' wire and 

 a thick "series" one, and the current passes along both, but in 

 inverse ratios to the relative resistances. They are thence 

 termed "compound," and are generally employed for incandes- 

 cent lighting, as they are more nearly self-regulating, provided 

 a regular speed is maintained, whereas with "shunt" machines 

 it may happen that if a great number of lamps are switched off, 

 too much current passes through the remainder, thus injuring 



