88 DYNAMO-ELECTRIC MACHINES. 



it will be proper to refer more particularly to their general construc- 

 tion and explain the principles of their action. There are in the 

 construction of dynamos some essential considerations, which are 

 necessary to insure success. The first is the choice of materials for 

 magnets and conductors. The second is the intensity of the mag- 

 netic field, which depends upon the material used and the arrange- 

 ments of the magnets. The third is the quantity or length of useful 

 conductor passing through the field. There are other considerations, 

 such as the number of coils, the length of conductor in each coil, 

 their insulation, the method of winding, their connection to the commu- 

 tator, and, in general, the whole distribution of the various parts, 

 which, when put together constitute a machine. 



As magnetic intensity varies inversely as the square of the distance, 

 it is essential that the field should be as condensed as possible. The 

 closer the field magnets are to each other the greater and stronger 

 the number of lines, or rays of magnetic force, passing between them. 

 It must not be understood from this, that the sizes of machines are 

 limited. On the contrary the larger the machine, if constructed under 

 right conditions, the greater the amount of electric energy developed 

 and the more economic, commercially, it becomes. The revolving 

 wheel of the Gordon machine, mentioned above, is nearly nine feet 

 in diameter, and weighs about eight tons. That machine however 

 is of the alternate current class, in which there seems to be greater 

 latitude for large dimensions than in those of the continuous current 

 kind. Gramme has made some large and powerful machines of the 

 continuous current class. Brush, also, has made some machines 

 that are claimed to be of 80,000 candle power, and will light 40 lamps, 

 of 2000 candle power each. The conductor should move as near to 

 the magnet as possible ; one eighth of an inch is the usual space 

 allowed in good machines. Even in that small space there is a great 

 loss of magnetic effect. From what has been said of the ratios of 

 magnetic intensity, if the wire conductor was one-eighth of an inch 

 diameter, the outside layer would only be affected to one-fourth the 

 extent, that it would be, if it could be made to run in the clearance 

 space between it and the field magnets. There is a consequent limit 

 to the number of superimposed layers of conductor wire covering an 

 armature. You will understand, that if a space equal to the thickness 

 of the wire, exists between the outside layer and the magnet, that the 



