94 PYNAMO-ELECTRIC MACHINES. 



electric, and what a great addition was made to the science of elec- 

 tric lighting by the invention of self-exciting machines. It will be 

 seen from this why dynamo, or .electro-magnetic machines, are em- 

 ployed, in preference to magneto-electric ones. It may be stated 

 here, that the first machines of Wilde were a combination of both, 

 to which their author gave the name of electro-magneto-dynamo- 

 electric machines. If there is anything in a name the length of this 

 ought to have been productive of good results. 



Magneto-electric machines possess some advantages over their 

 rivals. The intensity of the field is always uniform, whatever the 

 varying resistance of the circuit. There is no other resistance to be 

 considered beyond that of the armature. The whole of the current 

 generated, therefore, can be applied to useful work. A magnetic 

 field of uniform intensity is of very great importance in some opera- 

 tions, amongst which is light by incandescence. It became an ob- 

 ject, the attainment of which received considerable attention from 

 electricians, to devise a method of winding the magnet coils of dy- 

 namos to secure this, if possible. They have succeeded in a great 

 measure, some machines having been recently built, the currents 

 from which did not vary in intensity, whilst changing from 300 to 

 3 or 4 lamps, or the reverse. Edison claims to have accomplished 

 this also. Dynamos are almost exclusively used for industrial pur- 

 poses. De Merritten's is almost the only magneto-electric machine, 

 which claims. a place side by side with the others. The uniformity 

 of intensity and freedom from undulation of the current produced 

 by his machine, have contributed largely to its introduction and its 

 extended use. 



All currents generated by dynamos undulate. That is to say they 

 traverse the conductor in a series of wave-like motions. This mo- 

 tion varies considerably, being least in the Gramme and Siemens 

 types and reaching a maximum in the Brush, the current of which is 

 more a succession of rapid sledge-hammer-like blows than of gentle 

 undulation. The reason is that Brush, having few coils on his arma- 

 ture, is compelled to form them of a great number of turns. A coil 

 so formed is forced to bottle up, as it were, its energy, until it finds 

 an outlet through the commutator brush. It then rushes out with 

 such tremendous energy as to fly off to a great extent in the form of 

 sparks. The first machines of Siemens were constructed with eight 



