MAGNETO-ELECTRIC GENERATORS. 51 



important and most interesting class of apparatus from the 

 power and variety of their effects. 



From this rapid review it will be already seen that in- 

 duction machines may be divided into two great classes : 

 i, those which have for inducer a circuit traversed by a 

 voltaic current, and in which the action is developed by the 

 current itself; 2, those which have for inducer a perman- 

 ent magnet and require a mechanical movement to produce 

 their effect. But besides these two classes of apparatus there 

 is a third, the powers of which have recently been investi- 

 gated, and which, while belonging to the second class we 

 have just spoken of, present this curious difference, that the 

 apparatus do not require permanent magnets to excite them, 

 and that the inducer, made of soft iron, itself becomes a 

 magnet, and a most powerful magnet, under the influence 

 of the induced current it produces. The curious circum- 

 stance in these apparatus is that the initial cause is, so to 

 .speak, inappreciable ; it is a slight magnetization communi- 

 cated to the iron, either by the magnetism of the earth, or 

 :by a residual magnetism : in proportion to the working of the 

 .apparatus the initial cause and the effect are more and more 

 developed until the limit of maximum magnetization of iron 

 .is reached. These interesting machines, devised almost simul- 

 taneously by Wheatstone and by Siemens, have been im- 

 proved by Wilde and by Ladd, and have served to supple- 

 ment many other machines contrived on different plans, which 

 by this addition have acquired a very much greater power. 

 .Finally, to bring this historical sketch to a conclusion, we 

 shall add that in quite recent times the field of investigation 

 has just been extended in an unexpected manner by a new 

 class of magneto-electric machines founded on the continual 

 alternate reversal of the polarities successively developed in 

 the different parts of an annular electro-magnet by a power 

 ml magnet, and in the action exercised by this last on the 

 wires of the coils passing before it. To this class belong the 

 .machines of Gramme, of Siemens, of Brush, of Biirgin. and 



42. 



