MA GNETO-ELECTRIC ILL TJMINA TION. 



585 



waves of electricity. These alternate currents are converted into an 

 intermittent current moving in one direction only by means of a com- 

 mutator. The second armature revolves 1,800 times a minute, gen- 

 erating 3,600 alternately-opposed waves of electric force, which are 

 picked up and sent in one direction by a commutator, as in the former 

 case. 



It is evident that when a good friction contact is to be kept be- 

 tween pieces of metal moving at these enormous velocities, the wear 

 and tear is very great. For a long time, however, it was thought that 

 these difficulties were inherent to the magneto-electric machine, until 

 electricians found, first, that the almost instantaneous flash of the cur- 

 rent could be considerably lengthened out, and then that the succes- 

 sive waves generated could be so produced as to flow in the same in- 

 stead of in opposite directions. 



These important desiderata are supplied in a magneto-electric ma- 

 chine of a novel form, invented by M. Gramme. The principle is not 

 difficult to understand. Take a long bar of soft iron, E, E', Fig. 1, 



Fig. 1. 



sn 



.< mts- 



round which is coiled an insulated copper wire ; to this bar, forming 

 an electro-magnet, let a permanent magnet, S !NT, be presented, the 

 south pole being nearest to the iron bar. Now move the permanent 

 magnet in the direction of the arrow parallel with itself, with a uni- 

 form velocity, and always maintaining the same distance from the 

 bar. The south pole of the permanent magnet will produce a north 

 magnetic pole in the portion of the iron bar nearest to it ; and the 

 gradual displacement of this pole from one end to the other of the 

 iron bar, caused by the motion of the magnet, will induce in the sur- 

 rounding wire an electric current which may be rendered evident by 

 the galvanometer, G. This current will not be instantaneous : it will 

 continue to flow during the whole time the magnet is moving between 

 the two ends, E E', of the iron bar, and its time of duration may 

 therefore be varied at pleasure. 



This experiment shows that it may be possible, by proper arrange- 

 ments, to realize a machine which will furnish a continuous current of 

 electricity for as long as may be desired. We have only to imagine 

 the electro-magnet, instead of being the straight bar shown in Fig. 1, 

 bent into a circular form as at E, E', E", E'", Fig. 2. 



