312 



Professor J. A. Fleming 



[March 6, 



Two laminated straight iron bars (Fig. 16) are throttled at intervals 

 with secondary circuits, and have wound on one extremity a mag- 

 netising coil. The two bars are jilaced near ea'^h other and parallel. 

 The coils are so connected that the poles at any instant in the ends 

 of the two bars are of similar name. An alternate current is sent 

 through the coils joined in series. Under these circumstances a 

 series of alternate poles of similar names run up the bar parallel 

 with one another. A small, soft iron needle hung at any place 

 between the bars sets itself parallel to the bars, because at any instant 

 poles of similar names are abreast of one another at any spot in the 

 length of the bars. If, however, we shift one bar lengthways back- 

 wards or forwards through a certain distance, so as to bring opposite 



Fig:. 16. 



S N S tsl 



N i////iii 11 III III 



N 



Niy ///1 11 



N 



N 



N 



N 



S N S 



N m i II II I 



N 



S 



V= Nl 



Mr. Wright's experiments with throttled and alternately magnetised bars. 



poles abreast of each other throughout their journey up the bars, we 

 shall find a position such that the soft iron needle will set at right 

 angles to the bars when hung at any point in the space between 

 them. The distance by which we have to shift the one bar backwards 

 of the other to effect the change is evidently half a magnetic wave 

 length, and knowing the frequency of the alternations we can readily 

 arrive at a measure of the velocity of propagation of these altt- mate 

 poles in the bar. This velocity is evidently numerically equal to 

 the product of the frequency and wave length so obtained. 



§ 15. A very pretty apj)licatiou of the above principle has been 

 made in the electric meter of Messrs. Wright aud Ferranti for 

 measuring alternating currents. Before me stands one of these 

 meters. It consists of a pair of vertical electro-magnets, with 

 laminated iron cores, and each magnet bears at the toj) a curved 

 horn of laminated iron which is throttled by copper rings. These 

 curved horns, springing from the magnets, embrace and nearly touch 



