CONTINUOUS-CURRENT DYNAMOS. 143 



chiefly implied hard steel with a high percentage of carbon, 

 a material which reaches its highest excellence in crucible 

 cast steel. Gradually, however, the percentage of carbon 

 has been so far reduced, and the softness so much increased 

 that the same name of steel covers what is in effect as 

 nearly pure iron as it is practically possible to produce. It 

 is therefore mild or soft steel which is now so largely in use 

 for dynamo work, and, so far as the purposes of the elec- 

 trician are concerned, this is practically the same substance 

 as was previously used in the form of wrought-iron forgings, 

 the only difference being that whereas in the latter the iron 

 was worked up into slabs by welding under the hammer, it 

 is now produced in large quantities by melting in the 

 furnace or converter and is then poured out into ingot or 

 other moulds in which it solidifies. In the future we may 

 expect such steel to play an ever-increasing part, especially 

 for castings of a somewhat complicated shape which have 

 also to serve as the carriers of magnetic lines. 1 



Given then the necessary information with regard to 

 our iron, there follows an examination of the dimensions of 

 the magnetic circuit of the dynamo which we are designing. 

 The length and area of each portion is dealt with piecemeal, 

 and the necessary exciting power required by it separately 

 calculated ; the final summing up of the various items so 

 obtained enables us to draw a curve from which we can con- 

 fidently predict how many useful lines will pass through the 

 armature for any given value of the exciting power on the 

 field-magnet. 



The second important point of the paper of the Drs. 

 Hopkinson was their treatment of the magnetic leakage in 

 the case of two typical machines, the one of the Edison- 

 Hopkinson type, and the other a Manchester dynamo with 

 double horse-shoe field. In the first place, they experimen- 

 tally determined the proportion which the lines straying 

 beyond the limits of the armature bore to the number which 

 actually traversed it, and were usefully cut by the wires on 

 the surface : a factor (denoted by the symbol v) was thus 



1 See a paper by Kapp, " On Methods of Testing the Magnetic Qualities 

 of \xon," Journal List. El. Eng., vol. xxiii., No. no, 1894. 



