Magnetic Circuit of Dynamo Machines. 507 



the early part of the curve it is quite unsuitable. Pis r-^ x 



arnpere-turns in one coil of the machine, or the magneto- 

 motive force. 



The uses made by Herr Frohlich and Prof. Riicker of (15) 

 in a theory of compounding show how important it is to have 

 a simple empirical formula. Indeed in our patent specification 

 of 1882 we base the theory of compounding on the simplest 

 of all empirical formulae, 



E = aC (16) 



And it is by means of this formula that the theory can be best 

 put before students ; if our easy reasoning is properly grasped, 

 the practical electrical engineer will be able to use the results 

 graphically with actual characteristics in the manner employed 

 by the most experienced men at the present day. 



Instead of speaking of E the E.M.F. produced in the arma- 

 ture, which is proportional to N the magnetic field and n the 

 speed, as shown in (1) it is preferable to speak of 1ST the field 

 itself. And instead of referring merely to the current exciting 

 the field-magnets, it is preferable to speak of the ampere- 

 turns S 2 A 2 , or, better, of the magneto-motive force in C.G.S. 



4:77 , 



10 



We have tried in vain, during perhaps two months of very 

 hard work, to express the two constants a and b of (15) in 

 terms of the dimensions of a dynamo machine. We now 

 believe that this cannot be effected. We have already given 

 the reason why the linear law connecting the /3 and fi cannot 

 be used in obtaining a formula. In any case such a formula 

 would be very different from that of Frohlich. But in view 

 of the important relation (12) which we have established, that 

 when the machine is giving its best permanent output, — 



The air magnetic resistance of the space occupied by wind- 

 ing on the armature = all other magnetic resistance, 



it is not difficult to arrive at practical rules of working. It 

 will be noticed that in existing machines we may, in general 

 calculations, neglect the resistance of the clearance-space d. 



The most important fact to be kept in mind by constructors 

 of dynamos is, that the magnetic air-resistance is the governing 

 factor. Until half* the highest induction is reached we may 

 neglect altogether the resistance of the iron ; and indeed for 

 many calculations it is sufficient to take 



N=^|s 3 A a ; (17) 



units, ^ S 2 A 2 . 



