702 INDUSTRIAL APPLICATIONS. 



Electrodynamic machines have no practical interest since their 

 electromotive force would only have an appreciable value for ex- 

 cessive velocities. 



A machine on the Siemens' type might be considered as a mag- 

 netic one, as also the Thomson-Ferranti machine ; but they are only 

 used for alternate currents. 



Most of the present ^machines are magneto-electrical. If one 

 of the systems only contains iron, the coefficient C is null ; this is 

 the case with Froment's motor. 



The older machines, finally, are mixed, if we disregard the fact 

 that they naturally produce alternating currents, and that even with 

 a commutator the current cannot be considered as sensibly uniform, 

 Such are the machines of Pixii, of Clarke, of Nollet, of Me'ritens, 

 etc. The Gramme machines are mixed, and have a sensibly uniform 

 current, when the field is produced by magnets, or when the inducing 

 electromagnets are excited by an extraneous current. 



1258. Some particular cases give rise to remarkable properties. 



i st. If the function < (I) is a constant (magnetic machines, or 

 mixed machines with weak current), the electromotive force is pro- 

 portional to the velocity. For a given regime, the machine used 

 as electromotor is exactly comparable with an ordinary battery. 



2nd. If the function <f> (I) is of the form CI (electrodynamic 

 machines, or magneto-electrical machines with weak currents), and 

 the machine is used as motor, the current is given by the equation 



IR 1= E 



The machine only affects the expression for the current in the 

 form of a resistance proportional to the velocity; the efficiency is 



nCl nC 



u 



E R + nC J^ 

 nC 



Apart from friction, the velocity would continue to increase without 

 limit. 



If the machine is used as electromotor, it can only start provided 



or 



