698 INDUSTRIAL APPLICATIONS. 



wire, and rotated a piece of soft iron in front of it. The re- 

 searches of Masson and Breguet* have shown the remarkable effects 

 obtained by the induction of broken currents. The induction coil 

 which they used has come, by successive improvements, into general 

 use. 



For some years these machines have emerged from the labora- 

 tories, and have taken an important part in industry. They may 

 be divided into two groups, according as the current which they 

 produce is sensibly uniform that is to say, is always in the same 

 direction with small variations of intensity, or non-uniform, as the 

 current can change its direction, or undergo sudden changes. 



1254. The machines which give a uniform current or at any- 

 rate a current of constant direction when their motion is kept up 

 by external force become, on the contrary, motors when excited 

 by an external current. 



We shall first suppose it to be the case of perfect machines 

 such as Faraday's disc, in which the current is perfectly uniform. 

 In this case (550 and 557) the electromotive force of induction 

 E, other things being equal, is proportional to the velocity that 

 is, to the number n of turns per second, and to a function <(!) 

 of the intensity of the current, the form of which depends on the 

 mode of construction and on the play of the machine-; we may 

 write 



If a machine is used as a motor by exciting it by a constant 

 source, of electromotive force E , the intensity of the current in 

 a circuit of resistance R is given by equation 



current - which would correspond to the motor at rest, the work 

 R 



The work expended in unit time is \V = E I. If I is the 

 p> 



rent - which woul 

 R 



utilised may be written 

 U = E 

 and the efficiency 



__ = 

 ~~~ 



WE E 

 * MASSON and BREGUET. Ann de Chim. et de Phys. [3], Vol. iv., p. 129. 1842. 



