60 ELECTRIC LIGHTING. 



chemical applications, for which the effect is proportional to the 

 first power of the intensity, it is advantageous for the economy 

 of motive force not to drive the machine at high speed." 



If the increase of speed in magneto-electric machines 

 augments the intensity of the resulting induced currents, on 

 the other hand the increase in the intensity of the currents 

 opposes the rotatory movement. The more frequent sepa- 

 rations of parts attracting each other must give the motive 

 power a greater sum of mechanical resistances to overcome ; 

 but there is also in the very circumstance of the development 

 of electrical work effected a mechanical reaction, which is 

 the consequence of the transformation of the physical forces. 



Everybody knows the beautiful experiment of Foucault's, 

 which consists in making a copper disc revolve between the 

 poles of a powerful electro-magnet. While the magnet is 

 not in action the disc may be turned with any required 

 velocity, but as soon as the electro-magnet comes into action 

 the disc becomes more and more heated, the resistance 

 offered to its rotation increases considerably, and soon 

 becomes so great that the speed of the machine cannot be 

 further increased. The calorific effect produced has there- 

 fore necessitated an expenditure of force by giving rise to an 

 increase of resistance, and the measure of that additional 

 mechanical resistance must be the equivalent of the physical 

 action which has caused it. Now an action of this kind 

 must evidenly be produced in magneto-electric machines in 

 rapid rotation, and especially in dynamo-electric machines. 



There is yet an interesting question to study, namely, to 

 ascertain the duration of induced currents, the period which 

 elapses between the closing or opening of an inducing cir- 

 cuit and the appearance of the induced current, and how 

 the intensity of the current behaves at the different phases 

 of its appearance. Blaserna and Mouton have made some 

 very interesting researches on this subject, of which we shall 

 indicate the principal results. 



According to Blaserna the time which elapses between the 



