674 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK v. 



If the instrument employed is an magneto-electric induction apparatus, 

 in which the current changes its direction at each revolution, each of 

 the carbons is alternately positive and negative ; the wearing away is 

 the same. In every case it is obviously necessary, in order to obtain a 

 continuous source of light, to maintain the points of the two cones 

 at a sensibly constant distance ; and this is attained by means of 

 apparatus called regulators. 



The principle by which the regulators of the electric light work 

 is that the current itself regulates the distance between the carbons ; 

 it is especially charged to bring together the points, and to keep them 

 at a suitable distance. For this purpose it is made to traverse the coils 

 of the bobbin of an electro- magnet, and an arrnature of soft iron 

 comes in contact with its poles when the current has a sufficient 

 intensity that is to say, so long as the extremities of the carbon 

 cones are sufficiently near to give rise to a light of suitable intensity. 



If we once understand the principle of the regulators, the first 

 idea and first realisation of which are due to Le'on Foucault, there 

 will be no difficulty in understanding the mechanism and working 

 of the apparatus in general use. 



First let us speak of Duboscq's regulator, which has been 

 invented for using the continuous current furnished by the battery. 

 This clever and experienced constructor had in view chiefly the 

 scientific applications of the electric light ; and those who have 

 attended the public lectures on physics at the Sorbonne and 

 elsewhere may remember having seen it at work in the experiments 

 or in projecting microscopic objects on the screen. The carbon poles 

 thus supplied the place of the absent rays of the sun. 



Fig. 441 represents this regulator. 



candY are the two carbon points between which the luminous 

 arc leaps. The current which causes the production of the light 

 leaves the positive pole of the battery to enter by the binding screw 

 K, passes through the wire of the bobbin of the electro-magnet BB, the 

 rod T passes on from c to c', and thence, by the rods T' and s, to the 

 screw R', which is in communication with the negative pole to the 

 battery. 



A movable contact K, placed opposite the soft iron nucleus of the 

 electro-magnet, is attracted by the poles of the latter when the 

 current preserves a sufficient intensity, that is to say, when the carbons 



