VOLTAIC ARC LAMPS. 159 



wheel / on the wheel d it compels the wheel S to move a 

 little in the direction opposite to the former. But to effect 

 this it is necessary that the barrel L should have as much 

 power as L'. In any case the motion can only be very small. 

 The wheels a and b must be of one piece, but they must 

 move with gentle friction on the axle gh in order that the 

 wheel S may turn with making them share its motion. If 

 this much has been understood, it only remains to examine 

 the mode of action of the apparatus. 



The arc being established between the two carbons, the 

 attractive action of the electro-magnet is counterbalanced by 

 the antagonistic spring, so that only one of the arms of the 

 escapement anchor engages the fly o'. As the carbons con- 

 sume, the armature F is the less attracted the more the arc is 

 lengthened, but no sudden movement occurs ; the armature 

 drawn away by the spring rolls on the jointed curve x, Fig. 

 38, and at last the hammer liberates the fly o', and engages 

 the fly o ; the carbons are then approximated until the in- 

 tensity of the current is sufficient to re-establish the power of 

 the electro-magnet. If the carbons are too near, the arma- 

 ture F is attracted the more, and the escapement anchor will 

 set free the fly o, which will cause a separation of the 

 carbons. 



Let us add, that the carbons are capable of being moved 

 in two ways by hand in order to fix the position of the 

 luminous point in the first instance. Thus, the upper carbon 

 may be moved independently of the lower one, the whole 

 system may be raised or lowered, the separation of the car- 

 bons continuing unchanged; and this is required for pro- 

 perly centring the light in projections. 



An important improvement has lately been made in this 

 apparatus. It had been found that changes in the intensity 

 of the current modified the condition of the magnetic core, 

 and that the magnetic power persisted more or less. The 

 antagonistic spring regulated by a screw, seen on the right in 

 Fig. 38, was therefore unable for a given electric intensity to 



