VOLTAIC ARC LAMPS. 157 



armature, but on the extremity P of a piece jointed to a fixed 

 point x, and having its edge shaped to a particular curve, 

 and in rolling pressing on a projection, which thus represents 

 a lever of variable length, as in Robert Houdirts electric dis- 

 tributor. The armature must then always remain thus fluc- 

 tuating between the two limiting positions, for at each moment 

 the antagonistic force opposed by the spring to the attraction 

 of the electro-magnet is compensated by the effect of the 

 lever thus arranged. Or in other words, the position of the 

 armature every instant expresses the intensity of the current. 

 Whilst the intensity preserves its desired value, which is co- 

 relative to the distance maintained between the carbons, the 

 armature is balanced in such a manner as to prevent any 

 movement of approach or recession ; the moment the current 

 becomes too strong or too weak, there is a recession or 

 approach, for the lever T attached to the branch of the arma- 

 ture lever F produces these effects by the oscillation of an 

 escapement anchor t fixed at the end of the lever T, which 

 engages and disengages a double clockwork mechanism, the 

 action of which we shall now study. In this, the fly vanes o 

 and o' act as detents. 



This mechanism represented on a larger scale, in Fig. 39, 

 is set in motion by two spring barrels L L', each of which 

 controls its own trainwork of wheels, the last piece bearing the 

 fly vane mentioned above. The system controlled by the 

 barrel L tends to separate, and the other to bring them 

 together; but in order that these mechanisms working in oppo- 

 site directions may act on the movers of the carbons, a special 

 mechanical contrivance was necessary, and Foucault had re- 

 course to an arrangement invented by Huyghens, consisting 

 of two sun and planet wheels f and e, fitted to a wheel s, 

 which rotates on the axle gh. It is this wheel which governs 

 the motion of the double wheel acting on the racks H and D ; 

 but it can come into action only when the wheel work in con- 

 nection with the flies o o' is liberated by the electro-mag- 

 netic effect and the movement of the detent /. When in con- 



