VOLTAIC ARC LAMPS. 



I 79 



magnetic coil is at A, above the upper carbon-holder, and is 

 supported by the arm b fixed to a vertical piece c sliding 

 within a column so as to adapt the apparatus to different 

 lengths of carbons. Inside of the coil a magnetic core d 

 moves freely. This core 

 is hollow, and through 

 it passes the copper rod 

 // of the upper carbon- 

 holder, sliding freely for 

 its whole length. A sort 

 of collar h grasps it, 

 however, a little below 

 the magnetic core, and 

 is so arranged that when 

 it presses on a cross 

 piece //, forming .part of 

 the fixed system of the 

 arm b> it leaves the rod 

 // free, which then de- 

 scends by its own weight. 

 The collar therefore 

 maintains the rod only 

 when it is itself raised 

 up, and this takes place 

 almost constantly during 

 the working of the appa- 

 ratus, for a catch e fixed 

 on the magnetic core 

 supports it above; but 

 in its rise it cannot go 

 beyond a certain limit which can be adjusted, for a screw .v 

 has a head large enough to hold it by its upper edge. 



The magnetic core itself is kept up by a cross piece on 

 which act two spiral springs, the rods of which serve at 

 the same time as guides. The carbon-holders have no 

 particular feature ; one is fixed to rod // the other to an 



12 2 



FIG. 46. 



