1887. J On Radiant Matter Spectroscopy, 113 



Fig. 1, B. 



band, e f, the disk can be set in rapid rotation. At each revolution a 

 stationary object behind one of the apertures is alternately exposed and 

 hidden twelve times. A commutator, g (shown enlarged at fig. 1, B), 

 forms part of the axis of the disk. The commutator is formed of a 

 hollow cylinder of brass round a solid wooden cylinder. The brass 

 is cut into two halves by a saw cut running diagonally to and fro 

 round it, so as to form on each half of the cylinder twelve deeply cut 

 teeth interlocking, and insulated from these on the opposing half 

 ^cylinder by an air space about 2 mm. across. Only one half h hJi, 

 of the cylinder is used, the other, i i i, being idle : it might have been 

 cut away altogether were it not for some little use that it is in saving 

 Ohe rubbing-spring, j, from too great friction when passing rapidly 

 over the serrated edge. To a block beneath the commutator are 

 attached two springs, one, k, rubbing permanently against the con- 

 tinuous base of the serrated hemicylinder, h h, and the other, j, rub- 

 bing over the points of the teeth of h h. By connecting these springs 

 with the wires from a battery it will be seen that rotation of the com- 

 mutator produces alternate makes and breaks in the current. The 

 spring, j, rubbing against the teeth is made with a little adjustment 

 sideways, so that it can be said to touch the points of the teeth only, 



i 2 



