SCIENCE. 



479 



Fig. 1. 



taken apart with great care. The commutator is formed 

 of a long cylinder made of hard rubber. It is furnished 

 with longitudinal metallic bands destined to connect the 

 surface condensers, and crossed by pieces of copper wire 

 bent at the ends, the object of the latter being to unite 

 the condensers of tension. To this end, metallic wires, 

 fashioned like springs, rest upon the cylinder and are as- 

 sociated with the two armatures of each condenser by 

 very fine copper wires covered with gutta-percha. They 

 are attached to an ebonite plaque r on each side of the 

 cylinder, and the latter can be made to rotate rapidly 

 and continuously by means of a set of wheels. The final 

 springs are separated considerably from those preceding 

 them, in order to prevent the electric sparks from'dis- 



Fig. 2. 



charging between the tension poles of the rheostatic 

 machine and those of the secondary battery. 



The mica plates in the condensers are o" 1 18 in length 

 and o m 14 in breadth. The armatures are made of tin- 

 foil. The edges of the condensers are rendered adher- 

 ent by frames or simple ebonite plaques. These give 

 them more rigidity and cause them more readily to 

 maintain a vertical position, one beside the other, with- 

 out coming in contact. 



When the cylinder is so turned that the longitudinal 

 metallic bands come in juxtaposition with the springs, 

 the armatures in an even range with all the condensers 

 unite on one side, while those in an uneven range are 

 joined on the other, forming a single condenser of large 

 surface. The armatures discharge by causing the right 



hand boundaries, which can be easily distinguished in 

 the figure, to communicate with the poles of the battery. 



When, on the contrary, the cylinder is so turned that 

 its transversal pins are presented to the springs, all the 

 charged condensers are connected in a series or in ten- 

 sion. The armature of the furthest condenser on the 

 left, communicates with the last spring on the other side 

 of the cylinder and ends at branch T of the excitant. 

 The armature of the final condenser on the right com- 

 municates with the spring next to the last, and this 

 spring unites with the last metallic pin traversing the 

 cylinder. The last spring placed in the opposite side of 

 the cylinder communicates with the other branch T' of 



Fig. 3. 



the excitant. While the condensers are thus connected, 

 the pole, or battery, which charges the apparatus, is en- 

 tirely beyond the circuit. 



M. Plants has constructed rheostatic machines of 

 different sizes. The one here represented is supplied 

 with eighty condensers. The commutative cylinder is 

 is one meter long and o"'.i5 in diameter. 



When this cylinder is put in motion and the machine 

 connected with the battery of 800 secondary couples, we 

 perceive, as the charge begins to act upon the commu- 

 tator, long lines of sparks at those points where the me- 

 tallic contact is effected. It becomes a tube of sparkling 

 light and the effect is equally apparent when the discharge 

 in tension occurs. At the same time we obtain a long 

 spark at the excitant T T'. 



