278 



ELECTRIC LIGHT. 



arc placed in the current, and the machine depends 

 for the power of starting upon the small residual 

 magnetism which remains permanently in them. 



This machine produces a current in only 

 one direction, like the voltaic battery. Sub- 

 sequently M. Gramme so modified it as to 

 produce alternating currents, and this is the 

 form now used to supply the electricity for 

 the Jablochkoff light. 



The Brush machine (Fig. 17) presents two marked 

 differences from other machines, the first of which 

 consists in the peculiar method adopted for wind- 

 ing the armature. The latter is composed of a ring 



or endless band of iron, but, instead of having a 

 uniform cross-section, like that of the Gramme ma- 

 chines, is provided with grooves or depressions 

 whose direction is at right angles to its magnetic 

 axis or length. These grooves, which may be of 

 any suitable number, according to the uses for 

 which the machine is designed, are wound full of 

 insulated copper wire. The advantage of winding 

 the wire in grooves or depressions in the armature 

 is twofold : First, the projecting portions of the ar- 

 mature between the sections of wire may be made 

 to revolve very close to the poles of the magnets 

 from which the magnetic force is derived. By this 

 means the inductive force of the magnets is utilized 

 to a much greater extent than is possible in the case 

 of annular armatures as ordinarily used, which are 



FIG. 17. 



entirely covered with wire, and can not therefore be 

 brought very near the magnets. Second, owing to 

 the exposure of a very considerable portion of the 

 armature to the atmosphere, the heat, which is al- 

 ways developed by the rapidly succeeding magneti- 

 zations and demagnetizations of armatures in motion, 

 is rapidly dissipated by radiation and convection. 

 In the case of armatures entirely covered with wire, 

 the escape of the heat is very slow, so that they 

 must run at a comparatively low rate of speed, with 

 corresponding effect, in order to prevent injurious 

 heating. The second difference lies in the manner 

 of connecting the armature coils to the commutator,* 

 this being such that only the particular coils which 

 contribute to the production of the current are in 

 circuit at once. During the time they are passing 

 through the neutral points in the magnetic field, 

 Lhcy are cut out one after the other, and" thus, while 

 idle, do not tend to weaken the effects of the ma- 

 chine by affording a path to divert the current gen- 

 crated in the active sections from its proper channel. 



Whether a dynamo-ehctric machine can sus- 

 tain one light or many depends upon the char- 

 acter of the current which it furnishes, and this 

 in turn is determined by the manner of its con- 

 struction. If the current is required to leap over 

 sny five intervals, and thus produce five lights 

 in succession, it must possess a sufficient electro- 

 motive force. This is imparted to it by multi- 

 plying the thin wires forming the convolutions 



* A mpchanlcal arranjrement for gathering up the currents 

 MM causing them to flow in one direction. 



of the rotating armature. Each additional 

 convolution, like each additional cell in a vol- 

 taic battery, adds its electro-motive force to 

 that of all the others, and, though it also adds 

 its resistance, thereby diminishing the quantity 

 of current contributed by each convolution, the 

 current as a whole becomes endowed with the 

 power of leaping across the successive spaces 

 necessary for the production of a series of lights 

 in its course. The current is as it were rendered 

 at once thinner and more piercing by the simul- 

 taneous addition of internal resistance and elec- 

 tro-motive power. The machines, on the other 

 hand, which produce only a single light, have 

 a small internal resistance associated with a 

 small electro-motive force; in such machines 

 the wire in the rotating armature is compara- 

 tively short and thick, copper ribbon instead of 

 wire being commonly employed. Such ma- 

 chines deliver a large quantity of electricity of 

 low tension. Hence, though competent when 

 their power is converged upon a single interval 

 to produce one splendid light, their currents 

 are unable to force a passage when the num- 

 ber of intervals is increased ; thus by augment- 

 ing the convolutions of the machines we lose 

 quantity and gain electro-motive force, while 

 by lessening the number of the convolutions 

 we lose electro-motive force and gain quantity. 



