THE ELECTRIC LIGHT. 563 



leap being that which produces the light. Whether the current is to 

 be competent to pass through five lamps in succession, or to sustain only 

 a single'lamp, depends entirely upon the will and skill of the maker of 

 the machine. He has, to guide him, definite laws laid down by Ohm half 

 a century ago, by which he must abide. 



Ohm has taught us how to arrange the elements of our battery so 

 as to augment indefinitely its electro-motive force — that force, namely, 

 which urges the current forward and enables it to surmount external 

 obstacles. We have only to link the cells together so that the current 

 generated by each cell shall pass through all the others, and add its 

 electro-motive force to that of all the others. We increase, it is true, 

 at the same time the resistance of the battery, diminishing thereby the 

 quantity of the current from each cell, but we augment the power of 

 the integrated current to overcome external hindrances. The resistance 

 of the battery itself may, indeed, be rendered so great that the external 

 resistance shall vanish in comparison. What is here said regarding the 

 voltaic battery is equally true of magneto-electric machines. If we 

 wish our current to leap over five intervals, and produce five lights in 

 succession, we must invoke a sufficient electro-motive force. This is 

 done through multiplying by the use of thin wires the convolutions of 

 the rotating armature as, a moment ago, we augmented the cells of our 

 voltaic battery. Each additional convolution, like each additional cell, 

 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 integrated current becomes en- 

 dowed with the power of leaping across the successive spaces neces- 

 sary 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 electro-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 of the rotating armature is comparatively 

 short and thick, copper ribbon instead of wire being commonly em- 

 ployed. Such machines deliver a large quantity of electricity of low 

 tension — in other words, of low leaping power. Hence, though com- 

 petent when their power is converged upon a single interval to pro- 

 duce one splendid light, their currents are unable to force a passage 

 when the number of intervals is increased. Thus, by augmenting the 

 convolutions of our machines, we sacrifice quantity and gain electro- 

 motive force ; while, by lessening the number of the convolutions, we 

 sacrifice electro-motive force and gain quantity. Whether we ought to 

 choose the one form of machine or the other depends entirely upon the 

 external work the machine has to perform. If the object be to ob- 

 tain a single light of great splendor, machines of low resistance and 

 large quantity must be employed. If we want to obtain in the same 

 circuit several lights of moderate intensity, machines of high internal 



