32 ELECTRIC LIGHTING. 



the number of elements that arc joined by their dissimilar poles. 

 But as this isolation of the action of each element increases 

 the resistance of the battery as a whole, it may happen 

 under certain conditions of the external circuit that more 

 force may be lost by the increase of the total resistance of 

 the battery than is gained by the increase in the number of 

 its elements ; and this is why it is necessary when deter- 

 mining the arrangement of a battery to know exactly the 

 conditions of the external circuit on which the battepy is to 

 act, and more particularly the resistance of that circuit. 



The great principle which must guide us is that the battery 

 should be so arranged that its own resistance shall be equal to 

 that of the external circuit. If the 50 metres of telegraph 

 wire we have given be accepted as the resistance of a 

 Bunsen's cell, and if the resistance of a voltaic arc in the 

 ordinary electric lamps be estimated at 3,000 or 4,000 metres 

 of such wire, it will be seen at once that there is an advantage 

 in ai ranging for tension the elements of a battery us'ed for 

 electric illumination by the voltaic arc, and that at the same 

 time it is an advantage to have cells of large size, not in order 

 to give a greater electro-motive force, but to enable its action 

 to last longer and to increase its calorific power. If the 

 electric light is to be produced by incandescence, the resist- 

 ance of the circuit becomes very much less (about 33 metres 

 for the incandescent part) ; and there is a gain, on the 

 other hand, in arranging the battery for quantity or in series, 

 according to the number of lamps it has to supply and the 

 arrangement of the circuits. It is certain that if in the same 

 circuit a certain number of lamps be included, the battery 

 must have more tension than when the lamps are arranged 

 in distinct or derived circuits, and the mode of arrangement 

 to be given to the battery will still be determined by the total 

 resistance of the external circuit ; which, in that case, will 

 equal the resistance of one of the lamps divided by the number 

 of lamps to be lighted. It will be understood that in this 

 case the electricity supplied by the battery is simultaneously 



