1879.J on the Eledrir L'ujhf. ID 



current is inversely proportional to the resistance. A clear image of 

 the process is derived from the de2)ortmeut of water. When a river 

 meets an island it divides, passing riglit and left of the obstacle and 

 afterwards reuniting. If the two branch beds be equal in depth, 

 width, and inclination, the water will divide itself equally between 

 them. If they be unequal, the larger quantity of water will flow 

 through the more open course. Detaching one of these branch wires, 

 I send the whole current from our battery through the other, in which 

 a spiral of platinum wire is introduced. The spiral glows brightly. 

 I now connect the second branch, which also contains its spiral. The 

 current divides, and the consequence is that the first spiral falls while 

 the second rises in illumination. Augmenting the resistance of either 

 branch, an additional portion of the current is thrown upon the other, 

 increasing the light of its spiral. Introducing, instead of either spiral, 

 a piece of thick copper wire, nearly the whole of the current passes 

 through it, the glow of the spiral in the other branch falling to 

 darkness. And as in the case of the water we may have an indefinite 

 number of islands producing an indefinite subdivision of the trunk 

 stream, so in the case of electricity we may have instead of two 

 branches any number of branches, the current dividing itself among 

 them in accordance with the law which fixes the relation of current 

 to resistance. 



Let us apply this knowledge. Suppose an insulated copper rod, 

 which we may call an " electric main," to be laid down along one of 

 our streets, say along the Strand. Let this rod be connected with one 

 end of a powerful Voltaic battery, a good metallic connection being 

 established between the other end of the battery and the gas-pipes 

 under the street. As long as the electric main continues unconnected 

 with the gas-jHpes the circuit is incomplete and no current will flow ; 

 but if any part of the main, however distant from the battery, be 

 connected with the adjacent gas-pipes, the circuit will be completed 

 and the current will flow. Supposing our battery to be at Charing 

 Cross, and oui- rod of copper to be tapped opj)osite Somerset House, 

 a branch wire can be carried from the rod into the building, the 

 current passing through which may be subdivided into any number 

 of subordinate branches which reunite afterwards and return through 

 the gas-pipes to the battery. The branch currents may be employed 

 to raise to vivid incandescence a refractory metal like iridium or one 

 of its alloys. Instead of being tapjied at one point, our main may be 

 tapped at one hundred j)oints. The current will divide in strict 

 accordance with law, its power to produce light being solely limited 

 by its strength. The process of division closely resembles the cir- 

 culation of the blood ; the electric main carrying the outgoing current 

 representing a great artery, the gas-pipes carrying the return current 

 representing a great vein, while the intermediate branches represent the 

 various vessels by which the blood is distributed through the system. 

 To fix the matter in your minds, I will illustrate the arrangement 

 on a small scale. Before you is a battery with a thick copper wire 



c 2 



