Masterpieces of Science 



and the wire might give one an advantage over 

 the other, and that thus a residual or differential 

 current might be obtained. He combined wires 

 of different materials, and caused them to act in 

 opposition to each other, but found the combina- 

 tion ineffectual. The more copious flow in the 

 better conductor was exactly counterbalanced 

 by the resistance of the worst. Still, though 

 experiment was thus emphatic, he would clear 

 his mind of all discomfort by operating on the 

 earth itself. He went to the round lake near 

 Kensington Palace, and stretched four hundred 

 and eighty feet of copper wire, north and south, 

 over the lake, causing plates soldered to the wire 

 at its ends to dip into the water. The copper 

 wire was severed at the middle, and the severed 

 ends connected with a galvanometer. No 

 effect whatever was observed. But though 

 quiescent water gave no effect, moving water 

 might. He therefore worked at London Bridge 

 for three days during the ebb and flow of the 

 tide, but without any satisfactory result. Still 

 he urges, "Theoretically it seems a necessary con- 

 sequence, that where water is flowing there elec- 

 tric currents should be formed. If a line be imr 

 agined passing from Dover to Calais through the 

 sea, and returning through the land, beneath the 

 water, to Dover, it traces out a circuit of con- 

 ducting matter one part of which, when the 

 water moves up or down the channel, is cutting 

 the magnetic curves of the earth, while the other 

 is relatively at rest. . . . There is every 

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