380 EARTH MAGNETIC BY ROTATION. [SECT. xxxv. 



plates may be produced by the inductive action of the earth's 

 magnetism alone. If a copper plate be connected with a 

 galvanometer by two copper wires, one from the centre and 

 another from the circumference, in order to collect and 

 convey the electricity, it is found that, when the plate re- 

 volves in a plane passing through the line of the dip, the 

 galvanometer is not affected. But, as soon as the plate is 

 inclined to that plane, electricity begins to be developed by 

 its rotation; it becomes more powerful as the inclination 

 increases, and arrives at a maximum when the plate revolves 

 at right angles to the line of the dip. When the revolution 

 is in the same direction with that of the hands of a watch, the 

 current of electricity flows from its centre to the circumfe- 

 rence ; and, when the rotation is in the opposite direction, the 

 current sets the contrary way. The greatest deviation of the 

 galvanometer amounted to 50 or 60, when the direction of 

 the rotation was accommodated to the oscillations of the needle. 

 Thus a copper plate, revolving in a plane at right angles to 

 the line of the dip, forms a new electrical machine, differing 

 from the common plate-glass machine, by the material of 

 which it is composed being the most perfect conductor, 

 whereas glass is the most perfect non-conductor ; besides, in- 

 sulation, which is essential in the glass machine, is fatal in 

 the copper one. The quantity of electricity evolved by the 

 metal does not appear to be inferior to that developed by 

 the glass, though very different in intensity. 



From the experiments of Dr. Faraday, and also from 

 theory, it is possible that the rotation of the earth may pro- 

 duce electric currents in its own mass. In that case, they 

 would flow superficially in the meridians, and if collectors 

 could be applied at the equator and poles, as in the revolving 

 plate, negative electricity would be collected at the equator, 

 and positive at the poles ; that is to say, there would be a 

 deficiency at the equator and a redundancy at the poles ; 

 but, without something equivalent to conductors to complete 

 the circuit, these currents could not exist. 



