DYNAMO-ELECTRIC MACHINES. 83 



various scientists to utilize it for practical purposes. Faraday found 

 that by introducing a magnet quickly into the interior of a 

 coil of wire joined at its both ends to a galvanometer, that 

 an electric current was induced in the coil. When the mag- 

 net was allowed to remain at rest the galvanometer indicated no 

 current. When it was quickly withdrawn, a current was induced in 

 the opposite direction. Motion was theret^re necessary, either in the 

 magnet, or in the coil, to induce a current. The indications of the 

 galvanometer varied with the speed with which the magnet was intro- 

 duced or withdrawn from the coil. This is the governing principle 

 of the action of all generators, by mechanical means. He also tried 

 an experiment with a coil of thick wire connected with a battery. 

 On introducing this quickly into the interior of a larger and finer coil 

 connected as in the other experiment, to a galvanometer, a current 

 was generated and on withdrawing it a reverse current was observed. 

 He concluded from these two experiments that the cause operating 

 to produce the action on the galvanometer was the same. The 

 differences, if any, indicated by the instrument were due to the 

 difference of magnetic intensity in the coil and magnet, owing 

 to the greater magnetic effect of an intense current passing through 

 the introduced coil, than resulted from the introduction of the per- 

 manent magnet. In order to prove the existence of induced currents 

 generally, from magnetic influence, he caused a metallic circle to 

 rotate rapidly before the poles of a magnet, and this apparatus which 

 was one of extreme simplicity, was probably the first Magneto-Elec- 

 tric Machine in existence. It may perhaps be well to explain here 

 what this apparatus was, to show from what a simple beginning 

 the complex machines known as Dynamo Electric-Machines, have 

 sprung. It consisted of a disc of copper revolving between the poles 

 of a horse-shoe magnet fixed horizontally, connected in circuit by one 

 end of the conductor rubbing on the axis, on which the disc 

 rotated, the other rubbing on the periphery, directly above or below 

 the axis. By turning the disc in one direction a positive current was 

 sent through the circuit, and by turning in the opposite direction a 

 reverse current was observed. 



One of the first experimenters who tried to turn Faraday's discov- 

 ery into practical use, was M. Pixii, a maker of mathematical instru- 

 ments in Paris, who in 1832, only two years after the principle of 



