1884.] on Volta-Electric and Magneto-Electric Induction. 125 



magneto-electric machine. For this purpose he arranged an apparatus 

 similar to the one I have here, which is simply a permanent magnet, 

 so fixed that discs of metal or other substances can be rotated 

 between its poles. Two wires leading one from each terminal of a 

 galvanometer were applied to any desired part of the revolving disc, 

 and the deflections on the galvanometer noted. The first experiments 

 were made with a very large compound permanent magnet, and with 

 what would now be termed a quantity astatic galvanometer. His 

 discs of metal were twelve inches in diameter, and about one-fifth of 

 an inch in thickness, fixed upon a brass axis. He experienced diffi- 

 culty in making contact between the terminals of the galvanometer 

 and the edge and other parts of the revolving disc, as also in main- 

 taining uniform velocity of rotation. With a much smaller magnet 

 and a more sensitive galvanometer, the results were more striking. 

 Thus, with his accustomed simplicity, he demonstrated the production 

 of a permanent current of electricity from an ordinary magnet, at the 

 same time asserting that with powerful magnets and rapid rotation of 

 a copper disc, very strong currents would be produced. What a 

 wonderful lesson this apparently simple machine teaches, for it is 

 able to exert the power, which has its origin within itself, on external 

 matter, without in any way exhausting or diminishing that power ! 



The apparatus that I employed for my experiments is the same 

 that I shall use this evening. On this stand is fixed an electro- 

 magnet, the poles of which are so placed that the rim of the disc 

 under experiment can be freely revolved between them. The cores 

 of the electro-magnet are 1*75 inch in diameter, and are wound 

 with twelve layers of silk-covered copper wire of high conductivity, 

 • 028 of an inch in diameter, each layer having sixty-one turns ; each 

 core has 732 turns, making a total of 1464 turns on the two poles. 

 The total resistance of the wire is 13 • 75 Ohms, and through this flows 

 the current from twelve Leclanche cells. The same galvanometer is 

 used as in the other experiments, being brought in circuit with the 

 disc by means of the axis on which the disc revolves, and a metal 

 brush which forms a rubbing contact on the rim of the disc ; this, 

 working on a fixed centre, can be readily shifted to any part of the 

 circumference of the revolving disc. The disc is revolved by this 

 cone-shaped pulley, worked lathe fashion, and connected by an endless 

 band to a small grooved pulley fixed on the same axis as the metal 

 disc. We will now make a few experiments with the copper disc, 

 which is now revolving at the speed of 1000 revolutions per minute, 

 and the connections are so made that the current will be taken from 

 the rim of the disc just as it passes between the poles of the magnet. 

 On pressing this spring the circuit is completed, and you observe the 

 effect of the current on the scale of the galvanometer ; it is about 

 100 divisions. We will now remove the contact to about one-fourth 

 of the diameter of the disc or top position, so that the current will 

 be taken at that distance from the poles of the magnet as the disc is 

 approaching the poles ; now, on completing the circuit, you observe 



