ELECTRICAL APPARATUS. 163 



In the case of magneto-electric machines, changes of construc- 

 tion, which are of a more fundamental nature, have been made 

 within the last few years. The first magneto-electric machines 

 were made in 1832 (by Pixii and by Dal Negro) almost immedi- 

 ately after the announcement by Faraday of his discovery of 

 magneto-electric induction, that is to say, of the production of an 

 electric current in a conductor forming a closed circuit, when an 

 alteration takes place in the total magnetic force acting through the 

 area bounded by the circuit. It is evident that such an alteration 

 can be produced in either of two ways, namely (i), by a change in 

 the magnetic force itself, or (2), by a change in the shape or position 

 of the circuit. The first is the principle applied in the induction 

 coil, and in some magneto-electric machines, including that of 

 Pixii ; the second is that on which the action of most magneto - 

 electric machines chiefly depends, though in many of them the 

 two principles are applied conjointly. The first step in the 

 modern improvements of these machines was the introduction by 

 Siemens and Halske (1857) of an arrangement of the " armature " 

 (a piece of soft iron wound with insulated copper wire, by the 

 movement of which between the poles of a magnet the variations 

 of magnetic force acting across the circuit are produced) whereby 

 nearly the whole force of a large number of steel magnets could be 

 utilised. In 1866 Wilde showed that enormously increased effects 

 could be obtained by causing a Siemens's armature to revolve 

 between the poles of a large electro-magnet, the magnetism of 

 which was developed by the current of a smaller machine pro- 

 vided with permanent steel magnets. In the same year, S. A. 

 Varley, and in the following year, Siemens and Wheatstone 

 almost simultaneously constructed machines in which permanent 

 magnets were entirely dispensed with, and the current caused by 

 a very feebly magnetized electro-magnet was made to strengthen 

 the magnetisation of the magnet which produced it, and so in 

 turn to cause a stronger current whereby still greater magneti- 

 sation was developed, and so on, until the resistance of the con- 



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