ELECTRO-MAGNETIC MACHINES. 273 



currents. The invention of the magneto-electric machine was by no means 

 a sudden inspiration, but the gradual result of a scries of experiments and 

 discoveries, the first of which, dating from 1820, may be said to be QErsted's 

 observation, that a magnetised needle is deflected by the approach of an elec- 

 tric current as well as by that of a magnet, clearly proving that magnetism 

 and electricity have some relation to one another. In the same year 

 Arago discovered that a coil of insulated wire wound round a core of soft 

 iron, converts it into a powerful magnet (i>., an electro-magnet) when a 

 current passes through the coil. It was in 1830, however, that our country- 

 man Faraday proved the creation of a current by the action of a magnet on 

 a coil of wire, and his experiment proved shortly as follows* : If a coil of 

 wire be wound on a hollow core, and a permanent bar magnet be introduced 

 into the hollow core, whilst introducing it a current may be proved (by a 

 galvanometer), to be induced in the coil flowing in a certain direction, A B, 

 which ceases as soon as the magnet is at rest in the centre of coil. On the 

 withdrawal of the magnet a second current is induced flowing in the oppo- 

 site direction, B A. Therefore it is clear that if a magnet be incessantly 

 approached to and withdrawn from a coil of wire a constant succession of 

 currents will be produced, and if a charged coil (*>., a coil connected with 

 the poles of a voltaic battery) take the place of the magnet a precisely 

 similar result will be obtained. Now it will have been noticed that two 

 opposite currents are constantly being formed, and as the object is to obtain 

 a continuous flow of electricity in one given direction, or, in fact, divert or 

 reverse the current instantly on its formation to make it practically the 

 same current, for this purpose a commutator is used, and as for most pur- 

 purposes a commutator is one of the essentials of a magneto-electric machine, 

 we will here give a description thereof. (See fig. 275.) The machine is 

 composed of a cylinder, consisting of two metallic conducting halves, 

 separated by a non-conducting layer. Whilst it is at rest the alternating 

 currents, from being connected with the halves by the current, will pass to 

 the two contact springs, and thence through the circuit. Now if (as is the 

 case) the current is constantly changing, as has been noticed, the inverse 

 current will at the first change pass through the same channels, but in 

 another direction ; but if at the instant of the reversal of the current the 

 cylinder be revolved, the current flowing the reverse way will be guided 

 through other channels respectively, instead of the original channels, and ihe 

 direction of the current being changed at the same moment as the current 

 itself, the two inversions neutralize themselves, and one constant current is 

 produced. In a magneto-electric machine the commutator revolves indenti- 

 cally with the magnet or armature, and the point at which sparks are being 

 constantly produced is where the contact is being continually broken and 

 made by the passage of the friction springs from over the non-conducting 

 layer. The first machine formed on the basis of Faraday's experiments 

 was Pixii's. It was composed of two uprights and a cross bar, to which is 

 attached, hanging poles downwards, an electro-magnet ; underneath this, the 



18 



