GALVANISM. 



Fig. 29. 



Fig. 30. 



a magnet. In fact, the mutual action between a 

 current and a magnet is of such a nature, that if 

 either be fixed and the other movable, there will 

 be a constant rotation of the one round the other ; 

 and a change in the direction of the current, or in 

 the pole of the magnet, will reverse the direction 

 of rotation. 



Amperes theory of magnetism furnishes the 

 key to these actions, and forms the link between 

 magnetism and current electricity. According to 

 Ampere, magnetism is due to currents circulating 

 round the particles of a magnet, and all in the 

 same direction. The want of magnetism in a steel 

 bar is simply owing to the currents flowing in all 

 different directions, and so neutralising each other. 



Fig. 29 shews a sec- 



.^^^ tion of a magnetic 



bar with its currents 

 all flowing one way. 

 These are manifestly 

 equivalent to one 

 strong current flow- 

 ing round the bar, as 

 in fig. 30, for the 

 internal currents destroy each other. Magnetisa- 

 tion is perfect when the currents are exactly 

 parallel, and coercitive force is the resistance to 

 tliis parallel disposal of the currents. Soft iron is 

 more perfectly magnetic when under induction 

 than steel, because iron has less coercitive force. 



If this theory be correct, then we should find 

 that circular currents should have magnetical 

 properties. Experiment confirms the inference. 

 When a current passes through a helix or spiral 

 of copper wire it becomes exactly like a magnet. 

 Each end has an opposite polarity ; if suspended 

 delicately, it will point north and south like a 

 magnet ; and spiral currents act on each other in 

 every way as magnets do. 



The nature of the poles is determined by the 

 direction in which the current flows round the 

 axis of the spiral. If, on entering the spiral, the 

 current flow in a direction opposite to the hands 

 of a watch, the spiral is said to be left-handed, and 

 the N. pole will be as indicated in fig. 31. If it 



Fig- Si- 



go with the hands of a watch, it is right-handed, 

 like a common screw, and the poles are as in fig. 

 32. This is in accordance with Ampere's rule ; 



Fig. 32. 



for, if we imagine one swimming with the current 

 and his face to the axis of the spiral, the N. pole 

 is always on his left. In general, then, if we look 

 along the axis at the pole of a magnet, it will be a 

 south or a north pole according as its currents are 

 seen to flow in the same or opposite direction to 

 that of the hands of a watch. 



Electro-magnets. Perhaps the strongest proof 

 of Ampere's theory is that a piece of soft iron, 

 placed within a spiral current, becomes for the 



time powerfully magnetic. If bars of steel be 

 placed inside, they are permanently magnetised ; 

 and a spiral current is thus a very ready and 

 powerful means of magnetisation. Where power- 

 ful magnets are required, this is by far the best 

 way of making them, and there is nothing easier : 

 a few turns of insulated copper wire conveying a 

 current will at once convert a steel bar into a 

 magnet by passing it over the bar. 



Magnets produced by the action of the current 

 on soft iron are termed electro-magnets. They 

 far outrival permanent magnets in strength ; and 

 their value lies in this also, that we can change 

 their poles, or make their magnetism come and go 

 in a moment, as if by magic. If the iron core be 

 very soft, its power is gone the instant we stop the 

 current. 



Electro-magnets are generally made of the 

 horse-shoe shape, as in 

 fig. 33, and it matters 

 not whether the coils 

 be wound all over the 

 magnet, or accumulated 

 at the two ends. The 

 power depends only on 

 the number of turns of 

 the coil, an d the strength 

 of the current. But 

 there is, of course, a 

 limit to the number of 

 turns ; for the advant- 

 age of increasing them 

 may be counteracted 

 by the weakening effect 

 of the long wire on Fig. 33. 



the current. The wire 



which is wound on the core should be thick, so 

 as to be of small resistance, and covered either 

 with silk or cotton. For ordinary sizes, the latter 

 is sufficient, as the electricity most suited to 

 develop magnetism is of small tension and easily 

 insulated. Usually, it is wound, not on the core 

 directly, but on two pasteboard cylinders or bob- 

 bins, which can be taken off the magnet at 

 pleasure. With a very thick core, a sufficient 

 number of turns, and a strong battery, we may 

 have any power of electro-magnet Some have 

 been made capable of sustaining several tons 

 weight By far the largest that has ever been con- 

 structed is that just made for Lord Lindsay. It is 

 said to weigh over 6 tons, and to have 14 miles of 

 copper wire, J inch thick, wound on its core. An 

 equally colossal battery is now under construction 

 for it. The battery will consist of 150 Grove's 

 elements, each exposing a square yard of platinum 

 surface. This gigantic size has never before been 

 approached. 



The application of electricity to the moving and 

 regulating of clockwork will be found described 

 in the number on HOROLOGY. 



ELECTRIC TELEGRAPH. 



By far the most wonderful and important appli- 

 cation of voltaic electricity is the electric telegraph. 

 The idea of transmitting a signal to a distance, 

 by electricity conveyed along an insulated wire, 

 had suggested itself long before it was practically 

 carried into effect An attempt was made to 

 employ frictional electricity for this purpose, and 

 the discharge of a Leyden jar was even effected 



