GALVANISM. 



Extra Currents. Not only does a current in- 

 duce electricity in a neighbouring circuit, but it 

 also acts inductively on itself. If we break a 

 current passing through a long wire, we observe a 

 bright spark at the part broken. This is very 

 feeble if the wire be short, but very brilliant if it 

 be long, and especially if it be wound in a coil. 

 Obviously, it is not owing to the current being 

 strong, for we know it is weaker the longer the 

 path it has to traverse. The cause has been 

 found to be due to extra currents in the wire, pro- 

 duced by the various parts of the wire circuit 

 acting inductively on each other. They are direct 

 when the current stops, and inverse when it begins. 

 The effect of the inverse current, induced in the 

 circuit when it is joined, is to oppose and so 

 retard the instantaneous passage of the main 

 current ; while the direct extra current, which 

 follows the break of circuit, has a very weakening 

 effect on the tension of the secondary current, if it 

 is allowed to pass by spark or otherwise. This is 

 because it has itself a very high tension, and it 

 tends to prolong the current in the primary wire, 

 and so impair the suddenness of the break, on 

 which the tension of the induced current depends. 



The condenser is an important addition to the 

 coil, and serves to get rid of the extra current, 

 which, as we have said, forms in the primary wire. 

 It is placed in the sole S, and does not meet the 

 eye. It is simply a number of sheets of tinfoil 

 separated by layers of oiled silk, the odd ones 

 being all connected in one set, and the even in 

 another. Each set communicates with an end of 

 the primary wire, and acts like a coating of a 

 Leyden jar. The whole, in some way or other not 

 well explained, absorbs the extra current, and 

 suppresses its injurious effects on the secondary 

 current. 



Some enormous induction coils have been made 

 in recent years, the most powerful being one made 

 for Professor Pepper. There are in it more than 

 two miles of primary wire, and no less than 150 

 miles of secondary. The iron wire core alone 

 weighs over a hundredweight, and the whole coil 

 about three-quarters of a ton. It is said that, 

 with a powerful battery, a spark of fully two feet 

 can be got from it. 



The electricity of the induction coil is more of 

 the nature of the frictional than of the galvanic 

 excitement. The sparks which pass between the 

 'points' are very like those of the machine, 

 especially if the points be some distance apart. 

 As with common electricity too, the physiological 

 effects are very marked, and are a familiar object 

 of exhibition. The beautiful luminous effects 

 obtained by passing ordinary electricity through 

 rarefied air or different gases, are shewn yet more 

 brilliantly with the induction coil. 



On the other hand, the induced current has 

 little power to deflect a magnetic needle, or to 

 effect chemical decomposition. All this shews 

 that the electricity of induction may have enormous 

 intensity or tension, and yet no great quantity ; 

 for we know that magnetic and chemical effects 

 are due to electrical quantity only. 



In fact, the secondary current is as deficient in 

 quantity as it is superior in intensity to the 

 primary current. Or, in the language of the day, 

 the total energy of the primary current is exactly 

 equivalent to the total energy of the secondary : 

 the kinetic (or actual current) energy of the former 



is transformed into potential energy- of the latter ; 

 the potential energy of the primary is small, as is 

 the kinetic energy of the secondary. Induction is 

 a signal example of the conversion of kinetic into 

 potential energy. 



MAGNETO-ELECTRICITY. 



We have already seen that the action of the 

 primary in induction coils may be exactly re- 

 placed by that of a magnet. The approach and 

 withdrawal of a permanent magnet induces two 

 powerful opposite currents in a coil. If, then, 

 we had some means of rapidly removing the- in- 

 fluence of the permanent magnet, as we had 

 of the primary coil, by breaking its current, we 

 should have induced electricity from magnetism, 

 and altogether independent of galvanic batteries. 



We shall now describe the usual mode by which 

 such magneto-electricity, or magnet-origin elec- 

 tricity, is obtained. Fig. 41 represents one of the 

 simplest forms of magneto-electric machines. NS 

 is a large fixed steel 

 magnet, and BB is 

 a bar of soft iron, 

 which, with the 

 pieces of soft iron 

 under the coils C 

 and D, forms a sort 

 of bent armature to 

 the magnet. Now, 

 it is evident that, 

 as the armature 

 turns round from 

 being in line with 

 NS, there will be 

 first a gradual de- 

 crease of magnetic 

 power in its cores, 

 during a quarter of 

 a turn, and then 

 a gradual increase 

 during the next 

 quarter, till CD is 

 again in line with 

 NS. But the de- 

 crease of magnetic 

 influence, or the 

 withdrawal of it, induces a current in a coil ; and 

 the increase or approach of a magnet induces an 

 opposite current in a coil Thus, then, as CD 

 turns round from the line of the poles, currents 

 will be induced in the coils, if the two ends of 

 the wire are joined. They will not be produced 

 abruptly, but in a constant stream, as the change 

 of magnetic influence is gradual ; and during the 

 first quarter of a turn they will be all in one 

 direction. During the next quarter there is a 

 gradual increase of magnetism in the cores, and 

 this would produce currents in an opposite direc- 

 tion, were it not that there is also a change of 

 poles. This double reversal, therefore, gives 

 currents induced in the same direction as during 

 the first quarter. So, then, during the first half- 

 revolution from the line of the poles, a con- 

 stant stream of currents is generated all in one 

 direction. In the next half- revolution, the whole 

 state of matters is of course reversed, and we shall 

 have a current flow in the opposite direction. We 

 have merely to introduce a commutating piece, F 

 (which allows the springs, H, K, alternately to 



Fig. 41. 



