CHAP. VII., 5.] 



ELECTRICITY. DR FARADAY. 



181 



(815.) 

 CTniform 

 nature of 

 jlectricity 

 from what- 

 jversource 



(816.) 

 Induction 

 of electric 

 currents 

 magneto- 

 electricity. 



poles of the battery and became enfeebled with dis- 

 tance from them, they used a language not quite 

 rigorous indeed, yet expressing the actual pheno- 

 mena with that general accuracy which we can alone 

 expect in the first stages of so new and difficult an 

 inquiry. " The sum of chemical decomposition is 

 constant for any section of a decomposing conductor" 

 is Dr Faraday's expression. 1 So is the sum of illumi- 

 nations arising from light radiating from a point, 

 when taken across any section of its path, yet the 

 influence is said to vary inversely as the square of 

 its distance from the origin. The part of Dr Fara- 

 day's conclusions, however, most open to excep- 

 tion, is what refers to electric action at a distance, 

 which he conceives to depend solely upon induction 

 acting on intervening particles, which induction may 

 take place along curved lines. It is indeed true that 

 he has shown, by a beautiful experiment, that the in- 

 terposition of different substances between an excited 

 electric and a body capable of being electrified by in- 

 duction, occasions different degrees of excitement in 

 the latter, even when the interposed bodies are glass, 

 sulphur, and other " non-conductors ;" and this he 

 justly refers to a peculiar power or property of bodies 

 called "specific inductive capacity." But this is 

 rather different from the general proposition above 

 referred to. 



In conclusion of this part of the subject, I must 

 add that Mr Faraday has, with great pains and suc- 

 cess, demonstrated the fundamentally identical nature 

 of electricity from whatever source derived, and how- 

 ever differing in its usual manifestations ; such as 

 electricity of the pile, of the common machine, or that 

 induced by magnetic, thermal, and animal electricity. 

 These have all common properties, producingthe shock, 

 the spark, and magnetic, chemical, and heating effects; 

 and, except two, also producing sensible attraction 

 and repulsion. But the disproportion of the effects 

 of electricity, varying so much in intensity when re- 

 ferred to unit of quantity, is astonishing and para- 

 doxical. The electricity which so silently and speedily 

 decomposes a single grain of water would, when its 

 intensity is sufficiently exalted, produce, according to 

 Mr Faraday, " a very powerful flash of lightning," 

 or 800,000 times the contents of a well charged Ley- 

 den battery. Again, zinc and platinum wires half an 

 inch long and one-eighteenth inch diameter, dipped 

 into slightly acidulated water, produce in three se- 

 conds as much electricity as a man can easily bear 

 in the form of a shock. 



II. Induction of electric currents from other cur- 

 rents and from magnets. This splendid research, 

 which dates from 1831, constitutes the discovery of 

 magneto-electricity. 



The discovery by Ampere of the attraction and 

 repulsion of conductors conveying electric currents 

 rapidly followed (as we have seen in Art. 796) Oer- 



sted's discovery of the power of electricity to affect 

 the magnet, and the corollary from it of the mag- 

 netizing agency of electricity. This being achieved, 

 very striking analogies led to the expectations 1. 

 That a wire conveying a current ought to excite by 

 induction a current in another wire near it ; and, 2. 

 That a magnet ought, under some circumstances at 

 least, to be capable of exciting electric action. But 

 attempts in these directions had repeatedly and sig- 

 nally failed, and for a reason which Mr Faraday first 

 rendered apparent. 



Having made a compound helix of two copper 



wires wound parallel to one another, but not touch- 7? lt; . a "f ec ~ 

 j 11 j -.LI.' J.-L. j.i i- trie induc- 



ing, and rolled one within the other upon a cylin- tion. 



der, he found that when he transmitted a continu- 

 ous voltaic current through one wire, a momen- 

 tary current (tested by a galvanometer) took place 

 in the independent helix opposed in direction to that 

 of the primary current ; but it ceased to exist in- 

 stantaneously, although the primary current conti- 

 nued to act ; and it was only on the cessation of 

 that current that a new momentary induced current 

 appeared, but in the contrary direction to the previ- 

 ous one. The same effect occurred when a wire 

 conducting a current was mechanically brought into 

 the presence of another wire ; the approximation of 

 the two induced an oppositely directed current, their 

 separation a similar one. Whilst the wires were 

 immovable no induced current took place. This 

 he termed Volta- Electric Induction. 



Mr Faraday next took a ring of soft iron, disposing 

 two copper- wire coils round opposite portions of the 

 ring. In passing a current through one coil, and 

 thus magnetizing the ring, a current was induced in 

 the other copper coil, but, as in the former case, only 

 for an instant. When the primary current stopped, 

 and the magnet was unmade, an opposite current 

 shot through the secondary coil. 



The transition to the next experiment was natu- 

 ral, but highly important. The primary coil wasM a g net - 

 suppressed ; and the piece of soft iron embraced by electricit y- 

 the secondary coil was now magnetized by the in- 

 ductive action of a powerful bar magnet, with 

 which contact was alternately made and broken. 

 At the instant of making contact a momentary 

 current of electricity was produced in the remain- 

 ing coil, and on breaking it a reversed current, 

 also of instantaneous duration. No current ex- 

 isted whilst the magnet continued to be applied. 

 The direction of the current at making contact 

 was opposite to that which would have produced 

 the magnetism present in the iron core ; on break- 

 ing contact the current was similar to that which 

 would have magnetized the iron. The electricity 

 momentarily induced in the coil was tested by its 

 action on the galvanometer, by its power to mag- 

 netize steel, to convulse a frog, and finally by the 



(818.) 



(819.) 



1 Researches, Art. 504. 



