178 



1 MH'LATOKY 1 " I:OKS. ELECTRICITY. 



[IHDOCTIOH. 



till further illustrate thb position by referring to the 

 .r.lmary process of addition in algebra, whereby num- 

 ber* of different signs, on being added together, give a 

 total value midway between thi-ir iii.livi.lual amount. 

 Thus the addition, algebraically, of + a to a, wouM 

 afford nothing in amount. In the name way, electrio 

 o.|mli!>num wan similarly considered as existing in bodies 

 at lero, whilst iU disturbance was simply an addition to 

 the electrical quantity of bodies in a positive electrical 

 state, and an abstraction from those which assumed the 

 negative condition. 



This has bwn termed the single theory ; and so exten- 

 sive was its employment at one time, that all its terms 

 are still to be found in electrical works and lectures. In- 

 deed, it u often a matter of considerable amusement to 

 a strictly philosophical person, to hear those who 

 strenuously adopt the double theory, using constantly 

 such terms as "current," "passage," and the like, 

 which can only properly be used under the supposition 

 that a single electric force exists. 



We shall find, however, that although the single 

 theory has the advantage of great simplicity, and ready 

 adaptation to nearly all the circumstances of electric 

 disturbance, it yet fails to explain some of the most 

 important phenomena, especially such as are involved in 

 electrical induction. 



The double theory supposes the existence of two 

 electricities, exactly opposed to each other in every 

 respect, just as we observe in the opposite poles of a 

 magnet When electricity is excited, this theory sup- 

 poses that the two electricities are parted ; that they 

 have a mutual tendency to recombme ; and that all 

 electric phenomena result from one or other of these ten- 

 dencies, and, generally speaking, from the combined 

 action of the two. 



In the double theory, the terms "positive" and "nega- 

 tive" are used iu a physical sense only. They express no 

 idea of quantity, but rather of position. The opposite 

 electricities, indeed, are supposed to be exactly equal in 

 quantity and intensity ; and this is one chief cause of 

 tln-ir constant tendency towards recombination, and, 

 consequently, to reproduce the natural or normal state 

 of all bodies in an unexcited state, electrically speaking. 

 The distinction has been carried even still further ; and 

 the terms "vitreous," or "glass," and "resinous," or 

 "rosin-produced" electricities, are employed, for reasons 

 which will be best explained by our readers trying the 

 following experiment. 



Experiment 22. Suspend a feather, by means of a 

 silk thread, so that it may hang freely, and at a distance 

 from any other substance. Let an excited glass tube be 

 brought near it, when the feather will be immediately 

 attracted, and afterwards repelled. Remove the glass 

 tube, and then present a roll of sealing-wax, which has 

 been rubbed with a dry flannel, and the feather will 

 immediately fly towards the wax. 



This would be explained, under the single theory, by 

 saying that the glass had positively electrified the fea- 

 ther, and that the wax, being in negative or deficient 

 electrical state, had attracted, and so the excess of elec- 

 tricity of the feather had combined with the deficiency 

 of the wax, and a electrical equilibrium had thereby 

 been restored. 



Under the douhie theory, we should say, that the 

 <u kind of electricity of the excited feather com- 

 bined with the retmtnu kind of the wax, and hence the 

 result. Our readers will thus perceive, that the most 

 simple way of placing before them the antagonism of 

 the two theories, is that of assigning quantity as the 

 leading feature of the single, and kind, or quality, as 

 that of the double theory. That similarly electrified 

 bodies repel each other, is, of course, admitted by the 

 advocates of each theory as a fact ; but the cause each 

 assigns is in accordance with that adopted to explain 

 the results of our last experiment. 



We do not intend to take our readers through the 

 almost interminable arguments which have been used on 

 both sides. It has been our good fortune to have read 

 most of the works, and to have heard the public lectures, 



of the leading electricians of the day ; and the only 

 conclusion at which wo have arrived, is, that whilst the 

 subject in dispute has strong and able arguments on 

 either lido, those who take part in it are almost con- 

 st-nitty led to compromise their views as some occasional 

 or accidental circumstance arises. As we before ob- 

 served, the employment of the terms of each theory, 

 indifferently at least, indicates extreme uncertainty as 

 to the real state of the case. 



However, the double theory is the most usually re. 

 cognised at the present day ; and we shall use it in 

 explaining many of our future experiments. Tim 

 strongest objection which suggests itself to our mind 

 is, that it has no analogy to the theories of the other 

 undulatory forces. It is true that magnetic phenomena 

 countenance the idea ; but we must bear in mind, that 

 the connection of magnetic and electric affections is so 

 intimate, as to leave us scarcely room to doubt whether 

 any diiferonce really exists. If, however, we compare 

 the double theory of electricity with our theories of 

 heat and light, the analogy at once fails ; and no one 

 has yet been bold enough to suggest the idea that there 

 are two kinds of heat or two of light The investiga- 

 tions which have lately been made into the correlation of 

 forces, certainly tend to induce the adoption of the 

 single theory. However, this is one of a class of sub- 

 jects which can only be properly understood by a much 

 more extended knowledge of force, motion, and matt, r, 

 than we at present possess. We shall, therefore, re- 

 frain from urging any further views on the subject upon 

 our readers ; and having endeavoured to state the case 

 fairly, shall make free use of each theory, in our 

 future pages, as convenience of expression or illustra- 

 tion may render desirable. Wo shall thus stand free 

 from the charge of inconsistency or dogmatism. 



It will now be proper that wo should examine the 

 doctrine of electrical induction, and use the Leyden jar 

 and battery for the purpose of illustrating it 



INDUCTION THE LEYDEN JAR AND 

 BATTERY. 



IN attempting to convey an adequate idea of the phe- 

 nomena included in electrical induction, considerable 

 difficulty arises, because there is no analogy to which we 

 can refer, except that of magnetic phenomena, which 

 can assist us only to a limited extent We shall, there- 

 fore, rely on a careful and attentive study on the part 

 of our readers, if we endeavour to present to them, 

 as far as possible, a logical exposition of the doctrines 

 or laws which modern science has determined to be the 

 basis of this department of natural philosophy. The 

 term ' ' induction" has been chosen by electricians as ex- 

 pressing a similar idea, in electric science, to that which 

 the same word would convey in the conversation of 

 daily life. 



We endeavour to induce a person to fall into our 

 ideas by presenting him such facts or arguments as will 

 be sufficient to bring him into our mode or course of 

 thoughts. Similarly, when we wish to induce a certain 

 electrio state in any body, we present such an electrified 

 condition as will produce the effect we desire. There is, 

 however, this difference we produce an exactly opposite 

 state to that we present ; because, if we electrify any 

 body positively, we intend to produce on its surface a 

 negative electrical state, and rice tJcrnJ. Hence, we 

 depend on the fact, that action and reaction are not 

 only equal, but that they, in certain cases, are natu- 

 rally producible. Our readers will more fully under- 

 stand our statements when they reduce them to a prac- 

 tical form by actual experiment 



Tim most eligible instrument we can choose for the 

 purpose of illustrating our subject, is that of the Leyden 

 jar and battery. The early ex]M>rimenters in electrical 

 science, were not acquainted with the fact, that a greatly 

 increased effect might be produced by means of an 

 arrangement such as that to which wo allude. It is 

 stated, that the discovery was accidentally made by M. 

 Muschenbrooek, who, having allowed a chain from the 



