MAGNETISM, ANIMAL. 



MAGNETO-ELECTRICITY. 



418 



placed between UM poles ol the magnet ia repelled toward* either side, 

 Uderea divide* into two streams which pass off huritoutally, MM CO 

 either aide, in the equatorial direction. 



It i* important to observe, that the nature of the medium in which 



(ubsUnce i* placed, has an influence on the magnetic condition of 

 matter ; thu* a rod of glass suspended horixouUlly in water, which is 

 davmagnetic. will point axially, but if the same rod be suspended in a 

 solution of aulphate uf ir. n. which ia magnetic, it will point equa- 

 torially. So alao, a tube filled with a solution of sulphate of iron, will 

 appear to be magnetic if placed in pure water, but if placed in a 

 stronger eolution of aulphate uf iron, it will be dia-magnetic. The 

 came body may alao be magnetic or dia-magnctic, according as it forms 

 the t ~-r or electro-positive constituent of a compound, or the acid 

 or electro-negative constituent. Iron for example, in the crystals of 

 green vitriol act* as a base, and make* the crystals magnetic, but in 

 the yellow pruasiate of potash, where the iron is not a base but an electro- 

 iMgstiT* nonrtitusnt. the crystals are dia-magnetic ; so also bichromate 

 of potash ia dia-majmetio, the chromium acting the part of an acid, 

 while aulphate of chromium is magnetic, for here chromium act* 

 a* a baa*. Tyndall and Knoblauch (' Phil. Mag.' 1860), have shown 

 that UM arrangement of the component particles of a body will 

 determine whether it be magnetic or dia-magnetic. For example, 

 carbonate of iron ia magnetic ; if reduced to fine powder and formed 

 into a cylinder by means of gum-water, it will point axially, but if 

 compressed into a plate, this plate will aet with its face* parallel to 

 the equatorial direction. In fact, the part* brought nearest together 

 by compreaaion, were those which exhibited the dia-magnetic, or the 

 magnetic action moat strongly. In this way the phenomena, dis- 

 covered by Pluck er, were accounted for that in doubly refracting crystals 

 the optic axi* assumes a definite direction under the influence of the 

 electro-magnet ; for it is along this axis that the greatest condensation 

 of the particle* has probably taken place. There are many other re- 

 markable circumstances connected with dia-magnetum, for which wo 

 must refer to Faraday's original memoirs, which have been published 

 in a collected form, and also to Tyndall'* papers, especially those in the 



Philosophical Transaction*' for 1856, 1856, in which it is shown 

 among other result*, that the dia-magnetic repulsion, as measured by 

 the torsion balance, ia as the square of thu intensity of the current. 



MAli.SK.I.-M. ANIMAL. [MMkOUUt.] 



MAGNETO-ELECTRICITY. If a wire which join* the two ends 

 of a voltaic battery [GALVANISM] and consequently through which a 

 voltaic-current i* pausing, be arranged in such a manner as to 

 be parallel with a second wire, the two ends of which are con- 

 nected with a sensitive galvanometer, no action on the second 

 wire will be perceived so long as the current is allowed to pass uninter- 

 ruptedly through the first ; but if the current through tie first wire, 

 called the primary current, be suddenly arrested by breaking off it* 

 connection with the battery, a current of momentary duration will bo 

 produced in the second wire ; this which is called the tecondary current 

 i* said to be direct, or in the same direction as that of the battery wire. 

 On again connecting the first wire with the battery a momentary 

 current or electric wave U again sent through the second wire ; but 

 it is now inverse ur in a direction opposite to that of the primary 

 current* 



The term mlta-(lrr1ric-idtiftim was applied by Faraday to these 

 rhrnftnttm, Their effect* may be augmented by employing coils in 

 such a way that that which convey* the primary current, known as the 

 primary eeil, shall be placed in the axia of the coil in which the secondary 

 current is generated, the end* of this the itcondarg eoU being connected 

 with the end* of UM galvanometer. The moment the primary coil is con- 

 nected with the battery the needle will receive a powerful impulse; but 

 will regain it* original position after a few oscillation*, although the 

 current through the primary coil ia kept up ; but no sooner is the 

 primary coil disconnected from the battery than the needle will be 

 powerfully impelled in an opposite direction. Similar phenomena may 

 be produced if the primary coil, while connected with the battery, be 

 made suddenly to approach to, or be removed from the secondary coil, in 

 connection with the galvanometer. In approaching the coil the second- 

 ary current is inverse ; but in withdrawing it, it ia direct. If instead 

 of the galvanometer, a small helix bet connected with the secondary 

 coil, a steel wire may be magnetised by the induction of these instan- 

 taneous secondary current*. If a Leyden jar be discharged through 

 an insulated primary coil a secondary current will be generated in the 

 other helix. Since then voltaic and friction*! electricity may be made 

 to produce magnetism, it was natural to suppose that magnetism could 

 be mad* to produce electricity. We owe to Faraday the method ol 

 producing this result. An experiment will illustrate the condition* 

 required to produce it. The end* of a copper helix are connected 

 with a galvanometer by wire* several feet in length in order that the 

 instrument may not be directly acted on by the magnet which is to be 

 employed. Now if a permanent magnet be introduced into the axis of 

 the helix, a current in the wire i* instantly produced, aa marked by the 

 deflexion of the galvanometer needle; if the magnet be kept at rest with 

 in the h*Ux. the needle after a few otcUlation* returns to cero ; but the 

 act of withdrawing UM magnet is sufficient to deflect the galvanometer 

 needle ia UM opfraeite direction and to the same extent a* before 

 UM direction of UM currant depends on which pole of the magnet is 

 introduced into UM helix, and aUo upon whether the helix be left- 



landed or right-handed. On placing a bar of soft iron iu tho helix no 

 current is produced ; but if the opposite pole* of two bar-magnets be 

 brought in contact with the ends of the soft iron, a momentary cur- 

 rent is produced. 



The secondary currents thu* obtained by magnetic induction have 

 considerable intensity, and if when the current is pasting the circuit 



M broken a bright spark will be seen at the point of interruption. 



iuhmkorffa coil is an improved form of apparatus for exhibiting these 

 secondary currents. Tho primary or inner coil A A' is a stouter and 

 shorter wire than the secondary coil, B B ; this is a very long thin 

 wire insulated by silk, each layer of coils being carefully insulated 

 from the adjacent ones. In a ten-inch coil the primary wire is 

 132 feet long, and the secondary wire 26,246 feet long. A bundle of 

 soft iron wire, u, i* placed in the axis of the coils, while at + and -are 

 binding screws for connecting tho primary coil with a voltaic battery. 

 The primary coil U broken at c and </, d being a small armature of soft 





iron, faced below with platinum, the upper surface of e being als o 

 faced with platinum. When c and il are in contact the current cir- 

 culate* through A A ; but in doing so the iron core M becomes 

 magnetic, attract* rf, the effect of which is to interrupt the current 

 through A A, and the magnetism in u disappears ; whereupon the 

 hammer d falls upon r, the current through A A is again made to flow, 

 d is again attracted, the current is again interrupted, and in this way 

 the voltaic battery serve* aa a means for making and breaking the con- 

 tact, and so rapid is the action that the hammer rise* and foil* several 

 hundred times in a minute, and a powerful current is induced in 

 the secondary coil B B by each of these momentary currents in A A, the 

 effect of which is a continuous stream of sparks between the insulated 

 ends of the secondary wire e f, and shocks of the most painful and 

 dangerous character are produced. The power of the instrument may 

 be greatly increased by connecting the primary wire with an arrange- 

 ment called the eondentrr, consisting of oiled silk covered with tin-foil, 

 folded between two other surfaces of silk and packed in a flat wooden 

 case ; the two coatings are connected with binding screws attached to 

 c and d in the primary current. Tho effect of this condenser is to 

 increase the intensity of the secondary current, so that with a large 

 coil and careful attention to the insulation of tho conducting wires, 

 powerful sparks ten or eleven inches in length have been obtained. By 

 means of this coil, electricity of low tension may be made to equal in 

 intensity that of on ordinary plate machine, while the quantity is 

 greatly increased. If the secondary coil be allowed to discharge in an 

 exhausted receiver the phenomenon of tho auroral light may be 

 obtained in a very beautiful manner, through an interval of several feet. 

 By arranging the discharge in various ways, aa explained by Mr. 

 Qassiot (Phil. Trans., 1868, 1859) in long tubes partially exhausted or 

 containing gases of different kinds, the luminous portion is traversed 

 by a aerie* of dark band*, which are rendered much more distinct by 

 the presence of a small portion of phosphorus vapour. [ELECTRICAL 

 Euo.] The magnet has a powerful action on these stratified bands and 

 luminous discharges. 



\\ hen the terminal plates of a voltaic battery are connected by means 

 of a long wire coiled into a helix, a brilliant spark is produced on 

 breaking contact, arising from a current passing through the wire 

 in the aome direction as that from the battery. The inductive action of 

 the battery on the wire may be diverted by placing a second helix within 

 the primary coil or exterior to it. By placing an iron core within the 

 heliacal conducting wire the current becomes sufficiently intense on 

 breaking contact to give a powerful shock, although the battery be of 

 very small power. Professor Henry ha* shown (' Philosophical Maga- 

 zine,' 1840) that a succession of induced current* may be formed by 

 the action of coils upon each other. If, for example, the coil in con- 

 nection with the battery be in the form of a flat spiral, and we call 

 thia A, and two other flat coll* running into each other, namely, B 

 and o, be arranged so that B shall rest on A and o on the table, 

 there will be a momentary current in D and o every time the 

 connection of the coil A with the battery is broken. If two other 

 coils D and E, be arranged so that D shall rest on o and it on the table, 

 the induced current in o will produce s current of the third order in 

 D E. If a flat coil r be placed over K with its ends united by a 

 small helix a, a current of the fourth order will be obtained. In this 

 arrangement the secondary current B o will bo direct ; the current of 

 the third order D E inverse ; the current of the fourth order r o 

 direct. The coils must be carefully insulated, and with care current* 

 even up to the seventh order may be obtained, the currents being 

 alternately direct and invent*. Similar currents are generated when- 

 ever the primary circuit ia completed, but in such cose the direction 

 of the current is reversed. These induced current* give shocks, 

 magnetise steal, and effect chemical decomposition*. 



