POLAR CLOCK. 



POLARITY. 



eoo 



Poison, or Chambre Ardeute. The powara of thui court, however, 

 were abused, and Toy shortly after iu institution it was suppressed. 



Wlit-rever evidence has ben left sufficiently positive to admit of an 

 inquiry into the nature of these secret poisons, it has bean found that 

 they contain ingredients well known at the present day. It is only 

 where the sciences of chemistry and medicine are not sufficiently 

 known, or where they are neglected in the inquiry, that secret poisoning 

 can take place. The test* are now ao accurate, and the investigations 

 so strict, that it is no longer possible to administer any poison, vege- 

 table or mineral, of which traces cannot be discovered in the body of 

 the person poisoned, though of course it may not always lead to the 

 discovery of the person by whom it was administered. 



(Beokmann, //atari/ of /nrentinia; Beck, Medical Jwuprudaux ; 

 Adams, Edinburgh Medical and Surgical Journal, voL xxxiii.) 



I 1 - 'I.Alt CLOCK. A portion of the light which passes to the earth 

 under a sky which is clear, or nearly so, is more or less polarised. The 

 maximum effect occurs in a piano passing through the earth's axis at 

 90* from the sun : hence the amount of polarisation varies in different 

 parts of the sky with the position of the sun. On this relation 

 between the hour-angle and the plane of polarisation of light from the 

 sky. Professor Wheatstone has based his polar clock. It consists of a 

 tube, with the axis parallel to that of the earth passing with gentle 

 friction through an. hour-dial. Within the tube is a polarising appa- 

 ratus, which indicates the position of the plane of polarisation by the 

 vanishing of the coloured rays; and as the plane of polarisation is 

 always 90* from the sun, the index hand attached to the tube, and 

 properly adjusted, will always point to the hour on the disappearance 

 of the colours. The dial is connected with an upright, which moves 

 on a hinge joint, so that by means of a quadrant and a clamping screw 

 it can be adjusted to the latitude of the place of observation. 



I'til.AK LIGHTS. This subject, that of the Aurora Barealit and 

 A tatmlit, will be treated of under the appellation in which they were 

 included by Humboldt, TERRESTRIAL LIUIIT. 



POLARITY signifies, in general, a disposition in a body or in 

 an elementary molecule of a body to place its mathematical axis 

 in some particular direction ; frequently also it denotes in a body the 

 existence, either naturally or induced, of two points possessing con- 

 trary properties. 



If iron-tilings be strewed over a mass of natural -loadstone, it will 

 be found that there are two points on its surface at which the tilings 

 are most abundantly attracted, and where they dispose themselves 

 nearly in the direction of a line imagined to be drawn through the 

 mass. [MAUNKTISJI.] Then, if the loadstone be cut in the form of a 

 sphere, having this line for a diameter, the symmetrical arrangement 

 of tin- iron tilings with respect to this line affords an indication 

 that the particles of the loadstone may be symmetrically disposed 

 about the line ; and, from an analogy with the axis and poles of the 

 earth, this line is called the axis of the loadstone, and its extremities are 

 called the poles. If the mass of loadstone be cut in the form of a prism, 

 the length of the latter being in the direction of the axis, and if the 

 prism be suspended by its centre of gravity, it will be found to take 

 one particular direction with respect to the horizon and the meridian 

 of the observer. The two extremities of the prism so formed have 

 received the denomination of poles, and the term is now applied to the 

 opposite extremities of any body or molecule, when it assumes or can 



> ought into a particular direction. 



What has been said respecting the properties of a prism formed of 

 the natural loadstone, is true of a magnetised bar of steel [MAUXKT], 

 and the poles or opposite extremities of either material are found to 

 possess a contrariety of character. One extremity always tends 

 towards the northern part of the horizon only, and the other towards 

 the southern part ; and if two such prisms or bars are formed, and 

 suspended by their centres of gravity, on bringing the northern or 

 southern pole of one near the like pole of the other, they exercise 

 upon each other a mutual repulsion ; but if either pole of one be 

 brought near the opposite pole of the other, they mutually attract 

 . . . 



Since in magnetised bars the poles of contrary names thus attract 

 each other, and that the earth may be considered as a body possessing 

 boreal magnetism towards the north, and austral magnetism towards 

 the south, it is evident that the magnetism which exists in the 

 northern extremity or pole of a suspended bar (as a compass needle) 

 must be atulral, and that which exists in the southern extremity must 

 be tonal. 



If a cylinder of wood or metal be insulated on a glass stand, and it 

 be then brought near a body which has been electrified by the usual 

 machine, it will be rendered polar ; that is, one end will possess the 

 vitreous or positive electricity, and the other the resinous or negative 

 electricity, and near the middle the cylinder will be in a neutral state. 

 These conditions may be rendered evident on electrifying a pith ball, 

 insulated by means of a silk thread, and presenting it to the cylinder, 

 when it will be attracted towards one end and repelled from the other. 

 It appears, from the effect of the cylinder on the electrified ball, that 

 the particles of fluid of the same kind repel each other, and those of 

 unlike kinds attract each other. 



Polarity is also obtained by galvanic electricity. In an ordinary 

 battery, the fluid, by chemical action on the zinc, produces a separa- 

 tion of the two kinds of electricity ; that which is called positive is 



carried to the copper plate, and the latter communicates it to the cine 

 plate with liii-h it is connected. This action is repeated at every pair 

 of plates in the battery ; and from the last xiuc plate the ele< 

 enters the conducting wire, or that which is employed to connect the 

 opposite (extremities of the battery : thus the nnc extremity con- 

 stitutes the positive pole of the battery. At the same time an opposing 

 current of negative electricity passes from the copper, through the 

 fluid, to the cine, from thence to the next copper plate, and so on to 

 the last, which is in connection with the conducting wire at that 

 extremity of the battery ; this copper end is called the negative pole of 

 the battery- [GALVANISM.] 



The attractive power in a magnetised steel bar increases from each 

 extremity to about one-quarter of an inch from thence, where it is the 

 greatest, and it then diminishes gradually towards the centre ; this 

 distribution is similar to that of the induced electricity in the insulated 

 cylinder. But experiments show that if a prism of loadstone or a 

 magnetised bar be divided into several parts, perpendicularly to its 

 length, each part is a complete magnet, having poles of contrary deno- 

 minations at its extremities; and this condition is accounted for l.y 

 Coulomb in the following manner : He supposes that every molecule 

 of loadstone is a small magnet possessing opposite polarities at its 

 extremities, the axes of all being parallel to or coincident with the 

 magnetic axis of the mass ; and that a similar disposition of the mole- 

 cule is induced in a bar of steel when the magnetic power is communi- 

 cated to it. The austral polarity of each molecule, while in the mass, 

 is destroyed by the boreal polarity of that which is contiguous to it in 

 the common directions of their axes ; but on separating, as above, the 

 parts of the loadstone or magnetised bar, that extremity of each which 

 is farthest from the northern pole of the bar exhibits the austral 

 magnetism appertaining to the like extremities of the molecules ; and 

 that extremity which is nearest exhibits boreal magnetism. 



A magnetised steel bar, when tried by means of a small compass- 

 needle, is often found to exhibit, at different places in the direction of 

 its length, a change from boreal to austral magnetism, and the contrary. 

 These places are called consecutive poles. 



The intensity of the force, either of attraction or repulsion, exercised 

 by one of the poles of a magnet on any body is inversely proportional 

 to the square of the distance of such body from that pole ; and if a 

 very small compass-needle, supported or suspended in the usual way, 

 be brought near a magnetised bar, it must settle, between those oppo- 

 sing forces, in the direction of a tangent to some curve line passing 

 through the two poles of the magnet. This is called the magnetic 

 curve, and the direction of the tangent at any given point may be thus 

 investigated : 



Let N be the north and s the south pole of a magnet ; let P be any 

 given point at. which the centre of gravity of a small suspended needfe 

 may be placed, and join p N, P s. Let the attraction of N on p be 



expressed by - and be represented by PD; also let the repulsion 

 exercised by a on p be expressed by _L, and be represented by P o/in 



the direction of s p produced. Imagine the parallelogram CB to 

 be formed; then, by mechanics, p<j, its diagonal, will represent the 

 resultant of the forces acting on a particle at p ; it will therefore be 

 the direction of the needle and of a tangent to the curve at that point. 

 Let/^QPcbe represented bySj ^QPHbyS", and let fall CD per- 



pendicularly on P<J : then by trigonometry, cog ' 9 is the value of P D, 



PS* 



or the equivalent of the force represented by p c when reduced to the 

 direction P(j; and _ in the value of <JD, or of the force PB ( = QO) 



P N 



when reduced to the same direction. The sum of these, or <?ofl ' 8 + 



PS* 



;o "' i , is the value of P Q, and represents the whole force of the 

 magnet on the point p in the direction of that line : hence, 



po 



cog -' *' 



OS- * C08 ' ff 

 PS J . PN 3 



PS* PH* 

 But, by geometry, r<? = P O 5 <J c' + 2 P (J. <J D which, by substi- 



