1852.J 



ON THE PHYSICAL LINES OF MAGNETIC FORCE. 



31 



tbe temperature of the mercury is supposed to be 32° in botli 

 cases. 29,800 inches at 62° is but 29,791 inches at 32°, there 

 is consequently a real discrepanc}'' of 0,131 inch between the 

 standard barometric pressure in the two countries, and trifling as 

 this quantity may appear to be, it makes the French boiling-point 

 nearly a quarter of a degree Fahrenheit higher than the English 

 (''0°.24), and like the use of different scales and measures, is one 

 of the anomahes in science which it may be hoped will be 

 removed at no distant day. Let us supjjose then that the mer- 

 cury in a thermometer, graduated as described, stands at 1 1 5.7 

 divisions in ice, and at 61 0.9 in steam of the standard elasticity. 

 Then, if Fahrenheit's scale be adopted, we have 501.2 divisions 

 to represent 180 degrees, whence each division =0°.2784 Fah- 

 renheit, and the exact temperature corresponding to any division 

 is kllo^vn. The term stand:ird thermometer is imjsroperly applied 

 to any instrument which does not extend from the freezing to 

 the boiling point, and of which the perfect equality of the sub- 

 division has not been established, it is however commonly apphed 

 to instruments on which unusual pains have been bestowed, and 

 which have been compared with a true standard. Thermometers 

 manufactured and sold by wholesale ha\e not the slightast pre- 

 tensions to this character. It is essential that every obseiTer 

 should verify for himseltj at least the freezing point marked on 

 the scale of his instrument, by immersing it in pounded ice and 

 watei', up to the division 32' on the scale. The best instruments 

 will frequently deviate a little from the truth, owing to a change 

 which tlie capacity of the bulb undergoes in course of time, the 

 cause of which is still very obscure ; an error of one or even two 

 degrees is by no means extraordinarj- in common instruments. 

 In Canada it is chiefly the graduation below the freezing point 

 which the observer has to suspect, and it is desirable where a 

 standard thermometer cannot be leferred to for comparison, to 

 test an instmment by immersing it in a mixture of 1 part com- 

 mon salt, 2 parts snow, thoroughly mixed together and stirred up 

 in a deep cup, when it should sink to — 4°; this temperature is 

 however not absolutely constant like the others, but sufficiently 

 so for a useful practical test in the absence of better means. 

 The observer should not in these experiments trust to a single 

 reading, but take a considerable number, with an inter\-al of a 

 minute between them. The cup itself may be placed in an 

 exterior vessel filled with snow. Thermometers are not in general 

 much in error at the summer tempei'atures. 

 (To he continued.) 



On the Physical Lines of Magnetic Force ; by Piof. Faraday. 



On a former occasion, certain lines about a bar magnet were 

 described and defined (being those which are depicted to the 

 eye by the use of iron filings sprinkled in the neighbourhood of 

 the magnet,) and were recommended as expressing accurately 

 the nature, condition, direction, and amount of the force in any 

 given region either within or outside of the bar. At that time 

 the lines were cortsidered in the abstract. AVithout departing 

 from or unsettling anything then said, the enquiry is now entered 

 upon of the j'ossible and probable physical existence of such 

 lines. Many powers act manifest^ at a distance ; their physical 

 nature is incomprehensible to us : still we may learn much that 

 is real and positive about them, and amongst other thing-s some- 

 thing of the condition of the space between the body acting and 

 that acted upon, or between the two mutually acting bodies. 

 Such forces are presented to us by the phenomena of gravity, 

 light, electiicity, magnetism, (fee. These when examined will be 

 fomid to present remai-kable differences in relation to their respec- 

 tive lines of forces ; and at the same time that they establish the 

 existence of real physical lines in some cases, will facilitate 

 the consideration of the question as applied especially to magnet- 

 ism. Wh<ii\ two bodies, a, h, gravitate towards each other, the 



line in which they act is a straight line, for such is the line which 

 either would follow if free to move. The attractive force is not 

 altered, either in direction or amount, if a third body is made to 

 act by gravitation or otherwise upon either or both of the first 

 two. A balanced cylinder of brass gravitates, to the earth with 

 a weight exactly the same, whether it is left like a pendulum 

 freely to hang towards it, or whether it is drawn aside by other 

 attractions or by tension, whatever the amount of the latter may 

 be. A new gravitating force may be exerted upon a, but that 

 does not in the least att'ect the amount of power which it exerts 

 towai-ds b. We have no evidence that time entera in any wa}- 

 into the exercise of this power, whatever the distance between the 

 acting bodies, as that from the sun to the earth, or from star to 

 stai\ We can hardly conceive of this force in one particle by 

 itself; it is when two or more are present that we comjsrehend it ; 

 yet in gaining this idea we perceive no difterence in the character 

 of the power in the ditfercnt ptai-tieles ; all of the same kind are 

 equal, mutual, and alike. In the case of gravitation, no effect 

 which sustains the idea of an independent or physical line of 

 force is presented to us; and as far as we at present know, 

 the line of gravitation is merely an ideal line representing 

 the direction in which the power is exerted. Take the sun in 

 relation to another force which it exerts upon the earth, namelj", 

 its illuminating or wanning power. In this case rays (which are 

 lines of force) pass across the intemiediate space; but then we 

 may affect these lines by different media applied to them in their 

 course. We may alter their direction either by reflection or 

 refraction ; we may make them pursue curved or angular courses. 

 We may cut them off at their oi'igin, and then search for and 

 find them before they have attained their object. They have a 

 relation to time, and occupy eight minutes in coming from the 

 sun to the earth : so that they may exist independently either C)f 

 their source or their final home, and have iu fact a clear distinct 

 physical existence. They are in extreme contrast with the lines 

 of gravitating power in this respect ; as they are almost iu respect 

 of their condition at their terminations. The two bodies 

 terminathig a line of gravitating force are alike in their actions 

 in eveiy respect, and so the line joining them has like relations 

 in both directions. The two bodies at the terminals of a ray are 

 utterly unlike in action ; one is a source, the other a destroyer of 

 the line ; and the line itself has the relation of a stream flowing 

 in one direction. In these two cases of gravity and radiation, the 

 diflFerence between an abstract and a physical hne of force is 

 immediately manifest. Turning to the case of static electricity 

 we find here attractions (and other actions) at a distance, as in 

 the former cases ; but when we come to compare the attraction 

 with that of gravity, very striking distinctions are presented 

 which immediately affect the question of a physical line of force. 

 In the first place, when we examine the bodies bounding or termi- 

 nating the lines of attraction, we find them as before, mutually 

 and equally concerned in the action ; but they are not alike : on 

 the contrary, though each is endued with a force which, speaking 

 generally, is of the like nature, still they are in such contrast 

 that their actions on a third body in a state hke either of 

 them are precisely the reverse of each other, — what the one 

 attracts the other repels; and the force makes itself evident 

 as one of those manifestation of power endued with a dual and 

 antithetical condition- Now with all such dual powei-s, attraction 

 cannot occur unless the two conditions of force are present and in 

 face of each other through the lines of force. Another essential 

 limitation is, that these two conditions must be exactly equal in 

 amount, not merely to produce the effects of attraction, but in 

 every other case ; for it is impossible so to arrrange things that 

 there shall be present or bo evolved more electric power of the one 

 kind than the other. Another limitation is, that they must be in 

 phj'sical relation to each other ; and that when a positive and 

 a negative electrified surface are thus associated, we cannot cut 



