MAGNETISM. 





Fig. 5- 



attracted by a strong steel magnet. But the effect 

 of magnetism on other bodies was unknown till 

 the discovery of an unlimited magnetic power in 

 the galvanic current. The electro-magnet (to be 

 afterwards described) was used in the hands of 

 Faraday, to shew that all bodies are more or less 

 magnetic ; though, in general, the magnetism is 

 so feeble as to be utterly insensible to the most 

 powerful steel magnet. Faraday, in 1845, fi rst 

 proved that all the metals, such as gold, silver, 

 copper, &c. as well as the liquids and gases, are 

 influenced by the powerful magnetism of the 

 electro-magnet. He also established, by a series 

 of beautiful experiments, that there is a notable 

 difference in its effects on different bodies. 



If a small piece of iron wire be hung by a silk 



fibre between the 

 poles of an electro- 

 magnet (fig. 5), it 

 will come to rest 

 with its length in 

 the line of the 

 poles, ab, or axi- 

 ally, as Faraday 

 termed it. 



But if a small 

 rod of bismuth be 

 suspended in this way, it will set with its length 

 along cd, that is, across the line of the poles, or 

 equatorially. The reason is, that iron is attracted 

 by both poles, while bismuth is repelled by both. 

 Iron, and substances which behave like it, Fara- 

 day called paramagnetic ; bismuth, and those like 

 it, he called diamagnetic. Ordinary magnets are 

 sufficient to shew the paramagnetism of iron, 

 nickel, and cobalt ; but the coercitive force of 

 other substances is so feeble that they require a 

 much more powerful magnetism. The following 

 substances are found to be paramagnetic : Iron, 

 nickel, cobalt, manganese, paper, sealing-wax, 

 plumbago, red lead, zinc, sulphate, shellac, charcoal. 

 These are diamagnetic : Bismuth, antimony, zinc, 

 tin, mercury, lead, silver, copper, gold, alum, glass, 

 nitre, sulphur, resin, water, wood, leather, caou- 

 tchouc. They are given in the order of their 

 coercitive force. By inflating soap-bubbles with 

 different gases, Faraday found they also were 

 magnetic, but to a much feebler degree than any 

 solid or liquid substances. 



TERRESTRIAL MAGNETISM. 



Our earth is an immense magnet, with its two 

 magnetic poles situated towards the north and 

 south ends of its axis. This 

 *?. is the cause that a magnetised 

 needle, delicately poised, as 

 shewn in fig. 6, takes up a 

 position nearly north and 

 south. The distances of the 

 earth's poles are so great that 

 their actions on the poles of 

 the needle may be regarded 

 as equal forces acting in oppo- 

 site ways. Such forces can 

 have no other effect than 

 merely to turn the needle round its axis, so as to 

 have its length in their direction. For this reason, 

 a common steel needle, magnetised and made to 

 swim on water, will shew no tendency to move 

 either towards the north pole of the earth or 



Fig. 6. 



j towards the south, but will simply set in their 

 line. 



The value of the magnetic needle as a guide, in 

 the absence of other directing marks, is obvious. 

 To the navigator it is inestimable, and led, cen- 

 turies ago, to the invention of the mariner's com- 

 pass. 



But the needle does not point due north and 

 south, and the amount by which it deviates from 

 that direction is called the declination of the 

 needle. In more precise language, the declination 

 is the angle between the magnetic and the geo- 

 graphical meridian at any place. An instrument 

 furnished with a very delicate needle and the 

 means of ascertaining the true meridian of a 

 place, is called a declinometer. 



It is of the utmost importance to the mariner 

 to know the amount of this deviation of his needle, 

 or compass, from the true north ; and what makes 

 the indications of his guide less reliable is the fact, 

 that the declination is not the same for all places 

 on the earth's surface. In some places, there is 

 no declination at all that is to say, the needle 

 points due north. In this country, it points to the 

 west of true north, while in India it is found to 

 point to the east of it. 



Charts have been drawn having these important 

 changes accurately marked by lines traced through 

 all places where the amount of declination is the 

 same. These lines are called isogonic or equal- 

 angle lines, and the charts isogonic charts. Look- 

 ing at such a chart, we see that the needle points 

 to the west of the true north in Europe and Africa, 

 in the Atlantic Ocean, in the eastern part of the 

 two Americas, and the western part of Asia and 

 Australia. These form what may be called the 

 western declination hemisphere, while the other 

 parts of the earth's surface form the eastern. An 

 irregular line of no declination separates the two 

 hemispheres, running through the eastern con- 

 tinent of North America, and a corner of South 

 America, and coming round through Australia, 

 the west of Asia, and the east of Russia. Curi- 

 ously enough, the line is slowly changing place, 

 the Asiatic line of no declination advancing 

 towards Europe. 



Nor does the declination vary only with the 

 place on the earth's surface, but, at the same place, 

 it varies with \hzyear, the season of the year, and 

 even the hour. At Paris, for instance, in 1580, 

 when observations first began to be recorded, the 

 declination was n E. ; it gradually receded till, 

 in 1669, it was o. In 1700, it had gone 8 to W. 

 and continued its westerly sweep till, in 1814, it 

 was as much as 22 34' W. Since then, it has been 

 slowly diminishing, and it was, in 1864, down to 

 1 8 57' W. at Paris. In England, the greatest 

 declination was, in 1815, 24 27' W. ; and, in 

 1865, it was 21 6'. Thus, in half a century, it 

 has receded, in England, about 3$. At present, 

 it is about 19 45' W. ; the annual rate of decrease 

 being about 8'. 



There is also a periodical variation with the 

 season of the year. Observations shew that, from 

 April to July, the amount of westerly declination 

 decreases ; while, during the rest of the year, there 

 is a slight turning to the west 



The relative position of the sun during the day 

 also influences the needle. From three to eight 

 o'clock in the morning, there is a gradual motion 

 of the needle to the east of its mean position. At 



259 



