284 SEC. 9. MAGNETISM. 



1126a. Apparatus by Th. Petrouphevsky, Professor at the 

 University of St. Petersburg, for measuring the distance of the 

 Magnetic Poles in straight magnets from their ends. 



Imperial University of St. Petersburg. 



A straight magnet is suspended horizontally at a point where one of the 

 poles is approximately supposed to be. Another straight magnet, much 

 shorter than the first, is placed in the same horizontal plane, perpendicularly 

 to the suspended needle. A slow movement can be given to this second 

 magnet along a divided scale, horizontal and parallel to the suspended needle. 

 The mutual repulsive action of the two magnets is greatest when the direc- 

 tion of the axis of the small magnet passes through the magnetic pole 

 sought, provided that the suspended magnet remains perpendicular to the 

 second magnet. To fulfil this condition an auxiliary magnet is used, which 

 ie. placed on the other side of the suspended magnet, and which is to produce 

 an equal, but contrary, effect to the second magnet. This method of finding 

 the poles has been described in Russian in the " Messenger of Mathe- 

 matical Sciences" (Wiestin), and since, in Poggendorff's Anualen, Bd. 152, 

 s. 42. It requires the use of three distinct appliances: 1st. A bifilar 

 apparatus for suspending cylindrical magnets. 2nd. A measuring apparatus, 

 having a small magnet that can be moved parallel to itself while remaining 

 perpendicular to the direction of said movement. - The apparatus has two 

 divided scales, and is supplied with a microscope, two levels, and two small 

 telescopes of short focus. These two appliances are placed alongside, and 

 parallel to one another, and, approximately, in the magnetic meridian. The 

 third (No. 3) apparatus consists of a divided scale placed perpendicularly to 

 the suspended magnet. Along this may be moved a small magnet, of which 

 the function has been explained above. The second apparatus enumerated 

 was constructed by Mr. Brauer, of St. Petersburg; from the designs of Pro- 

 fessor Petrouchevsky. 



1126b. Apparatus for so-called " Normal " Magneti- 

 sation, by Th. Petrouchevsky, Professor of Physics at the Uni- 

 versity of St. Petersburg. Imperial University of St. Petersburg. 



An iron cylinder for magnetising, placed symmetrically in a magnetising 

 coil, may receive very different distributions of the free magnetism, accord- 

 ing to the length of the coil, but the distribution does not depend on the 

 strength of the current. The inventor has discovered that the distribution of 

 the remanent magnetism, or rather the distance between the poles, remains 

 alsvays the same, independently of the length of the coil or the strength of the 

 current. At a fixed length of the coil, the magnetic poles of the electro- 

 magnet and the poles of the remanent magnetism coincide, so that the electro- 

 magnet, placed horizontally and perpendicularly to the magnetic meridian 

 before a sensitive magnetic needle, acts upon it in the fixed direction (say, 

 the direction of the meridian itself) during the circulation of, and after the 

 cessation of, the current ; in the latter case, the poles of the permanent magne- 

 tism act. The poles are then almost at the ends of the magnetising spiral, 

 the length of which is about 0*8 of the iron cylinder, independently of the 

 length and diameter of the cylinder. This arrangement of the poles of the 

 electro -magnets is called " normal." The apparatus used in these researches 

 are composed of two very distinct parts. The first apparatus contains two 

 brass cylinders, surrounded with wire spirals ; by turning one of these 

 cylinders, the number of spirals can be lessened on the one, and increased on 

 the other. Inside one of these cylinders is placed the iron to be experimented 

 upon. This apparatus is set upon a table covered with marble plates moving 



