390 Intelligence and Miscellaneous Articles, 



produces the magnetization.. It is clear that the effect observed is 

 in part due to the action of the glass plate. 



To allow for this latter action the bismuth may be removed from 

 a small portion of the glass plate, and placing the analyzer at the 

 other end of the apparatus we may examine the transmitted ray. 

 It is found that the plane of polarization of the light which has 

 passed through the glass turns through 24' when the direction of 

 the current is changed ; the rotation, as is known, is in the direction 

 of the current. But in the first experiment the glass plate, having 

 been twice traversed by light, should have produced a rotation of 

 48' ; observation gave 30'. We should conclude from this that 

 bismuth has turned the plane of polarization through 18' in the 

 opposite direction of the current which excites the electromagnet. 

 We may compare this result with the effect produced by a steel 

 mirror placed under the same conditions ; experiment shows a 

 rotation of 22' in the opposite direction of the current which excites 

 the electromagnet. 



If we cover again the steel mirror of the plate which carried 

 the bismuth mirror, we observe a rotation of 28' in the direction of 

 the current, from which we may conclude that we shall have the 

 special effect of the glass plate by adding to this rotation that pro- 

 duced directly by the steel, which gives for the effect of the plate 

 50' instead of 48', the number found above ; the verification is 

 therefore satisfactory. Experiments at present being made have 

 shown me that the electrical resistance of bismuth increases when 

 it is placed in a magnetic field. — Journal de Physique, August 1884. 



ON THE DISTANCE OF THE POLES OF MAGNETS. 

 BY W. HALLOCK AND F. KOHLRAUSCH. 



Most measurements of bar-magnets, of terrestrial magnetism, 

 and galvanism, in so far as the distribution of the magnetism in a 

 bar comes into account, may be arranged in such a way that, for 

 ordinary purposes, we need go no further than for the knowledge 

 of the position of the " poles." By this we understand here those 

 points in which, in actions at a distance, we may assume the free 

 magnetism, provided that the fourth power of the ratio of the length 

 of the magnet to the distance from the magnet vanishes in compa- 

 rison with unity. Ordinary long magnets have this property. 



The distance of these poles from each other usually occur in 

 corrections, which are sufficiently near if the polar distance is 

 known to within a few percents. The question how far this is 

 given by the form of the magnet plays an important part in elec- 

 trical and magnetic measurements, for it influences their simplicity 

 and, under some circumstances, even their accuracy. 



One of us some time ago had endeavoured, by measurements 

 with the tangent-galvanometer made in the Physical Institute in 

 Gobtingen, to determine the distance of the poles, and in connexion 

 with these experiments had caused M. Schneebeli* to make some 



* Progr. der Mdger, Polytechnik in Zurich, 1871, p. 15 of the separate 

 impression. 



