662 CONSTANTS OF MAGNETISATION. 



to say, the excess of the value for the coefficient for a vacuum 

 over the coefficient for air, also in respect of vacuum. Becquerel 

 used this relation for a great number of liquids and of gases. 



Let j be the apparent coefficient of a substance in air, k its 

 value in vacuum, and k the coefficient of air ; we have 



Without altering the rest of the experiment, the air is replaced 

 by another medium (water, for instance), the coefficient of which is 

 k"', the new value 2 gives, in like manner, 



The coefficient of water in respect of air is then 



/Ci ATo ^ K ~" A- 



1224. One arrangement used by Faraday, and then by Weber,* 

 consists in placing a bar of the substance to be investigated (bis- 

 muth, for instance) in what is the uniform field of a long cylin- 

 drical coil. Between the bar and the magnetising coil is interposed 

 a second coil, in which the wire is wound in opposite directions 

 on the two halves. The two ends of this coil are connected 

 with a delicate galvanometer. When the bar is displaced in either 

 direction along the axis, its magnetic state is unchanged; but the 

 two poles develop in each half of the coil induced currents which 

 add themselves. By combining this motion of the bar with that 

 of the needle we may work by multiplication. By giving to the 

 bar a rapid backward and forward motion, and rectifying the cur- 

 rents by a commutator, we may obtain a permanent deflection. The 

 effect produced is then compared with that which a bar of soft 

 iron would give. By this method Weber found that for the same 

 weight the moment of bismuth is 456,000 times as small as that 

 of iron. 



The difference in the inductive effects of two concentric coils, 

 according as a magnetic core is or is not introduced into the axis, 

 renders it possible to determine the coefficient of magnetisation of 

 the core; the direct method would give no result with bismuth, 

 but it becomes applicable if a differential method is made of it. 



* WEBER. Electrodyn. Maasbestim., loc cit. 



