USE OF GALVANOMETERS. 577 



If the field F is perpendicular to the frame, the experiment may 

 be adjusted so that there is no deflection, and then 



FGI. 



Lippmann's galvanometer (862) is particularly suited for the 

 investigation of parts of very intense fields, because it may be 

 reduced to very small dimensions. By measuring the. change of 

 pressure corresponding to the action of the field on a current I, 

 which traverses a layer of mercury of thickness , we shall have 

 the component F of the field perpendicular to the bath by the 

 equation 



The measurement of the thickness e and of the current I enables 

 us to determine F in absolute value. For comparative experiments, 

 the field F is proportional, in all cases, to the difference of pressure /. 



M. Leduc* used this arrangement, and even made the apparatus 

 more sensitive by estimating the difference of pressure by a column 

 of mercury. 



We have seen, finally (857), how the simultaneous use of a 

 tangent galvanometer, and of- a movable frame, enables us to de- 

 termine separately the intensity of the current which traverses the 

 two apparatus, and that of the field in which they are placed, by 

 the deflections of the needle, and of the frame, when the dimensions 

 of the instrument are known. 



By a suitable arrangement (858 and 859), we may also obtain 

 the same result by observing the deflection of the frame by the field, 

 and that which it produces, on an adjacent needle, situate in a prin- 

 cipal position in respect of the original plane of the frame. These 

 latter methods are particularly suited for determining the terrestrial 

 field. 



1150. INDUCED CURRENTS. The measurement of induced 

 charges determines variations of the flow of force in the surface 

 of a circuit, and therefore the mean value F m of the field on this 

 surface. This method was first used by Verdet,t to measure the 

 intensity of the field, in which he placed substances having rotatory 

 magnetic power ; this is particularly suitable for powerful fields, such 

 as those produced by electromagnets or industrial machines. 



* A. LEDUC. Comptes rendus, Vol. xcix., p. 186. 1884. 

 t VERDET. Ann. de Chim. et de Phys. [3], Vol. XLI., p. 395. 

 VOL. II. P P 



