ABSOLUTE AND RELATIVE SENSITIVENESS. 205 



827. When two intensities are to be merely compared, it is 

 unnecessary to know the dimension of the coils ; the factors which 

 depend on it disappear in the ratio being common to the two terms. 

 We thus attempt more particularly to obtain instruments the sen- 

 sitiveness of which is suited to the currents to be estimated, and 

 the coils are constructed of those dimensions which are most 

 advantageous from this point of view (733). 



To get the absolute measure of the current, we must calculate 

 the electromagnetic action (728), or the surface of the coil as a 

 function of its dimensions. Circular frames with small rectangular 

 sections, in which the wire is regularly coiled, are the only ones the 

 calculation of which does not present too great difficulties, and which 

 it is desirable to adopt in what are called absolute instruments. 



828. ABSOLUTE AND RELATIVE SENSITIVENESS. Whatever be 

 the method employed, if the intensity of the current is determined 

 as a function of a quantity (x) resulting from observation 



!=/(*), 



the absolute sensitiveness S a of the instrument is the ratio of the 

 increase dx of the variable to the corresponding increase d\ of the 

 current 



s -**- 



o^ : 



d\ /'(*)' 



the relative sensitiveness is the ratio of the increase dx to the relative 



d\ 

 variation of the current, 



829. GALVANOMETERS. Suppose that we cause a coil, traversed 

 by a current I, to act on a magnetised needle about an axis. When 

 the needle is infinitely small, if M is the projection of its magnetic 

 moment on a plane perpendicular to the axis of rotation, GI the 

 projection on the same plane of the action of the current at the 

 point where the needle is, /3 the angle of the two directions GI and 

 M, the moment of the couple which acts on the needle is equal to 

 IMG sin/?. The factor G x , which represents the action of unit 

 current, is the galvanometric constant of the coil. 



