238 MEASUREMENT OF CURRENTS. 



is very small the factor of correction is very small, and we might 



M 



determine the ratio -- approximately by the ordinary methods. 

 H 



The apparatus would indeed allow this correction to be made 

 directly. For if the needle is fixed in its original position inde- 

 pendently of the coil, and we read the deflection 8' of the coil 

 produced by the same current, the equation of equilibrium is 

 then 



comparing with equation (27), we get 



MG cos (8 + 8') tan 8 l - tan 8' 

 "HS cos 8' sin 8' 



859. The idea of Sir W. Thomson does not seem to have as 

 yet been realised in this simple form. The accuracy of the method 

 requires that the deflections of the coil and of the needle are 

 of the same order of magnitude. We could only attain this result 

 with a very light coil, having only a few turns of wire and supported 

 by a suspension with very feeble coefficient In these conditions 

 it wouid be difficult to make the coil sufficiently rigid to know 

 exactly its dimensions, and to determine its moment of inertia so 

 as to deduce from it the couple of torsion. These drawbacks 

 may all be avoided by using one of Weber's arrangements for the 

 tangent galvanometer. 



The needle is placed outside the coil in a principal position, 

 on the axis or on the mean plane, and at so great a distance 

 that the action of the current is materially diminished. The 

 coil may in that case have a great number of windings, and a 

 suspension may be used, the directing couple of which is easily 

 determined. 



Suppose, for instance, the suspension so adjusted that the mean 

 plane of the coil in equilibrium is in the meridian, and that the 

 needle is placed on the axis at a distance d from the centre. If 

 8 and 8' are the deflections of the needle, and of the coil, produced 

 by the passage of a current I, y and x the co-ordinates of the 



