CHAP, in.] ELECTRICITY AND MAGNETISM. 163 



deflection of 6 it is known to be approximately three 

 times as strong as a current which only turns the needle 

 through 2. But this approximate proportion ceases to 

 be true if the deflection is more than 15 or 20; for 

 then the needle is not acted upon so advantageously by 

 the current, since the poles are no longer within the coils, 

 but are protruding at the side, and, moreover, the needle 

 being oblique to the force acting on it, part only of the 

 force is turning it against the directive force of the fibre ; 

 the other part of the force is uselessly pulling or pushing 

 the needle along its length. It is, however, possible to 

 " calibrate " the galvanometer, that is, to ascertain by 

 special measurements, or by comparison with a standard 

 instrument, to what strengths of current particular 

 amounts of deflection correspond. Thus, suppose it once 

 known that a deflection of 32 on a particular galvano- 

 meter is produced by a current of T ^ of a weber per 

 second, then a current of that strength will always pro- 

 duce on that instrument the same deflection, unless from 

 any accident the torsion force or the magnetism of the 

 needles is altered. 



199. The Tangent Galvanometer. It is not 

 for the reasons mentioned above possible to construct 

 a galvanometer in which the angle (as measured in 

 degrees of arc) through which the needle is deflected is 

 proportional throughout its whole range to the strength 

 of the current. But it is possible to construct a very 

 simple galvanometer in which the tangent^ of the angle 

 of deflection shall be accurately proportional to the 

 strength of the current. Fig. 89 shows a frequent form 

 of Tangent Galvanometer. The coil of this instru- 

 ment consists of a simple circle of stout copper wire 

 from ten to fifteen inches in diameter. At, the centre is 

 delicately suspended a magnetised steel needle not 

 exceeding one inch in length, and usually furnished with 

 a light index of aluminium. The instrument is adjusted 



1 See note on Ways of Reckoning Angles, p. IOQ. 



