GRADUATION OF GALVANOMETERS. 255 



necessary for a complete graduation of the galvanometer as far as 

 the limits of the scale. 



872. Poggendorff's method* consists in transforming the gal- 

 vanometer into a sine one to make the graduation. If the 

 instrument does not possess a horizontal circle, it is placed over 

 a graduated circle which can turn about a vertical axis. The coil 

 being at first parallel to the meridian, a constant current is passed, 

 and the deflection 3 gives an equation of the form 



The coil is then turned through the quantities a', a", a" from the 

 initial position, and the corresponding deflections S', S", 8"' of the 

 needle with the coil are observed. We have 



I sinS sin (a' + 8') _ sin (a" -f S")_ 

 H = <(oV)~ 



The values of <f>(8) are then proportional to the sine of the angles 







determined by experiment, which gives the table of the values of 



4>(*) / 



for the normal position of the coil. 



This method would enable us, for instance, to determine the 

 factor of correction of a tangent galvanometer. If the law of the 

 tangents was exact, the value of <(S) should in fact, to within a 

 factor, be equal to cos 8. 



873. Petrinaf used the method of shunt circuits by placing the 

 galvanometer wire as a shunt between two points of a homogenous 

 rectilinear conductor traversed by a current. If the resistance g 

 of the galvanometer is very great compared with the resistance 

 comprised between two points (868), the intensity I of the principal 

 current and that / of the galvanometer satisfy the ratio 



s 



The intensity I being constant, the intensity i is proportional to the 



* POGGENDORFF. Pogg. Ann., Vol. LVI., p. 324. 1842. 



t PETRINA. Holger's Zeitschrift, Vol. I., p. 171. 1842. 



