Constants of a Rectangular Galvanometer 



789 



giving the magnetic force at any point P in the plane of a 

 rectangular coil carrying a current ?', the perpendiculars pi &c. 

 having the meaning exhibited in fig. 1. 



Firr. 1. 



Equation (i.), which is readily obtained by integrating 

 Ampere's formula round the rectangle, may then be tested 

 by placing the needle of a mirror magnetometer at some 

 definite point in the plane of the rectangle, determining 



X' by, say, a graphic method, and comparing it with the 



value of F/z obtained by means of the magnetometer and a 

 current balance. 



It seemed, however, that such a coil, used as an absolute 

 galvanometer, possessed certain advantages, and as the 

 writer has not seen anywhere a detailed description of such 

 a galvanometer, a discussion of the chief points of interest 

 connected therewith is here presented. 



In all absolute instruments, the cylindrical type is chosen 

 on account of the relative ease with which a cylindrical surface 

 may be turned. But if the use of metallic materials is vetoed 

 on account of the possible presence of traces of magnetic 

 substance, the construction of a wooden bobbin which shall 

 reduce the danger of warping to a minimum is a matter of 

 some difficulty *. On the other hand, it is fairly easy to 

 construct a strong wooden rectangle so that the irregularities 

 in its surface shall be small compared with the unavoidable 

 experimental errors introduced in reading the deflexions of 

 the magnetometer, &c. ; but it is difficult to say, a priori, 



* Gray, 'Absolute Measurements,' vol. ii. p. 353. 

 Phil Mag. S. 6. Vol. 24. No. 143. Nov. 1912. 3 F 



