A DYNAMICAL THEORY OF THE ELECTROMAGNETIC FIELD. 



549 



making contact x, y, z are zero. After a time z disappears, and x and y reach 

 constant values. The equations for each conductor will therefore be 



PX + (p + h)x + (k+l}y = \Adt- 



(24). 



RY + (k+m)x + (r + o)y = \Adt-\Edt 



S(Y+Z}+(1 + n)x + (o +s)y= \Edt-\Cdt 

 GZ= \Ddt-\Edt. 



Solving these equations for Z, we find 



^...(25). 



(45) Now let the deflection of the galvanometer by the instantaneous 

 current whose intensity is Z be a. 



*Let the permanent deflection produced by making the ratio of PS to QR, 

 p instead of unity, be 6. 



Also let the time of vibration of the galvanometer needle from rest to rest 

 be T. 



Then calling the quantity 



'1 

 P~ Q~ R^ S~* "\P~Ql ' 



we find 



tan 6 TT lp 



.(26), 

 (27). 



* [In those circumstances the values of x and y found in Art. 44 require modification before 

 l>eing inserted in equation (24). This has been pointed out by Lord Rayleigh, who employed the 

 method described in the text in his second determination of the British unit of resistance in 

 absolute measure. See the Philosophical Transactions, 1882, Part II. pp. 677, 678.] 



