( 230 ) 



(C. G. S.), and since sufficiently extensive measurements of the 

 strength of the field in all parts of the slit are ^vanting■, H would 

 have to be estimated approximately. Rather than doing this we shall 

 follow an entirely difTerent \vay. 



It is possilile, namel}", to get to know r^ \\\ a ditFercnt manner. 

 For this the resistance to the motion of the string must be measnred 

 twice, first when a very great conductive resistance is inserted in 

 the gahanometer circuit and then after the condnctive resistance in 

 this circuit has been made as small as possible. In the first case the 

 resistance to the motion of the string consists of air-damping only, in 

 the second case of air-damping and electromagnetic damping combined. 

 The difference betweeji the two values gives us the value of the 

 electromagnetic damping. Remembering formula (I) we write: 



n ^=r — Va (23) 



When the value of ri has become known by means of r and Vn 

 H can be calculated. P'or this purpose we write formula (22) in 

 the form : 



r^&u-XlOOO 



II =- — (24) 



0.637 VI 



We have /> — 660, T— 500 and /=12.7, so that we may 

 write for H : 



ƒ/= 1G3.2 X ''i«' (25) 



H was computed for string 13. Plate A 34 shows a curve, 

 recorded, when suddenly a constant potential difference was established 

 between the ends of the quartz-thread, the galvanometer circuit being- 

 closed by a small external resistance, the amount of which may be 

 neglected. Analysis of the curve shows that q = 24.8, c = 89.4, 

 ?; z= 0.98 and <-/ = 0.964, from which we derive by means of the 



formula rrrr-^ri 5) that r == 0.0294. 

 cvd 



Formerl}^ we found r„ to be 0.0174, from which follows that 

 n =:z 0.0120. 



The conductive resistance of the quartz-thread is id = 9000 Ohms, 

 and from this we compute by means of formula (25) that i/^ 17600 

 [C. G. S.]. 



Two tables follow now. In table YI are found for three quartz- 

 threads the data, cnal)ling us to calculate the value of r by formula 

 (15). The speed of the sliding frame is always 500 niM. per second, 

 the galvanometer circuit being closed by a small external resistance, 

 the amount of which may be neglected. 



