( 229 ) 



in wliicli, as abov^e, i\ means llie velocity of the middle of the 



string expressed in centimetres per second, r the vek)city of the 



image of the string, expressed in millimetres distance per millimetre 



of time, V the velocit_y of the frame in millimetres per second and 



h the magniiication nsed. 



Önbstitnting in formnla (19) the vahie r as given in formnla (20), 



we have 



0,637 V VIH X 10-^ ,, , 



e =z Voltfi. 



b 



Calling the resistance in the galvanometer circnit m Ohms, the 

 intensity of the cnrrent /, caused in the circnit by the motion of 

 the (piartz thread and expressed in micramperes, is _, 



^ .X106 0,637. n//x 10-3 . 



w wh 



The ponderomotive force, experienced by the string, is expressed 

 by I [mm — iiA]. 



Hence if we pnt in formnla (21) v = l, f becomes = >v, so that 

 we have 



0,637 VIHX 10-3 

 n = - -^ [m.m-^A] .... (22) 



From the above formnla (22) it appeal's that, in order to cak^date 

 i'lj, we must know V, /, ir, l> and H. Of tiiese (piantities the four 

 lirst mentioned can l)e measured with sulticieni accuracy, wliile on 

 the other hand the value H of the intensity of the tield presents 

 some difticnlty. Oj)posite the mi(klle of the string there is an opening 

 in the pole shoes of the electromagnet, by whicii in that place the 

 intensity of the tield, although not zero, yet becomes much smaller 

 than in any other [)lace of the slit-shaj)ed space, in ^vhicil the quartz 

 thread moves. 



Measurements of the intensity of the held in ditferent parts of 

 this slit-shaped space showed that in those places whei-e the intensity 

 is greatest it may be put at about 22500 (0. G. S.), tiie current 

 of the field magnet being regulated at 2.7 Amp. With this current 

 of the field magnet nearly all our observations were made. The 

 average intensity of the field, calculated over the whole length of 

 the slit, appeared to be about lO"/,, less, i.e. about 20250 (C. (1. S.). 

 But this value for //is inadmissible since the places of least intensity 

 are found at the middle of the slit where the lines of force exert 

 the greatest infiuence on the motion of the string. For H we shall 

 have to reckon considerably less than the average value of 20250 



