( 236 ) 



accuracy, at any rate with our measuring arrangement, when tlie 

 microscope lias been mounted with the cross-wire eye-piece. And 

 hence q itself becomes inaccurately known. 



So we conclude that measuring q has no practical value when 

 we want to know the value of m for a strongly stretched oscillating 

 string. Moreover this value has already been determined for this 

 case in a satisfactory manner by the metiiod described in chapter 3. 



But the measurement of q does obtain practical \'alue when we 

 want to know tiie virtual mass of the image of a string, wliich has 

 written a curxe with a feel)le or moderate tension of the quartz- 

 thread ^). When analysing ditferent curves it is not sufiicient to use 

 the calculated real mass of the quartz-thread, since, as has already 

 been mentioned and will still moi-e clearly ap|)ear in the following 

 chaptei", the virtual mass of the image of the string is very con- 

 siderably modified by clianges in the tension of the quartz-thread. 



We finally remark that when the velocity v is great, also the 

 angles a and /? become large, by which the difference of the sines 

 diminishes for the same difference of the angles. This causes a 

 diminution of the accuracy with which q can be known. 



Also when q becomes very great the determination loses in accuracy, 

 since then for an equal value of /' the difference between sin a and 

 sin '^ greatly diminishes. But this (h'awback is of no practical conse- 

 quence, as in the analysis of a curve the value of o, as soon as it 

 gets beyond a certain limit, may lie put x without a large error. 



6. Analysis of some curves. 



We give in this chapter the results of the analysis of some cur\'es, 

 written, when a known, constant potential difference was suddenly 

 established between the ends of the quartz-thread; 



The first of the cur\es to be dealt with was recorded with a 

 rather feeble tension of the quartz-thread, i.e. with a rather sensitive 

 position of the galvanometer. 1 mm. ordinate = 1,87 X^^^^^ ^"ip- 

 or the sensitiveness c = 535. The speed of the sensitive plate is 

 T"=500 mm. per sec, so the value of 1 mm. absciss = 2 d. 



We call / = () the moment when the electric current is started. 

 Now the angles of inclination of the curve are measured at t = lo, 

 2 o, 3 Ö, etc. In the following table IX, in the first column the 

 values of t are expressed in thousandths of a second and in the 



1) How a separate calculation of p can be avoided here, will appear in the 

 following chapter. 



