INFLUENCE OF THE DURATION OF THE SWING. 



27 1 



corresponding to the distances a, b, and c (Fig. 171). We should 

 then have 



These equations give, as an experimental condition, 



We might take advantage of this relation to determine the position 



Fig. 171. 



of zero. The former equation will give the value of ^, and we shall 

 have 



a =a I+g 



T -4 /7** 



This method, however, has no interest when the damping is 

 considerable. 



889. INFLUENCE OF THE DURATION AND OF THE INSTANT OF 

 THE SWING. The use of the ballistic galvanometer in the various 

 cases which precede assumes that the impulse is given to the needle 

 in a very short time, and at the exact moment at which it passes its 

 position of equilibrium. It is important to examine what conditions 

 must be introduced into the formula if these conditions are not 

 exactly realised.* 



* O. CHWOLSON. Melanges de Phys. et de Chim., p. 403. Petersburg, 1881. 

 E. DORN. Wied. Ann., Vol. xvn., p. 654. 1882. 



