DETERMINATION OF TIME BY ELECTRICITY. 



57 



3. In a similar way the different periods in muscular 

 pulsation may be measured by means of an electric I 

 current. In order to understand this process, let us 

 suppose a sudden push to be given to a heavy pendulum. 

 The pendulum is thus caused to deflect from the 

 vertical position proper to it when 

 quiescent, the angle formed by its de- 

 rlection depending on the force of the 

 push which operated on it. Heavy 

 pendulums of this sort, called ballistic 

 pendulums, are used for measuring 

 the speed of gun-shots. A magnetic 

 needle which when suspended from a 

 thread assumes a direction from north 

 to south, may be regarded as a pen- 

 dulum in which, in place of the force 

 of gravitation, the magnetic attraction 

 of the earth determines its position 

 in a certain direction. If a sudden 

 push is given to a pendulum of this 

 sort, the force of the propulsion may 

 be calculated in this case also from 

 the degree of deflection. If a con- FlG 19 _ MEASUI:E- 

 tinuous electric current be conducted WKNT OF 

 to a magnetic needle, the current 



O ' 



being parallel to the needle, the latter 

 deflects and assumes a position at an angle to the cur- 

 rent, the magnitude of this angle depending on the 

 strength of the current. The magnetic needle assumes 

 a new position, the repelling force of the current and the 

 magnetism of the earth counterbalancing each other. 



O O 



If, however, the current, instead of acting continuously, 

 acts only for a short time, the mngnetic needle suffers 

 4 



ANGLES OF DEFLEC- 

 TION WITH MIUItOK 

 AND LENS. 



