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 

 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- | ^ 



flection 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 w^hich 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 orravitation, the mao-netic 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- 

 tinuous electric current be conducted 

 to a magnetic needle, the current 

 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 

 maofnetism of the earth counterbalancino- each other. 

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



acts only for a short time, the magnetic needle suffers 

 4 



Fig. 19. Mkasure- 

 imknt of smalt, 

 angles of deflec- 

 tion with mirkor 



AND LENS. 



