PENDULUM METHODS 



169 



Fig. 105. 



equalizes the elasticity constant proper to each of the two 

 perpendicular axes, making it the same in all directions.^ 



An ingenious mechanism gives the necessary impulse 

 to the pendulum at each station. For 

 convenience in reading, the ellipse caused 

 by the pendulum under this impulse 

 should be as nearly a circle as possible. 

 This mechanism consists of a crank on 

 point P (Fig. 106) capable of being dis- 

 placed along its vertical axis. It is 

 brought to its initial angular position by 

 a coiled spring and to its vertical position 

 by a plate spring. By the action of a 

 surface-operated electric motor placed above the pendulum 

 top, a half turn is given to the coil spring and simultane- 

 ously, by means of a ramp, the crank is 

 displaced on its vertical axis. P strikes 

 against a copper dome on the pendulum 

 and the crank is liberated from the action 

 of the motor, and under the influence of 

 the spring it describes an arc aM and rises 

 back to its former position. Point a, struck 

 by P, describes a tangential trajectory to 

 the arc. At the moment of release a is 

 going along the tangent M and the pendu- 

 lum has to describe the ellipse of major 

 axis NN'. If the impulse is suited the 

 path NN' will equal a circle MM'. 



Actually in the grating or grid the thin 



bars (reglets) or coordinate lines are fine V 



grooves cut in the spherical silver grating 



plate (Fig. 107). The pendulum point 



(Figs. 104 and 108) breaks circuit with 



the grating surface at these coordinate lines, the break 



being recorded by the electromagnet controlled pen in 



the surface receiver. This receiver (Fig. 109) is a 



^Loc. cit., pp. 72 et seq. The counterforce of the cross springs in the 

 suspension is analyzed here with the aid of Fig. 104. 



Fig. 106. 



