RECORDING TIME 



23 



shape of surface which is most generally useful is that of a cylinder 

 which can be set rotating about its long axis by clockwork or some 

 other means. In fig. 27 are reproduced two tracings taken by a tuning- 

 fork, which vibrated at the rate of ten per second, the cylinder being 

 made to rotate at two different rates. The distance between the 

 summit of one curve and that of the next curve represents the space 

 travelled over by the surface in -^ second. This distance in the 

 tracing a & is 2"85 cm., or in one second the surface travelled 28 - 5 cm. 

 In the lower tracing the rate is found to be 4*8 cm. per second, if we 

 measure the distance between one summit and the tenth following. 



The recording of time by means of a tuning-fork possesses the dis- 

 advantage that the vibrations soon cease, especially if the rate of vibration be 

 rapid. To obviate this, a method commonly employed is to record by means 

 of a chronograph (fig. 28), actuated by an electrical current made and broken 

 at some definite known rate by a special piece of apparatus. The chronograph 



Fig. 28. — A Time-marker or Chronograph. (McKendrick.) 



(fig. 28) consists of a small electromagnet and a movable armature, to which 

 is attached a writing-point. Each time the current is closed the armature is 

 attracted and the writing-point moves downwards. The rate of vibration of 

 the writing-point thus depends upon the rate of make and break of the current 

 employed. The current may be automatically closed in a regular manner in 

 several ways. Where the rate required is slow a pendulum clock is very 

 frequently used ; when a more rapid rate is required, a tuning-fork, to one 

 prong of which a platinum wire is attached, so that with each vibration the 

 wire completes a circuit either by touching a platinum surface or by dipping 

 into mercury. The tuning-fork is kept vibrating indefinitely by means of an 

 electromagnet. 



A very convenient time-marker is shown in fig. 29. It consists of a stiff 

 steel band s firmly clamped at one end by the metal cross-bar c. Attached 

 to it is a heavy weight w, by altering the position of which we are able to 

 modify, to a certain extent, the rate of oscillation. The oscillation of the 

 spring is communicated to the vertical bar e, and thus to a lever b e, to which 

 a writing lever l is attached. The writing lever l makes an angle with b e, so 



