378 Eev. F. J. Smith on a New Form 



use, several conditions have to be complied with ; some of 

 these are as follows : — 



(1) The moving surface on which the time-traces are made 

 should be both long and wide. It should be long, so that 

 periods greatly differing in their duration may be recorded 

 on the same surface close together. It should be wide, so 

 that a large number of time-determinations may be made and 

 compared side by side on the same plate. 



(2) The experimentalist should be able to vary the velocity 

 of the moving surface. 



(3) The surface should so move that all the time-traces may 

 be in straight lines, and the velocity of the surface should be 

 uniform during an experiment. The necessity for uniform 

 movement is forced upon one by the experience of the diffi- 

 culty found in subdividing a tuning-fork trace as recorded 

 by a pendulum-chronograph. When a time-record is made 

 on a surface which is moving at a uniform velocity, the diffi- 

 culty of accurately subdividing a single vibration at once 

 disappears. 



(4) The marking-points of the electromagnetic styli, 

 actuated by springs, when released from an electromagnet, 

 should make a sharp and definite mark ; also the time between 

 the breaking of the circuit and the marking should be as 

 short as possible, and it should be constant in value. 



These ends have been attained in the following manner : — 

 To a vertical carriage, running upon wheels, between two rails, 

 a sheet of smoked glass is fixed. The carriage is impelled by 

 means of a cord attached to a weight ; after the weight has 

 acted upon the carriage through a certain length of fall it is 

 arrested, and the carriage moves with the velocity it has 

 attained : this velocity is found to be practically constant for 

 the whole length of the trace. It will be noticed that this 

 arrangement embodies the principle of the Atwood machine. 

 It will also be noticed that the impulse given by means of the 

 falling weight gives a maximum velocity at the point where 

 its pressure is taken off. With respect to pressure given by 

 a spiral spring, as used in the well-known shooter employed 

 in physiological work, exactly the opposite of this takes place. 

 The writing-points of the electromagnetic styli and the tracing- 

 point of the tuning-fork are placed so as to mark the moving 

 surface. 



The distance between the markings of the styli is obtained 

 in terms of the length of the vibrations of the fork : from this 

 the time between two or more markings is determined. This 

 will become clear from the consideration of the following 

 detailed description of the instrument. 



