Discontinuous Wave-Motion. 205 



free end, which is allowed to swing down in the manner o£ 

 a pendulum. The stops A and B are placed approximately 

 in a vertical line below the point of suspension of the string. 

 As soon as the string impinges on the lower stop B, the 

 weight swings inwards, and the end B is thus effectually 

 fixed. The upper stop A, however, presents some difficulty 

 as it is practically in the same line as that joining B with 

 the point of suspension, and does not therefore, except at the 

 first impact, effectually fix the string at A. To avoid this 

 difficulty, a small cylinder of soft iron is fixed to the string 

 midway between A and its point of suspension, and an electro- 

 magnet is provided which, when the string reaches the 

 vertical position carrying the cylinder with it, draws the 

 latter inwards and then holds it. The stop at A is thus 

 rendered completely effective. The initial motion at any 

 point on the string between A and B and its subsequent 

 vibration are photographically recorded on sensitive paper 

 contained in a dark-slide, which moves downwards behind 

 an illuminated slit set across the string *. 



When the position of the stops A and B is such that the 

 string impinges upon both simultaneously, the impulses cross 

 one another midway between A and B, and the resultant 

 vibration is then necessarily symmetrical. The first six 

 records shown in the Plate refer to the motion at different 

 points when this condition is practically attained. The last 

 record, however, shows a different case, in which the string 

 is fixed at the stop A an appreciable interval after it is fixed 

 at B. This condition is attained by drawing A a little out 

 of the straight line joining B and the point of suspension. 

 The discontinuities cross elsewhere than at the centre of the 

 string twice in each period of vibration, and the vibration- 

 curves are then necessarily asymmetrical. This is evident 

 from the record shown. 



Theory. 



The theoretical form of the vibration-curve at any specified 

 point may be deduced from the velocity-diagram. For, the 

 velocity at any given point on the string is unaffected by 

 the motion of the discontinuities except when one of them 

 actually passes over it. The successive velocities and the 



* For facility of work, it is arranged that the weight and string are 

 released electromagnetically. Simultaneously an auxiliary pendulum is 

 released, which after an adjustable interval of time breaks a contact and 

 releases the photographic slide. 



