Tension of Liquid Films. 



277 



Fig. 4. 



starting from the point m, 

 and lay it on the liquid 

 surface, holding in the hand 

 the wax pellet and keeping 

 the free end of the silk in 

 a vertical position. Burst- 

 ing then the film formed be- 

 tween the silk and the solid 

 skeleton, the thread will 

 be immediately stretched, 

 and if we raise the weight 

 a little we shall see the 

 part immersed gradually 

 increase. The parts of the 

 thread which are in this 

 manner gradually added to 



those already submitted to the tension of the liquid should 

 be previously moistened, otherwise the film will burst. When 

 equilibrium is judged to have been established between the 

 tension of the thread and the weight it supports, the skeleton 

 is gently raised, being turned about the side a b, which during 

 this movement of rotation remains horizontal : when the film 

 is nearly vertical, the weight is let go, and the direction ob- 

 served in which it tends to move; if it descends, it is clearly 

 because the traction exerted by the liquid is too small; and accord- 

 ing to theory the radius of the circle ought to be increased, which 

 is effected by increasing the distance of the fixed point m from 

 the last part immersed, n, and submitting at the same time a 

 fresh portion of silk to the action of the liquid ; we can thus re- 

 tain the curve in its form of a semicircumference. If, on the 

 contrary, the weight is raised, we ought to diminish the radius 

 of the arc. We endeavour in this manner to attain equilibrium ; 

 and we ascertain whether the curve produced is semicircular by ex- 

 amining, with the aid of compasses, whether the maximum height 

 e o is equal to the semidistance of m n. When the weight remains 

 immoveable, if the film be made perfectly vertical, in general equi- 

 librium will still be maintained ; yet it is only due to the friction 

 exerted at the point n, where the thread quits the film. In fact, 

 disregarding this friction, we see that as the weight sinks, the 

 radius of the arc decreases. But in this case the tension of the 

 thread also diminishes, and the motion should continue in the 

 same direction ; if the weight ascends, on the contrary, the radius 

 of curvature, and hence also the tension of the thread, increases 

 more and more, which imparts to the motion commenced a gra- 

 dually increasing velocity. 



This being the case, as long as equilibrium continues, due 



