416 Mr. C. V. Boys's Experiments 



there are never more than three surfaces meeting in a line 

 where the angles are always 120°, or more than four lines or 

 six surfaces meeting in a point. Further, the mean curva- 

 tures of the films are always zero so long as no air is enclosed. 

 As Plateau mentions in his book, the screw-surface has no 

 mean curvature, therefore if a frame be made out of a helix 

 of wire with its ends connected by wire to a solid axis, such 

 a frame after being dipped will carry a helical surface of 

 soap-film. 



Exp. 14. — If, instead of a single helix of wire, two helices 

 are fixed to the same axis, but not quite symmetrically, so 

 that in any part the wire of one helix is nearer that above it 

 than the one below, two helical films will not be formed, but 

 there will be a single one in an intermediate position which 

 will be joined to the two wire screws by a pair of conical 

 screw-surfaces, these forming with the true screw-surface a 

 screw-shaped trough down which bubbles may be rolled or 

 up which they may be wound, as water is wound up by a 

 screw-pump (fig. 13). Further, if a series of small bubbles 

 are blown along the helical edge in which the three films 

 meet at angles of 120°, a spiral staircase is made of soap- 

 film, down which a bubble will run one or two steps at a 

 time, and from which it will escape uninjured when it reaches 

 the bottom. Of course bubbles lighter than air, in the same 

 way, will rest against the lower sides of a trough or roll up 

 instead of down the. screw-surface. 



Exp. 15. — One more experiment in which the rolling of 

 bubbles is the chief feature is worth describing. Three rings 

 of wire, seen in section in fig. 14, are joined together by wires, 

 shown dotted, and are carried by a central axis, which may 

 be made to rotate. After this frame has been dipped in the 

 solution of soap, and the three radial planes broken, it is 

 found to carry a circular trough, into which a series of 

 bubbles may be dropped, while at the same time the frame 

 may be kept rotating, so that the bubbles are rolling round 

 and round like marbles on the rim of a solitaire-board. A 

 corresponding frame might possibly be made of light wire, 

 which after dipping would rest on the bubbles in the first 

 frame, thus forming a working model of the ball-bearing. 1 

 have not, however, succeeded. 



Plateau has mentioned the fact (p. 166) that M. Chautard 

 has found a soap-film a convenient envelope for a gas which 

 is to be tested magnetically. He says that a spherical film 

 above one pole of an electromagnet is visibly disturbed if the 

 gas within has magnetic properties when the exciting current 

 from a battery of 25 to 30 Bunsen's cells is made and broken. 



