LIQUIDS AND AI^LIED EXPERIMENTS. 45 



method of measurement may be tested. In this case D is to be the same 

 for the charged and the uncharged state, i. c, to be constant, while the 

 water level ww is lowered in fig. 1 1 a by a definite amount of rotation of the 

 screw in fig. 13. Hence equation (3) becomes 



F' = 2.262 X 10' -^pg/D' (10) 



where I is the drop of the water level due to the play of the screw .S in fig. 13. 

 If 2i?' is the diameter of the v/ater level ww, in the cup, fig. 1 1 a, and 2B!' 

 the effective diameter of the screw (diameter of the solid cylinder plus the 

 thickness of the thread), and L its longitudinal displacement, equation (10) 

 becomes 



F=' = 2.262Xio«(-^) ,gLB' 



If, therefore, {R"/R'Y is small, say o.i, the apparatus is correspondingly 

 more sensitive, a result which is further enhanced by making {r/RY small, 

 i. e., using a smaller stem and larger disk. 



In case of the given float actually constructed 



2r = o.6o5cm, 27? = 6.65cm. 27?' = 8.23cm. 2i?"= 1.803 cm, 



so that for water 



\ 6.65X8.23 / "^ 



31. Experiments with the Tubular Float. — Experiments with a tubular 

 float, chiefly in the form fig. 11 b, were made at considerable length, 

 but only a few results need here be recorded. With the apparatus as 

 shown it was not convenient to go above 30,000 volts, and even with 

 this voltage the inconstancy of the electrical machine was an ever-present 

 annoyance. The chief purpose of the table, therefore, is to show the rela- 

 tive values of /, the depression of the water level for a distance apart D of 

 the disks of the condenser, when the upper disk was at a potential of V 

 and the lower at a potential zero. 



The measurements as a whole proceeded smoothly, the only difficulty 

 being the control of the electrical machine. The endeavor to increase the 

 potential of the electrometer above 35,000 volts did not succeed, while with 

 the appearance of brush discharge potentials fluctuated enormously at once. 

 When the machine works smoothly, however, the disk takes a stable posi- 

 tion for a sufficiently large D and may then be brought flush with the surface 

 (by lowering the water level) without difficulty. Care must be taken to 

 keep the centering screv/s above the water level (w in fig. 1 1 b) quite dry, 

 A little tapping is essential. 



In all the above experiments 3Z was much less than D, so that the disk 

 rose to a position of stable equilibrium below the point at which continuous 

 motion from the bottom to the top plate would have occurred. Only in 



