238 Intelligence and Miscellaneous Articles. 



considering the influence of rain, he observed that its effects are to 

 slightly increase the rate of descent by diminishing the coefficient 

 of friction, and by lowering the temperature, both as being itself 

 generally colder than the air on the ground surface, and also owing 

 to evaporation. He likewise observed that the rate of descent was 

 nearly doubled during the latter part of the winter, chiefly owing 

 to the effects of snow. 



XXIV. Intelligence and Miscellaneous Articles. 



ON THE ELECTROAREOMETER. BY W. MICHELSON. 

 rpHE following very simple, but so far as I am aware unknown, 

 *- experiment may serve as a good illustration of the theory 

 of condensers, and of the electrometer. 



If the internal surface of an ordinary glass areometer is made a 

 conductor, by being silvered for instance, it will, when dipped in 

 a conducting liquid, obviously form a condenser of variable capacity. 

 If the liquid and the internal coating are charged to different and 

 constant potentials, the energy of this condenser strives towards a 

 maximum. A force is at work which dips the areometer deeper 

 in the liquid. The areometer sinks until the hydrostatic reaction 

 of the liquid is so great that it can also hold in equilibrium this 

 added force. 



After a complete discharge the areometer reverts to its original 

 position. 



If we first of all disregard the changes of capillary force due to 

 electrification, and presuppose that the part of the areometer which 

 projects out of the liquid is very long in comparison with the dis- 

 placement of the areometer, the value of the force in question can 

 be easily calculated from a known formula* for the energy W of a 

 portion of a cylindrical condenser of the length I, and the radii a 

 and b (b > a). If the constant potentials of the coatings are A and 

 B, this force is 



L _dW_ K(A-B) 2 



V 4 log 6 ' 



a 



where K is the dielectric constant of the kind of glass of which the 

 areometer is constructed. 



If the densities of the two liquids at whose boundary the 

 areometer floats are s l and s 2 , then, putting the hydrostatic antag- 

 onistic force as equal to the above expression, we have for the 

 position of equilibrium a depression 



mm K(A-B)' 



iKb 2 g(s-s 2 )log- 



We see that the depression must be proportional to the square 

 * Maxwell, ' Electricity and Magnetism.' 



