CHAPTER III. 



HYDROSTATIC METHODS FOR THE ABSOLUTE ELECTROMETRY 



OF HIGH POTENTIALS. 



Part I. — Hydrometer Methods. 



26. Introduction. — The remarkable precision of weio^ht measurement, 

 which was shown in case of the experiments with the Cartesian diver dis- 

 cussed in the above chapters, suggested experiments along similar lines for 

 purposes where other forces (electrical forces, for instance) are in question. 

 It will be necessary in the course of the present work to measure very high 

 potentials. Hence the modification of the absolute electrometer in such a 

 way that the movable disk is supported by a hydrometer, or by a Cartesian 

 diver, floating in insulating liquids under known conditions, seemed to be 

 an interesting application. In fact, the direct test of the consequences of 

 Coulomb's law, for the case in which the movable conductor is a Cartesian 

 diver, is well worth the trial. 



27. Absolute Electrometer. — The first experiments were made by the 

 hydrometer method, and figs, ii, 12, 13 show the apparatus. In fig. 11 a 

 ceded is the condenser, dd being the guard ring and e the movable disk, both 

 being flanged on the circular edges toward each other, for stiffness. The 

 plate ec is supported by the screw 55 passing through the lateral arm of hard 

 rubber h. The hard-rubber handle a rotates the screw 55 and plate cc 

 around the vertical axis, so that it may be brought in contact with dd or 

 removed from it by as much as may be desirable. The complete and partial 

 turns of 55 are given by a graduated head (not shown) as in case of the ordi- 

 nary micrometer caliper. In this way the distance apart D of the plates 

 cc and dd is sharply determinable. 



In certain of the measurements below it is desirable to be able to raise 

 the guard ring from its position in the uncharged apparatus to the level of 

 the disk. It should therefore be adjustable in the vertical direction. 



The guard ring dd is the top of a box and is perforated at its bottom with 

 a brass tube ggg, 35 to 40 cm. long, closed by the plug k. The T-coupling 

 h communicates at i with a water reservoir for floating the hydrometer, 

 the top of which is the disk e. 



In fig. 1 1 A the float is a very thin tube of aluminum ff about 0.854 cm. 

 in diameter and nearly 30 cm. long. It is closed above and below and there 

 prolonged by a thin stiff wire m about 7 cm. long, terminating in the brass 

 sinker n. The w^hole arrangement efmn weighs about 15 grams and floats 

 with the tube vertical and disk horizontal in the charge of water wiv. This 

 float is very mobile in the vertical direction, so that if electrical forces are 

 strong enough it may actually be lifted into contact with the disk cc. One 

 of the methods of measurement presently to be given will depend upon this 

 possibility. 



39 



