ON STANDARDS OF ELECTRICAL RESISTANCE. 



511 



(§§ 17-21) ; more recently it has been used heterostatically, and is about to 

 acquire (§ 22) special organs adapted for beterostatic use ; as yet, bowever, 

 no species of tbe absolute electrometer promising permanence has come into 

 existence. 



§ 41. It is instructive to trace tbe origin of various beterostatic species of 

 electrometers by natiu-al selection. A body hanging, or otherwise symmetri- 

 cally balanced, in the middle of a symmetrical field offeree, but free to move 

 in one direction or the other in a line tangential to a line of force, moves in 

 one direction or the opposite when electrified positively or negatively. 

 Bohnenberger's arrangement of this kind has a convenient and approximately 

 constant field of force ; and his instrument was chosen in preference to others 

 which may have been equally sensitive, but were less convenient and constant, 

 and it became a permanent species. 



§ 42. Bennet's gold-leaf electroscope, constructed with care to secure good 

 insulation, electrified sufficiently to produce a moderate divergence, has been 

 often used to test, by aid of this electrification, tbe quality of the electrifica- 

 tion of an electrified body brought into the neighbourhood of its upper pro- 

 jecting electrode, causing, if its electricity is of the same sign as that of tbe 

 gold leaves, increase of divergence ; if of the opposite sign, diminution. By 

 connecting the upper electrode with the inner coating of a Leyden jar with 

 internal artificially dried atmosphere, the charge of the gold leaves may be 

 made to last with little loss from day to day ; and by insulating Faraday's 

 metal cage (§ 2) round the gold leaves and alternately connecting it with 

 the earth and with a conductor whose difference of potentials from the earth 

 is to be tested, an increase or a diminution of divergence is observed accord- 

 ing as this difference is negative or positive, the gold leaves being positive. 

 Hence (through Peltier's and Delmann's forms) the beterostatic stationary and 

 portable repulsion electrometers, described in the Eoyal Institution Lecture 

 on "Atmospheric Electiicity " and in Nicbol's Cyclopaedia, article "Electricity, 

 Atmospheric," already referred to, of which one species stUl survives in 

 King's CoUege, Nova Scotia, and in the Natural Philosophy Class Eoom of 

 Edinburgh University. The same form of the beterostatic principle applied 

 to Snow Harris's attracted disk electrometer gave tbe portable and standard 

 electrometers described above. 



§ 43. A modification of Bohnenberger's electroscope, in which the two knobs 

 on the two sides of tbe banging gold leaf became transformed into halves of a 

 circular cylinder, with its axis horizontal and the gold leaf hung on a wire 

 insulated in a position coinciding with its axis, producing a species designed 

 for telegraphic purposes, but which did 

 not acquire permanence by natural se- 

 lection, and is only known to exist in 

 one fossil specimen. In this instrument 

 the wire bearing the gold leaf was con- 

 nected with a charged Leyden jar, and 

 the semicylinders with the bodies whose 

 difference of potential was to be tested. 

 But various modifications of the divi- 

 ded-cylinder or divided-ring class with 

 the axis vertical and plane of motion 

 horizontal have done some practical 

 work, and one species, the new quad- 

 rant electrometer (§ 6), promises to be- 

 come permanent. 



