ON STANDARDS 0^ ELECTRICAL RESISTANCE. 491 



on the upper side of the maiu cover, t'^o of these legs being guided by a 

 straight V-groore at g to give them freedom to move in a straight line in- 

 wards or outwards, and to prevent any other motion. This brass piece is 

 pressed outwards and downwards by a properly arranged spring (Ji), and is 

 kept from sliding out by a micrometer-screw ((') txirning in a fixed nut. This 

 simple kinematic arrangement gives great steadiness to the fourth quadrant 

 when the screw is turaed inwards or outwards, and then left in any position ; 

 and at the same time produces but little friction against the sliding in either 

 direction. The opposite quadrants are connected in two pairs by wires, as 

 shown in fig. 5 ; and two stout vertical wires (I, m), called the chief electrodes 

 passing through holes in the roof of the lantern, are firmly supported by long 

 perforated vulcanite columns passing through those holes which serve to 

 connect the pairs of quadrants with the external conductors whose diiference 

 of potentials is to be tested. Springs («, o) at the lower ends of these columns, 

 shown in figs. 1 & 2, maintain metallic contact between the chief electrodes 

 and the upper sides of two contiguous quadrants (a & 5) when the lantern is set 

 down in its proper position, but allow the lantern to be removed, carrpng the 

 chief electrodes with it, and to be replaced at pleasure without disturbing the 

 quadrants. The lantern also carries an msidated charging-rod (p), or tem- 

 porary electrode, for charging the inner coating of the jar (§ 11) to a small 

 degree, to be increased by the replenisher (§ 12), or, it may be, for making 

 special experiments in which the potential of the interior coating of the jar 

 is to be measured by a separate electrometer, or kept at any stated amount 

 from that of the outer coating. When not in use this temporarj'' electrode is 

 secured in a position in which it is disconnected from the inner coating. 



§ 7. The main cover supports a glass column (q, fig. 2) projecting vertically 

 upwards through its central apertm-e, to the iipper end of which is attached a 

 brass piece (/•), which bears above it a fixed atti-actiug disk (s), to be described 

 later (§ 13) ; and projecting down from it a fixed plate bearing the silk-fibre 

 suspension of the mirror (t), needle (u), &c., seen in figs. 1 & 2, and fixed guard 

 tubes (v, lu), to be described presently. 



§ 8. The moveable conductor of the instrument consists of a stiff platinum 

 wire (x), about 8 centimetres (3^ inches) long, with the needle rigidly attached 

 in a perpendicular plane to it, and connected with sulphuric acid in the 

 bottom of the jar by a fine platinum wire hung down from its lower end and 

 kept stretched by a platinum weight under the level of the liquid. The 

 upper end of the stiff platinum wire is supported by a single silk-fibre so that 

 it hangs down vertically. The mirror is attached to it just below its upper 

 end. Thus the miiTor, the needle, and the stiff platinum stem constitute a 

 rigid body having very perfect freedom to move round a vertical axis (the 

 line of the bearing fibre), and yet practically prevented from any other 

 motion in the regular use of the instrument by the weight of its own mass 

 and that of the loose piece of platinum hanging from it below the surface of 

 the hquid in the jar. A very small magnet is attached to the needle, which, 

 by strong magnets fixed outside the jar, is directed to one position, about 

 which it oscillates after it is turned through any angle round the vertical 

 axis, and then left to itself. The external magnets are so placed that Avhen 

 there is magnetic equilibrium the needle is in the symmetrical position shown 

 in figs. 5 & 6 with reference to the quadrants *. 



§ 9. The needle (u) is of very thin sheet aluminium cut to the shape seen in 

 figs. 5 & 6 ; the very thinnest sheet aluminium that gives the requisite stiflf- 



* Eecently I hare made experiments on a bifilar suspension with a view to superseding 

 the magnetic adjustment, which promise well. 



