ON STANDARDS OF ELECTRICAL RESISTANCE. 503 



sure that when lie is using it it is pressed in properly to its Vs and down upon 

 the agate-plate. A long arm (d, figs. 8 and 9) (or two arms one above the 

 other), firmly attached to the sliding-tube, carries a pointer which moves up 

 and down with it. Two fixed guiding-cheeks on each side of this pointer 

 prevent the tube from being carried round too far in either direction when the 

 screw is turned : one of these cheeks is graduated so that each division is equal 

 in length to the step of the micrometer-screw ; this enables the operator to 

 ascertain the number of times he has turned the screw. These two checks 

 must never simultaneously press upon the sliding-pointer ; on the contrary, 

 they must leave it a slight amount of lateral freedom to move. If this 

 does not amount to -36 of a degree, the amount of " lost time " produced by 

 it will not exceed J^ of a division of the micrometer-circle, and will not pro- 

 duce any sensible error in the use of the instrument. A glass rod cemented 

 to the lower end of the tube prolongs its axis downwards, and bears the 

 continuous attracting- plate of the electrometer at its loAver end. 



The object aimed at in the mechanism just described is to prevent the nut 

 and other parts rigidly connected with it from any other motion than parallel 

 to one definite line, and to leave it freedom to move in this line, unimpeded 

 by any other friction than that which is indispensable in the arrangement 

 for keeping the sliding tube in its Vs. 



§ 25. If the inner tinfoil covering of the Leyden jar were completed up to 

 the guard-plate bearing the aluminium balance, the long arm of this lever 

 being in the interior of a hollow conductor would experience no electric in- 

 fluence, and no force from the electrification of the Leyden jar, or from 

 separate electrification of the upper attracting-plate, or, more strictly 

 speaking, the electric density and consequent electric force on the long arm 

 of the lever would be absolutely insensible to the most refined test we coidd 

 apply, because of the smallness of the gap between the moveable aluminium 

 square and the boundary of the square aperture in the guard-plate. But to 

 see the hair on the long end of the lever, and the white background with 

 black dots behind it, a good portion of the glass under the guard-plate must 

 be cleared of tinfoil outside and inside. Thus the electric potential of the 

 inner coating of the Leyden jar will not be continued quite uniformly over 

 the inner surface of the bared portion of the glass, and a disturbance affecting 

 chiefly the most sensitive part of the lever will be introduced. To diminish 

 this as much as possible without inconveniently impeding vision, a double 

 screen of thin wire fences, in metallic communication with the inner tinfoil 

 coating and the guard-plate, is introduced between the end of the lever and 

 the glass through which it is observed. 



§ 26. A very light spiral spring (?•) connects the ixppor attracting-plate with 

 a brass piece supported upon a fixed vertical glass column projecting down- 

 wards from the roof of the instrument. This brass piece bears a stout wire (s), 

 called the main electrode, projecting vertically upwards along the axis of a 

 brass tube open at each end, fixed in an aperture in the roof so as to project 

 upwards and downwards, as shown in fig. 9. 



§ 27. The top of the main electrode bears a brass shding-piece (t), which, 

 when raised a little, serves for umbrella and wind-guard without disturbing the 

 insulation ; and when pressed down closes the aperture and puts the electrode 

 in metallic connexion with the roof of the instrument. When the instru- 

 ment is to be used for atmospheric electi'icity (unless at a fixed station), a 

 steel wire, about 20 centimetres long, is placed in the hole on the top of the 

 sliding brass piece just mentioned, and is thus held in the vertical position. 

 A burning match is attached to its upper end, which has the efiect of 



