9 



electricity, hence acts a,s an inner coating of the jar ; (c) it 

 allows the needle to become charged without impeding its move- 

 ments. Over the mouth of the jar is the main cover, a circular 

 brass plate, screwed down to the raetal casing, and secured so 

 as to be air-tight and prevent the entrance of moisture. Over a 

 large circular hole in the cover stands the " lantern" ; the latter is 

 of brass and covers the mirror and its suspending arrangements. 

 It has a glass window in front and allows the ray of light to fall 

 upon the mirror and be reflected back on the scale. It also carries 

 the "gauge" and three electrodes, which project from the top. 

 Four quadrants supported by short glass pillars project downward 

 from the main cover. The quadrants are movable in radial slots, 

 and can be drawn out or pushed in toward the axis of suspen- 

 sion. When the instrument is in adjustment, the quadrants are 

 arranged symmetrically about tlie needle. One of the four is 

 capable of adjustment by turning a milled head screw ; the moving 

 of this quadrant also serves in adjusting the zero of the instru- 

 ment. Each pair of opposite quadrants is connected by a fine 

 wire, and from one quadrant of each pair rises an electrode, 

 which, insulated, passes through the top of the lantern. These 

 electrodes serve to connect the two conductors whose difference of 

 potentials is to be measured. The third electrode, seen projecting 

 through the top of the lantern, is for charging and discharging 

 the Leyden jar of the electrometer. 



Tlie Needle (Fig. 2) . — The needle consists of a flat piece of sheet 

 aluminum shaped like a double canoe paddle, and is fixed horizon- 

 tally to a stiff vertical platinum wire which passes through its centre 

 upward and downward. A small cross-bar is attached to the upper 

 end of this wire, and the whole is carried by a bifilar suspension 

 of cocoon fibres. To the lower end of the stiff platinum wire is 

 attaclied a very fine platinum wire carrying a small plummet, 

 which dips into the sulphuric acid at the bottom of the jar. By 

 tliis means the needle is brought in contact with the inside coating 

 of the jar and is charged. The platinum wire carrying the needle 

 has also a small circular mirror attached to it. This mirror is a 

 concave mirror of silvered glass, about !.'> millimetres in diameter, 

 and weighs about one-third of a grain. The concavity of the 

 mirror is such that reflected rays of light are brought to a focus 

 about one metre distant from the mirror. All the movable parts 

 a"e carefully guarded from external influences ; the platinum wire, 

 botli above and below the needle, is protected by a metal " guard 

 tube " through which it passes, and the mirror by a little cylindrical 

 hood projecting some distance from the suspension plate. 



