ELECTROMETERS. 



LECTURE II. 



THE instrument to which I will now direct your attention 

 is Thomson's quadrant electrometer. It is the most recent 

 and most complete development from the divided ring 

 electrometer which we examined yesterday. The semicircles 

 of the divided ring over which the movable needle swings 

 are in the quadrant electrometer replaced by four quadrants 

 of a hollow cylindrical box, within which is the movable 

 needle ; and the needle, instead of projecting on one side 

 only of the axis of suspension, as in the divided ring elec- 

 trometer, is symmetrical about that axis. 



In the quadrant electrometer we have first a white flint 

 glass bell- jar, surrounded and supported, mouth up, by a 

 metal casing. The outside is partially coated with tin-foil ; 

 the inside contains strong sulphuric acid a couple of inches 

 deep. This arrangement gives us, as we have already 

 seen in former cases, a Leyden jar ; and with the in- 

 side coating of the Leyden jar the movable needle is 

 connected. 



Secondly, we have the " main cover " of the instrument, 

 a circular brass plate, covering the mouth of the jar and 

 screwed down to the metal casing of the jar. Over a large 

 circular hole in the main cover stands what is called the 

 " lantern." The lantern is of brass, but with a window in 

 front. It carries the " gauge," and three electrodes project 

 from the top of it. [See Fig. 4.] 



The four quadrants (Fig. 5) are supported each on a short 

 pillar of glass projecting downwards from the main cover of 

 the electrometer. The supports of these glass pillars are 

 movable in radial slots, and the quadrants can be drawn 

 out from or pushed in towards the axis of suspension. 

 When the instrument is in adjustment they are pushed in 



