ELECTRICITY. 



523 



Prattical 

 Electricity- 





tube B is cemented near its middle into the common 

 brass cap CC. Hence it is evident, that the insulation 

 of the wire, and consequently of the gold leaves, will 

 be complete, until the inside as well as the outside of the 

 gla.s tube is coated with moisture. This electrometer 

 is shewn completed in Fig. 12. 



SECT. II. On Electrometers for Indicating and Measur- 

 ing Electricity. 



1. Henley's Quadrant Electrometer. 



Henley's This instrument is represented in Plate CCXLVII. 



quadrant Fig. 1 1, where A B is a smooth round stem, about 7 inches 



electrome- long, surmounted by a ball A. Below this ball, and at 

 one side of the stem, is fixed a semicircle of ivory C, ha- 



PLATE v i n g its lower quadrant divided into 1)0 degrees. A thin 

 ' piece of cane mn, 4 inches long, with a ball n at its lower 

 end, is suspended from a pin at m in the centre of the 

 graduated arch, and turns freely round that pin as a 

 center. When the instrument is electrified, the ball, 

 which naturally hangs in a vertical direction, is repel- 

 led by the stern, and indicates the electrical force by its 

 angular distance from the stem. This instrument may 

 be either placed on a stand, like D, or it may be fixed 

 at the end of a conductor, or upon the top of a I.eydtn 

 jar. It is quite obvious, that equal spaces on the qua- 

 drant cannot correspond to equal degrees of electricity. 

 M. Achard has shewn, that the quadrant shotdd l>e di- 

 vided according to a scale of arcs, the tangents of which 

 are in arithmetical progression. 



Brooke's 

 steelyard 



electrome- 

 ter. 



PLATE 

 CCXLIX. 



Pig. 13. 



2. Brooke'i Steelyard Electrometer. 



This ingenious, though complicated and expensive, 

 instrument, is founded on the principle, that the charge 

 of ajar or battery may be accurately determined by the 

 weight in grains, which the repulsive force of the accu- 

 mulated electricity is able to raise. This instrument, 

 which is represented in Fig. 13. Plate CCXLIX. is sup- 

 ported upon a base AB, about J)J inches in diameter, 

 and is completely insulated upon the glass pillar 1)1). 

 The arms G and g, which may be hollow tubes of cop- 

 per, have a motion round the brass wire H, so as to be 

 turned as far away as possible from the jar or battery. 

 The balls IK are made of hollow copper, so as to be as 

 light as possible. The arm G is screwed to a solid piece 

 within the ball F, and has a motion round an axis close 

 to the surface of the ball F, in a vertical plane. The 

 arm E has also a motion in a vertical plane round an 

 axis concealed behind the dial plate H, and by means of 

 a multiplying motion behind the dial plate, the index R 

 performs one complete revolution, while the arm E 

 move-; through 90, or from a vertical into a horizontal 

 position. By means of the apparatus NPH, which we 

 have not room to describe, the electrometer is connected 

 with the conductor, or the jar, or the battery, and all 

 the parts of it arc electrified positively if the conductor 

 gives positive electricity. The two balls I, K, will of 

 course repel one another, and the arm G will rise in 

 a vertical plane. In the same manner the ball L will 

 repel the other ball C, and the arm EC will rise in .1 

 vertical plane. The angular motion of EC is indicated 

 upon the outer circle of the dial plate, divided into 90 

 by the lower and longest arm of the index, and the force 

 of repulsion by which the arm GI is raised, is marked 

 in grains upon the inner circle, to which the shorter 

 arm of the index points. In order to graduate the in- 

 ner circle, a weight m is moved upon the arm GI, till 



it exactly counterbalances the weight in F at the other Practical 

 end of the lever. One end of the weight m will there- Electricity. 

 fore be the zero, or commencement of the scale. Then ^~^~^ 

 having shifted the weight m nearly to the ball I, find, electioffie- 

 by a pair of scales, the weight of the ball I, or the weight ter. 

 produced in consequence of the motion of m towards 

 I. Divide this space into the number of grains thus 

 found, and subdivide the space, corresponding to each 

 grain, into halves and quarters. These divisions may 

 be readily transferred to the inner circle of the dial 

 plate, by observing the position of the shorter index 

 when m is standing at any number of grains in the arm 

 GI. When this is done, Mr Brooke even thinks that the 

 arms G, g, and balls I, K, may be removed as unneces- 

 sary. This ingenious instrument, though never brought 

 to perfection by its author, has given rise to several ad- 

 mirable electrometers founded upon the same principle, 

 of which we shall give a particular description. 



3. Hauch's Improved Discharging Electrometer. 



This instrument is an improvement upon the elec- Hauch's 

 trometer of Mr Brooke, which M. Hauch has greatly improved 

 simplified. It consists of two levers with unequal arms, discharging 

 supported on separate glass pillars, one of which is a dectroni'- 

 littie higher than the other. The end of the longest ter> 

 ami of the upper lever being raised by the repulsion of 

 a ball with which it is in contact, acts upon the end of 

 the shortest arm of the lowest lever; and the end of the 

 longest arm of the lowest lever, by being thus raised, 

 touches the apparatus of the upper lever, and discharges 

 the jar or battery. Our limits will not permit us to 

 give a more detailed account of this instrument ; but 

 a drawing and description of it will be seen in the 

 Phil. Mag. vol. iv. p. 267, or in the Transactions of the 

 Royal Society of Copenhagen. 



4. Dr Robison's Comparable Electrometer. 



This ingenious instrument, which is also an improve- Dr Robi- 

 ment upon Brooke's electrometer, was employed by Dr son's com- 

 Robison in his experiments on the law of electrical ac- P rttble 

 tion. It is represented in Plate CCXLIV. Figs. 5. and electrome - 

 6. and has been fully described in p. 440, 441. 



PLATE 

 5. Coulomb's Torsion Balance Electrometers. CCXLIV. 



These ingenious instruments, three of which we 

 have given representations of in Plate CCXLIV. Figs. Coulomb ' s 



*? 11 i IH_A_ fif^\ri ir r: 



711. and Plate CCXLV. Figs. 1-3, have been \- 

 ready fully described in pages 441, 442, 450, 4.5], and trometers. 

 are undoubtedly the most accurate instruments that PtATE 

 have ever been made for measuring small quantities of CCXLIV 

 electricity. Fig. 71 i. 



6. Cuthbertton's Compound Electrometer. 



This instrument is represented in Plate CCXLIX. Cuthbert- 

 Fig. 14, where D, E are two glass pillars fixed in the ton s cam ' 

 base AB, 18 inches long and six inches broad. These j^omctc'r 1 '*' 

 supports terminate in brass balls D, E, under the last of 

 which is a brass hook for connecting it with the outside p*; ATE 

 of ajar: upon the top of a brass pillar Dl> is supported fjg {4 

 the metallic wire GH, terminating in two equal balls, 

 and balanced like a scale-beam on a knife-edged centre, 

 the ball b being so constructed as to allow the arm GH 

 to move in a vertical plane. A rectangular arm of 

 brass FCD is fixed at D. The arm G4 is divided in- 

 to 60 grains, and carries a slider <;, which can be set 

 at any number of grains. A common Henley's qua- 

 drant electrometer is placed at k. If the balls G and F 

 are electrified, and the slider g be set at 0, they will 

 repel one another ; G will ascend, and H will descend 



