ELECTRICITY. 



the escape of a conducting fluid, such 

 as water, through a narrow aperture, is 

 promoted by electrifying it. If a small 

 metallic vessel filled with water be sus- 

 pended from the prime conductor, and 

 there be placed in the water one end of 

 a glass syphon, with a capillary bore of 

 such a diameter as that the water will 

 scarcely drop from it ; upon turning the 

 cylinder of the machine so as to convey 

 electricity to the vessel and its con- 

 tents, the water immediately flows in a 

 stream, and, if the electrical charge 

 be veiy powerful, the descending current 

 will be seen to separate into several 

 branches. 



(71.) If a sponge, saturated with 

 water, be suspended from the prime 

 conductor, the water will at first only 

 drop gradually from the sponge ; but 

 when the conductor has become strongly 

 electrified, the drops will fall plenti- 

 fully, and, in the dark, will produce 

 the appearance of a luminous shower 

 of rain. 



(72.) Advantage is taken of the re- 

 pulsive property of electrified bodies 

 for the construction of an Electrometer, 

 or instrument adapted to measure the 

 intensity of the electricity they may con- 

 tain. Henley's electrometer (fig. 13.) 



Fig. 13. 



consists of a slender rod of very light 

 wood, r, serving as an index, terminated 

 by a small pith ball, and suspended 

 from the upper part of a stem of w^ood, 

 ,9, which is fitted to a hole in the upper 

 surface of the conductor. An ivory 

 semicircle, or quadrant, q, is affixed to 

 the stem, having its centre coinciding 

 with the axis of motion of the rod, for 

 the purpose of measuring the angle of 

 deviation from the perpendicular, which 

 the repulsion of the ball from the stem 

 produces in the moveable rod. The 

 number of degrees which is described 

 by the index, affords some evidence of 

 the quantity of electricity with which 

 the apparatus is charged ; though the 



instrument has obviously no pretensions 

 to being an exact measure of its inten^ 

 sity. 



(73.) One of the most delicate in- 

 struments for detecting the presence of 

 electricity is that which was invented 

 by Mr. Bennet, and is usually called 

 the gold-leaf electrometer ; although it 

 is, properly speaking, only an electro- 

 scope. It consists (fig. 14.) of two 



Fig. 14. ,' 



narrow slips of gold leaf, g, suspended 

 parallel to each other, in a glass cylin- 

 der, which secures it from disturbance 

 by accidental currents of air, and at- 

 tached to the end of a small metallic 

 tube, which terminates above either in 

 a flat surface, S, of metal, or in a me- 

 tallic ball. Two slips of tin- foil 1 1, are 

 pasted to the inside of the cylinder, on 

 opposite sides, in a vertical position, 

 and so placed as that the gold leaves 

 may come in contact with them, when 

 their mutual repulsion is sufficiently 

 powerful to make them diverge to that 

 extent. These slips of tin-foil terminate 

 in the foot of the instrument, and thus 

 are in communication with the earth. 

 A very minute charge of electricity 

 communicated to the upper end of the 

 tube, is immediately transmitted to the 

 gold leaves, which are thus made to 

 repel each other ; but if the repulsion 

 is such as to make them strike against 

 the tin-foil, their insulation ceases, and 

 their electricity is carried off; and 

 being now rendered neutral, they cease 

 to repel one another, and, collapsing, 

 resume their original position. 



(74.) The most perfect electrometer 

 for measuring very small quantities of 

 electricity, is the apparatus contrived 

 by Coulomb, and to which he has given 

 the name of the torsion balance. It is 

 represented in its simplest form in fig. 

 15, and consists of a cylindrical glass 

 jar, covered at the top by a circular 

 glass plate, with a hole in its centre. 

 Through this hole a single fibre of 

 c 2 



