ELECTRICITY. 



825 



press on the surface of the glass, they are opposed 

 to each other, one pressing on each of the sides. 

 They are attached to the cross piece on the top, and 

 their pressure is regulated by means of screws. A 

 similar pair is attached at the bottom, there being in 

 all four rubbers acting upon the plate. Each of the 

 rubbers carries a silk flap, which extends to nearly 

 a quarter of the circumference of the plate : one of 

 these may be seen at E. A part of the metallic con- 

 ductor is seen at H. Its construction is different 

 from that employed in the cylinder machine. The 

 knob, H, is attached to a large metallic sphere near 

 the back upright, which sphere gives out two brass 

 tubes, one of which is seen at G ; they are formed 

 into curves, and terminate at the edges of the plate, 

 one at G and the other a little below F. The ends 

 of these tubes carry two wires with the points (simi- 

 lar to those in the cylindrical machine,) which collect 

 the electricity from the surface of the plate. The 

 management of this machine is similar to that for- 

 merly described. 



IV. The effects of electrical attraction and repul- 

 sion may now be exhibited much more distinctly with 

 the aid of those considerable accumulations of elec- 

 tricity which we are enabled to form by the electrical 

 machine. A pith ball, or a fragment of gold leaf, is 

 very strongly and immediately attracted by the elec- 

 trified conductor ; and the instant after it lias come 

 into contact with it, it is repelled ; but it is now 

 attracted by the other bodies in its neighbourhood, to 

 which it communicates its own electricity, and then 

 is again in a state to be influenced by the conductor, 

 and to be again attracted ; and this alternation of 

 effects will continue as long as the conductor remains 

 charged. This alternation of attractions and repulsions 

 accompanying the transferring electricity by movable 

 conductors, is also illustrated by the motions of a ball 

 suspended by a silk thread, and placed between two 

 .bells, of which the one is electrified, and the other 

 communicates with the ground. The alternate mo- 

 tion of the ball between the two bells will keep up a 

 continual ringing. An elegant form of this apparatus 

 is shown, fig. 8, where a is a metallic stand, on the top 

 of which is fixed a brass cap, b. From the top of the 

 cap there rises a glass rod. b c, terminated by a brass 

 ball, c. Into this ball four wires with small brass balls 

 at the ends c d, c d, c d, c d. From these balls four 

 bells, e, e, e, e, are suspended, by means of wires or 

 small chains, and from the middle of the rods, c d, c d, 

 cd,cd, four small brass balls, /,/,/,/, are suspended 

 by means of silk threads. To set the bells a ringing 

 it is only necessary to give the ball, C, a supply of 

 positive electricity by connecting it with the prime 

 conductor of the machine. This little instrument 

 has been applied to give notice of changes taking 

 place in the electrical state of the atmosphere. 

 Another amusing experiment, depending on the same 

 principles, is the following : Suspend from the prime 

 conductor, by means of the chain d, fig. 10, a circu- 

 lar disk of sheet tin, or any other metal, and place 

 below it, at the distance of an inch or more, accord- 

 ing to the strength of the electricity, another metallic 

 disk, b, placed on a stand, a. If two little gilt paper 

 figures be placed on the under disk, and the machine 

 put in action, the little figures will begin to dance. 

 The reason is, that when the upper disk becomes 

 charged with electricity,it will attract the little figures, 

 which themselves become charged, and being repelled 

 fly to the bottom and pass it to the under disk, and 

 and this is again repeated as the upper disk is sup- 

 plied. The mutual repulsion of bodies that are 

 similarly electrified gives rise to many interesting 

 experiments. A small figure in the shape of a human 

 head, covered with hair, when placed upon the con- 

 ductor and electrified, will exhibit the appearance of 



terror from the bristling up and divergence of the 

 hair. Advantage is taken of the repulsive property 

 of electrified bodies for the construction of an instru- 

 ment adapted to measure the intensity of the electri- 

 city they may contain. This instrument is called an 

 electrometer. That of Henley consists of a slender 

 rod of wood, serving as an index, terminated by 

 a small pith ball, d, Fig. 6 and suspended from the 

 upper part of a stem of wood, b, b, which is fitted to 

 a hole in the upper surface of the conductor c. An 

 ivory semicircle or quadrant a, is fixed 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 movable 

 rod. The number of degrees wlu'ch is described by 

 the index affords some evidence of the quantity of 

 electricity with which the apparatus is charged, 

 though the instrument cannot be viewed as affording 

 an exact measure of its intensity. The gold leaf 

 electrometer of Bennet, or rather electroscope, which 

 is one of the most delicate instruments ever invented 

 for detecting the presence of electricity, consist of 

 two narrow slips of gold leaf suspended parallel to 

 each other, in a glass cylinder (which secures. them 

 from disturbance by the air), and attached to the end 

 of a small metallic tube, terminating above either in 

 a flat surface of metal or a metallic ball. Two slips 

 of tinfoil 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 

 these, when their mutual repulsion is sufficiently 

 powerful to make them diverge to that extent. 

 These slips of tinfoil terminate hi the foot of the 

 instrument, and thus are in communication with the 

 earth. A very minute charge of electricity, com- 

 municated to the upper end of the tube, is immediate- 

 ly 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 tinfoil, their insula- 

 tion ceases, and their electricity is carried off, and 

 becoming neutral, they resume their original position. 

 Fig. 7 represents two of these electrometers, a and b, 

 with their metallic caps, c, c. The figure represents 

 them as supporting an instrument invented by De 

 Luc, called the electric pile. This instrument consists 

 of a long glass tube, terminated by two caps, d d. 

 Within the tube are enclosed discs of paper, gilded 

 with Dutch leaf, and zinc, alternately, the column 

 consisting of eight or ten hundred pairs. With this 

 simple instrument electric bells have been kept con- 

 stantly ringing for more than a year. The end which 

 is terminated by the gilded paper is the positive, and 

 the other the negative end of the instrument. It has 

 been used as an electroscope and for other purposes. 

 The most perfect electrometer, however, is that in- 

 vented by Coulomb, and called by him the torsion 

 balance. It consists of a cylindrical glass jar, covered 

 at the top by a circular glass plate, with a hole in its 

 centre, through which descends nearly to the bottom 

 of the jar, a single fibre of the web of the silkworm, 

 with a needle of gum-lac or a piece of straw coated 

 by sealing-wax, affixed to its lower extremity. The 

 needle is terminated at one end by a small pith ball, 

 and at the other by a disc of varnished paper, to 

 serve as a counterpoise to the ball. The upper end 

 of the silk fibre is attached to a kind of button, having 

 a small index, and capable of being turned round 

 upon a circular plate divided into degrees. One side 

 of the jar is perforated towards its bottom to allow 

 of the insertion of a short horizontal bar, having a 

 small metallic sphere at each of its ends, the one 

 being within, and the other upon the outside of the jar ; 

 and the former being so situated as just to allow the 

 ball of the suspended needle to come in contact with 



