Electricity.

Date:
[1829]
Catalogue details

Licence: Public Domain Mark

Credit: Electricity. Source: Wellcome Collection.

    29/150 (page 39)
    When it rains or snows, the small cover, fig. 9, is to be screwed on the top of the instrument, as by this its insulation is preserved. This indicates not only the electricity of fogs, but also that of serene weather, and enables the observer to dis¬ cover the kind of electricity which reigns in the atmosphere; and, in some degree, to form an estimate of its quantity ^ and that under two dif¬ ferent points of view, the degree of intensity, and the distance from the earth 'at which it first begins to be sensible. A conductor exhibits signs of electricity only when the electric fluid is more or less condensed in the air than in the earth. Though the air resists the passage of the electric fluid, it is not absolutely impermeable to it; it suffers it to pass gradually, and generally with more ease in proportion as its mass is less. It is, therefore, interesting to discover at what height it is necessary to be elevated, in order to find a sensible difference between the electricity of the earth and that of the air. 163. Mr. Brooke of Norwich constructed an electrometer of a very ingenious description, and certainly valuable in its application, of which he has given a full account in .ibis Miscellaneous Experiments ; the limits, however, within which our article must be confined, will not allow of our giving his lengthened description of the instrument. We shall, therefore, present our readers with a short account of another electro¬ meter, the invention of professor Robison of Edinburgh, which in our opinion is, in its chief essentials, much superior to that of Mr. Brooke. This beautiful and delicate instrument is repre¬ sented by fig. 10, where A, a finely polished brass ball of a quarter of an inch diameter, is fixed on the point of a common sewing needle about three inches long, and as slender as can be pro¬ cured of that length. On the other end of the needle is fixed a ball of amber, glass, or other non-conducting substance, of about ^ or fths of an inch in diameter. This ball is so fixed as that the needle does not quite reach to its surface, though the ball F must be completely perforated. From the electric ball there passes a slender glass rod, F, E, L, bent at right angles at E, so that the part F E may be about three inches long, and the other extremity L, immediately opposite to the centre of the ball A. A piece of amber C, so cut as to have two parallel cheeks, is fixed .on the extremity L of the glass rod. For the prin¬ cipal part of the instrument, a strong dry silk thread is to be prepared by dipping it perpen¬ dicularly in melted sealing-wax, till it be fully penetrated by the wax, so as to retain a thin coating of it. 164. The thread, thus coated, must be kept extended, so that it may be quite straight; it must be made perfectly smooth by holding it before a fire, and rolling it on a smooth table. It is then to be passed through a small cube of amber, that has two holes drilled in two of its opposite faces, perpendicularly to the stalk. By these holes the cube is suspended, so as to move readily, on two fine brass pins, between the cheeks of the piece of amber at L. The waxed thread is about six inches long, and is equally divided by the amber cube. To the end B is fixed a ball of some conducting substance, as of polished metal, or gilt cork, a quarter of an inch in diameter. The other extremity D passes through a cork ball, so as to move with a slight friction. 165. The construction of this part of the elec¬ trometer is such that when FE lies perpendicu¬ larly to the horizon, and the stalk B D, with its balls, is allowed to hang freely, the ball B just touches the ball A, as represented in fig. 11. The ball F is fixed to one end of a glass rod FI passing perpendicularly through the centre of a graduated circle GH.O, and furnished at the opposite extremity I with a knobbed handle of boxwood. H K is the stand of the electrometer, in the head of which is a hole in which the rod FI slides smoothly but not easily. There is also adapted to the glass rod FI an index N H that turns round it. This/index is placed so as to be parallel to a line LA drawn through the centre of the ball A. And, as the circle is divided into 360 degrees, 0 being marked above, and 90 on the right hand; the index will point out the angle which the line L A makes with the vertical line. It is convenient to have another index on the rod FI turning with some friction round it, and extending considerably beyond the circle GHO. 166. When this electrometer is used for com¬ puting the quantum of electricity belonging to any body, it must be connected by means of a wire with the substance to be examined : the wire must be fixed into the hole of the ball F, which is in part filled by the needle; and indeed it must come in contact with the needle. The index must now be turned round by the handle I until •it stand at 90°. In this position C B is horizon¬ tal, and the ball B contiguous to A. If in this position the balls B and A are electrified, they do not separate till the index be turned back towards 0. In some part of this space they will separate, and the point must be carefully noted, as this is the measure of their electric power while in contact. By turning still more towards 0, the separation is increased. An assistant must now turn the long index till it be parallel to the other, and consequently to CD. On loading the cork ball D with grain weights till B D be nicely balanced in a horizontal position, and computing for the proportional length of CB and C D, we obtain an exact measure in grains of the electric force with which the balls B and A separate in this position. 167. The electrometers already described have all been found highly useful to the practical electrician, according to the peculiar nature of the course of his experiments; but for ascer¬ taining the actual repulsive and attractive powers of very faintly electrified bodies, they are per¬ haps all excelled by the ingenious invention of M. Coulomb, a French philosopher. The con¬ struction of this delicate instrument, which is called the torsion balance, will be best un- derstood by a reference to the plate, where we have given a representation of it in its complete state, and of some of its parts on an enlarged scale. Fig. 1, Plate IV., is a cylinder of flint glass twelve inches in diameter, and twelve in height, covered with a plate of glass, which is made- to fit to it by a projecting fillet on the lower sur¬ face and having in it four round perforations, of