One magnet is suspended in the magnetic meridian by a silk thread, and, by the aid of a mirror attached, it describes on the cylinder, moved by clockwork in the centre pier, all the variations in the magnetic declination.
The other two components of the magnetic force of the earth are given by the other magnets. That recording the vertical variations rests on two agate edges under a glass shade, while the horizontal component magnet is suspended by a double silk thread, under the shade to the right of the picture, being retained by the tension of the thread in a position nearly at right angles to the magnetic meridian.
The clock box in the centre covers the three revolving cylinders bearing the sensitive photographic paper, and to each magnet is attached a semicircular mirror, which reflects the rays from a gas jet to one of the cylinders, and thus describes by a curved line the oscillations of the magnet. A second semicircular mirror is fixed to the pier on which the instrument stands, and consequently describes a straight line, or zero, from whence the curves are measured.
To avoid errors attending sudden changes of temperature, underground vaults are always chosen for magnetic observations, and also on account of light being more easily and perfectly excluded.
ATMOSPHERIC ELECTRICITY.
Since the performance of Franklin’s famous kite experiment, by which he determined the identity of lightning with the electrical discharge from a machine, much attention has been devoted, not only to that form of atmospheric electricity which displays itself in the thunder-cloud, but to the electric condition of the air in all states of the weather. These researches have established the fact that the air is always in an electrical condition, even when the sky is clear and free from thunder-clouds. The instruments employed for ascertaining the kind and intensity of atmospheric electricity are called Electroscopes. Fig. 65 shows a modification of Saussure’s Electroscope, the basis of which is a narrow-mouthed flint glass bottle with a divided scale to indicate the degree of divergence of the gold leaves or straws. To protect the lower part from rain, it is covered by a metallic shield about five inches in diameter. Bohnenberger’s Electroscope indicates the presence and quality of feeble electric currents. Peltier’s Electrometer yields the same result by the deflection of a magnetic needle. This latter has been in use at Brussels for thirty years, and at Utrecht for twenty years, and is highly recommended.
65.
Electroscope.
Scale about 1/7.
Singer’s Atmospheric Electroscope is an efficient form of the instrument in which an ordinary gold-leaf electrometer has attached to its circular brass plate a brass rod two feet in length, with a clip at its upper extremity to receive a lighted paper or cigar fusee. The electricity of the air in immediate contact with the flame, causes, by induction, electricity of the opposite nature to accumulate at the upper extremity, where it is constantly carried off by the convection currents in the flame, leaving the conductor charged with the same kind and power of electricity as that contained in the air at the time of the experiment. The principle of this method was initiated by Volta, and has been extended and applied by Sir William Thomson in his Water-dropping Collector, which consists of an insulated cistern from which water escapes through a jet so fine that it breaks into drops immediately after leaving the nozzle of the tube. The result of this is that in half a minute from the starting of the stream the can is found to be electrified to the same extent as the air at the point of the tube. The scale value of each instrument has to be separately determined by repeated comparative experiments, and involves much delicacy of manipulation.
It is chiefly important for the ordinary observer to know that the occurrence of thunder and lightning should be always noted in the column headed “Remarks.”