Art. LVII.—On a Form of Electro-Magnetic Seismograph adapted for Indicating or Registering Minute Shocks.—
[Read before the Wellington Philosophical Society, 25th November, 1871.]*
The author has prepared an instrument for this purpose in which an electric circuit is broken, instead of being (as in the system adopted by M. Palmieri) completed by an earthquake.
The following is a specification of this instrument:—A solid block of metal, having a fine platinum wire attached, which projects about half an inch from it horizontally, is connected with a vertical galvanometer, and this with a single pair or small battery of pretty constant power.
A very fine platinum wire depends from a point above in such a position that it presses slightly upon the wire just specified by its lower extremity, a small bob being attached thereto to steady it. A fine screw, or other proper adjusting apparatus, is employed to regulate the pressure (i.e. area of contact) of these wires upon each other; the top of this long wire being connected with the other. pole of the battery. This apparatus when properly set is capable of indicating very feeble shocks. A gentle wind of varying power playing upon the substantial building in which it was first used kept the indicator constantly in motion. When the shock receiving part of the instrument (a massive pile of wood bearing the metal block), in order to avoid effects of wind, was placed under ground, it was ascertained that the impact of two pounds weight of stone, falling from a height of five feet upon ground situated fifty feet from this part of the instrument, affected the indicator very determinately. The intervening ground was clay. In conclusion, the author describes the method in which he arranges a series of such shock-receivers, with batteries for denoting direction of shake, so as to secure economy with effectiveness, and by which constancy in battery power can be dispensed with.
[Footnote] * The original paper, of which this is an abstract, was sent to the Editor of the “Chemical News.”