Davis's manual of magnetism : including galvanism, magnetism, electro-magnetism, electro-dynamics, magneto-electricity, and thermo-electricity . board; one of theseis connected with a short coil round the magnet, theother with a long coil. By making this long coil offine wire, the instrument compares currents differingin their intensity. Two batteries are first estimated,as to quantity, by their magnetizing power throughthe short coil. Their relative intensity is then showncompared with their quantity, by their magnetizingpower through the long coil, their intensity being inmathematical propor
Davis's manual of magnetism : including galvanism, magnetism, electro-magnetism, electro-dynamics, magneto-electricity, and thermo-electricity . board; one of theseis connected with a short coil round the magnet, theother with a long coil. By making this long coil offine wire, the instrument compares currents differingin their intensity. Two batteries are first estimated,as to quantity, by their magnetizing power throughthe short coil. Their relative intensity is then showncompared with their quantity, by their magnetizingpower through the long coil, their intensity being inmathematical proportion to the conducting power ofthe wire for each, and therefore to the amount ofelectricity which passes. In comparing the powerof different batteries, — a matter now of some prac-tical importance, — this instrument gives a rapid anduniform result. 308. Instead of using a long coil of wire, sur-rounding the magnet, in estimating intensity, thecurrent may be passed through a detached coil offine wire, and through the short coil of the instru-ment, which would give a similar result. 309. Axial Magnetometer. — Another form of Fig. 13a. the magnetometer is represented in Fig. 133. Inthis, the axial attraction of a double helix is made ELECTRO —MAGNETIC TELEGRAPH. 193 use of, as in the Axial Galvanometer invented by , and described in Sillimans Journal, vol. , 136. In other respects, the construction is en-tirely different from his. A double coil of fine wiremay be added, as in Fig. 131, to compare intensitycurrents. 310. Electro-magnetic Telegraph. — Within afew years, the electric telegraph, the most importantapplication of galvanism ever made, has been con-trived and brought into practical use, A slightsketch of the history of this invention, drawn partlyfrom Yails book, The American Electro-MagneticTelegraph, and partly from the original sources,will be of interest here as an inkoduction. Thisis the more needed, as there has been great confu-sion as to the
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Keywords: ., bookcentury1800, bookdecade1850, booksubjectmagnetism, bookyear18
