This useful instrument, the invention of which is based upon Oersted’s discovery of the deflection of the magnetic needle, by the action of conducting wires conveying galvanic currents, seems to have furnished to most of the inventors of telegraphs, the main spring of communication. It was a very natural suggestion, as being the most convenient and ready mode of obtaining the required motion, by making and breaking the galvanic circuit. Thus Steinheil, Wheatstone and Bain have availed themselves of this _one idea_ to effect that part of the telegraphic operation which may be called the _galvanic_, in contradistinction to the _mechanical_ parts, which last have varied considerably with different operators. The construction and operation of the galvanometer may be understood by reference to the figures 27, 28, 29. A A, fig. 28, are two long coils of covered copper wire, a side view of which is shown in fig. 27. These coils are connected with the binding screws, L L, attached to the frame, or box, holding the coils. Two coils are used for the convenience of allowing the pivot sustaining the magnetic needle to pass between them; one coil might be used, by leaving room enough between the wires for a socket for the pivot, but the arrangement, represented, is the most readily constructed. A side view of the instrument, figure 27, shows the arrangement of the needles, two of them being generally used to increase the operation of deflection, and to neutralize the influence of the earth’s magnetism. The pair of needles is usually denominated, an _astatic_ needle, or a needle without directive power; as the current traversing a conducting wire gives different directions to needles placed above and below the wire, the action upon the two needles thus placed is combined, by arranging their poles in opposite directions. When the current is in the direction indicated by the arrows in figure 27, the north pole of the needle, within the coil, is carried in a direction from you, as you face the drawing, and the north pole, without the coil, in a contrary direction. The operation upon the south pole is the reverse. Changing the direction of the galvanic current, reverses the motions. It is usual to apply the force of torsion, or of a _hair_ spring, or of the superior weight of one extremity of the needle, to act against the deflective force of the current, and to attach a graduated scale to the instrument, fixing it between the uppermost needle and the coils as in figure 29. Instead of deflecting the needle, the coils themselves may be deflected, as in the galvanoscope of Prof. Page, invented in January, 1837, and described by him in the 33d vol. of Silliman’s Journal, page 376. The object of this contrivance was to enable him to use powerful magnets and lighter coils. This modification of the galvanoscope, Mr. Bain has preferred as the means of operating his telegraph. [Illustration: FIG. 27.] [Illustration: FIG. 28.] [Illustration: FIG. 29.]