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The annexed engraving is a perspective view of a Magnetometer, invented by James Swaim, of Philadelphia, who has taken means to secure a patent. A is a small rosewood block ; B represents an electro magnet placed in the block. The extremities of the coil surrounding this electro magnet, are attached to the binding screws, K K. The poles of the screw an arm or indicator is fixed so as to I turn with the screw, I. Under the spring a fixed graduated disc, J, is placed. As the screw, I, is raised or lowered, the indicator points to a degree of the disc, and thus the extent of elevation or depression of the screw can be accurately observed. At H a similar screw to I is inserted through the spring into a nut in the block, A, and an indicator, a, is attached to the screw. A second fixed gradu-ted disc, F, is placed on the spring, and under the indicator. The extent of elevation and depression of the screw, H, is indicated by the position of the indicator, a, on this fixed disc. The operation of this apparatus is as follows :—The extremities of the wire, K K, are to be placed in the course of any galvanic circuit, or are connected with the poles of any battery, the current of which it is desired to measure. The current passing through any such circuit will induce, in the electro-magnet, a power of attraction proportionate to the force of such current. The attractive power of the magnet will be shown by the distance through which it will attract the armature, and the resistance of the spring, G that such attraction will overcome. The screw, H, regulates the upward limit of the armature. The screw, I, regulates the re-acting force of the spring, G. By moving these screws, H and I, until the electro-magnet is just able to cause the armature to vibrate, when the galvanic current passing electro-magnet pass through the top of the block, A, and the armature, E, is suspended immediately across these poles from a straight spring, G. One extremity of this spring is fastened by screws to the block, A. At the point where the spring rises from the block, a hole is made in the spring, and a screw, I, is inserted through this hole into a nut fixed in the frame. To the frame of this around the electro-magnet is broken and closed by the key, L, the respective positions of the hands or indicators will mark the relative distance and re-acting force, which can be overco me by any particular current or battery. This apparatus is especially intended for application to telegraph lines, where it is often of great importance to determine the relative force of currents passing along main or airlines of wire,with a view of detecting breaks, defective insulation, c. When used for that purpose, the electro-magnet, B, should be covered with a long fine wire similar to that used on receiving magnets, and the parts should be constructed of great delicacy, so as to indicate very slight variations in the strength of different currents. M is a binding screw connected with the binding screw, K. Between these two screws a bad conducting substance is interposed in order to diminish the force of powerful currents so as to enable the same spring to measure the relative strength of strong currents as well as weaker currents. The small block or frame, A, may be diminished in size, and placed immediately across the legs, C C, of the electro-magnet, instead of longitudinally along with the coils. By this arrangement the size of the apparatus may be made smaller and more portable. This is a neat, useful, and ingenious instrument. For further information apply to Wm. M. Swain, President of the Magnetic Telegraph Co., 101 Chesnut st., Philadelphia.
