LVANOMETER. ) The delicacy of the instrument may he increased in many ways: making the magnet light. and short, the wire in sev eral turns instead of a single loop, suspending the magnet by a tine fibre with little if any torsional resist a /10P. Illqking the magnet 'astatic.' An 'astatie' magnet is really a combination of Iwo ( nr More) rigidly fastened to a stiff rod or 'staff.' and so placed that the directive magnetic notion of the earth is destroyed as far as possible. This is done hy placing the magnets in opposite directions, lint in the same plane, and making them as nearly as possible of the same strength. In suspending this compound system in the galvanoscope it is so arranged that one magnet (or set of similar magnets) comes inside the coil of wire, while the other magnet (or set of magnets), turned in the opposite direction, lies outside the coil.
In another type of instrument a flat coil of wire is suspended by a wire which has torsional rigidify het Weell the two poles of a horseshoe magnet, so that the line joining the poles is par allel to the faces of the coil. When a current is passed through the coil it will turn so as to tend to place its faces perpendicular to the line joining the poles. thus including more lines of
force. The rigidity of the suspension prevents the coil from turning far; and the turning moment due to the action of the magnet on the current is balanced by the moment of restitution of the twisted wire. This is the prineiple of the siphon recorder, of the D'Arsonval galvommieter, etc. Still another form of instrument differs from the one just described in having the magnetic field due to the permanent magnet replaced by one dime to an electric: current through parallel coils of wire, one on each side of the suspended coil, with their faces it right angles to those of the latter. This instrument is called an 'elee trodynamometer.' It should he noted that Tlowland has shown by direct experiment that an eleetrostatie charge carried at a high speed has the same effect on a magnet as a enrnmt; the direction of the equiva lent current depending whether a positive or a negative dla•ge is carried.