THE D'ARSONVAL GALVANOMETER is quite dif ferent in its underlying principle from the in struments already described, for instead of hav ing the magnet suspended and deflected under the influence of the current in a surrounding coil of wire, the coil itself is suspended between the poles of a compound horseshoe magnet. This coil is made of fine copper wire, wound on a rectangular frame of thin copper, and suspended by a fine wire of silver or cop per, through which the current flows to the coil. The other end of the coil is con nected to a similar wire, which leads to one of the binding posts, the supporting wire being connected with the other. The coil can thus oscillate freely in the space be tween the two mag nets, and is in a strong magnetic field.
When a current flows through the coil an op posing field is set up, and the coil, being free to move, is deflected. The motion of the coil can be determined either by a light pointer or means of a mirror and a reflected beam, as in the case of the Thomson galvanometer. The D'Arsonval galvanometer is, perhaps, at the pres ent time the most used of any form of galva nometer, since it is not affected by any external magnetic influences, and is easily adjusted. It is also aperiodic, or 'dead beat,' the coil com ing to rest almost instantly and thus saving much time to the observer. For these and other reasons this galvanometer is extensively used for making tests and measurements, and certain modifications have been introduced, so as to ren der the apparatus portable and indicate current and electromotive force directly by means of a pointer and scale. These instruments form the
most accurate ammeters and voltmeters.
Tilt.: BALLISTIC GALVANOMETER iS intended to measure currents of extremely brief duration, such as those produced by the discharge of a condenser or by induction, and a magnetic needle is employed that has a period of vibration amounting often to several seconds. Instead of coming to rest after its deflection by the current, the needle will continue to oscillate, as there are no damping devices, and as the needle itself has considerable mass. When used to measure a momentary current, the deflection does not begin to move practically until the current has passed, and then the throw of the needle is noted. This instrument is used to determine the capacity of a condenser and to measure self-induction. Con sult Kempe, Handbook of Electrical Testing (6th ed., London, 1900) ; Thompson, Elementary Les sons in Electricity and Magnetism (New York, 1901), contains a full elementary description of galvanometers and the theory of their action.