ELECTROMAGNETIC DAMPING Introduction In this unit we are going to introduce you to an electrical instrument called galvanometer. You will learn in detail about it and you will be able to explore many interesting features about its construction and working. Galvanometers are instruments which are intended primarily to indicate the existence of a current – steady or transient, in a circuit. These may be capable of measuring the current, only under certain circumstances, when calibrated. The construction of galvanometers is based on the interaction between coils carrying current and magnets. These may be in general divided into two types as given below according to as the moving part is a magnet or a coil. Moving-magnet galvanometer: In this type of galvanometer, the current is passed through a fixed coil which produces a magnetic field under which a magnet moves. Moving-coil galvanometer: In this case the current is passed through a movable coil placed between the poles of a powerful magnet. The coil swings under the action of a deflecting couple acting upon it. We will discuss this particular type in detail here. Moving coil galvanometers can be further classified as: Weston galvanometer or pivoted-coil galvanometer In this galvanometer, the coil swings between two pivots and the deflection of the coil is read by a pointer attached to it which moves on a circular scale. This type of a galvanometer is less sensitive but more convenient. These are extensively used in various bridge type circuits working with direct current, for obtaining null-point. You might have seen or worked with such a galvanometer, in particular, while performing an experiment with Carey Foster’s Bridge. D’ Arsonval galvanometer or suspended-coil galvanometer Here the coil is suspended freely and the deflection of the coil is read using a mirror attached to it. You will learn about this type of galvanometer in detail. It shows very distinct and interesting behavior as a dead-beat galvanometer, a critically damped galvanometer and a ballistic galvanometer, under different circuit conditions. You will find the details about these types of behavior under the theory of this experiment. Let us now look into the details of construction of this type of galvanometer. Figure 1 shows the sketch of D’ Arsonval galvanometer or suspended-coil galvanometer It consists of a narrow rectangular coil C of many turns of fine insulated copper wire suspended so as to move freely in a narrow annular space between the pole- pieces of a