Construction Of Direct Current Ammeters And Voltmeters

Essay, Research Paper

Construction of Direct Current Voltmeters and AmmetersTheory:Whenever working with electrical systems, the measurement of both current and voltage can be very important. Without accurate measurements of current and voltage, calculations of resistance and other pertinent values could not be completed. This need for measuring electric values in circuit lead to the construction of ammeters and voltmeters. The ammeter measures the electric current flowing into its terminals, and the voltmeter measures the potential difference applied to its terminals (Ohanian 715). Current, the flow of an electric charge, is measured in amperes (A), with 1 Amp being equal to 1 Coulomb of charge moving past a point in 1 second. The current is therefore related to voltage where current is flowing through a conductor or some other component of a system across which is a difference in electric potential. Therefore, electric potential is measured in volts (V). A galvanometer is a device used in the construction of both ammeters and voltmeters (Serway and Faughn 623).The galvanometer is defined as an instrument used to determine the presence, direction, and strength of an electric current in a conductor. When an electric current is passing through the conductor, the magnetic needle tends to turn at right angles to the conductor so that its direction is parallel to the lines of induction around the conductor and its north pole points in the direction in which these lines of induction flow. In general, the extent to which the needle turns is dependent upon the strength of the current. In the first galvanometers, a freely turning magnetic needle was hung in a coil of wire; in later versions the magnet was fixed and the coil made movable. Modern galvanometers are of this movable-coil type and are called d’Arsonval galvanometers (after Ars ne d’Arsonval, a French physicist). If a pointer is attached to the moving coil so that it passes over a suitably calibrated scale, the galvanometer can be used to measure quantitatively the current passing through it. Such calibrated galvanometers are used in many electrical measuring devices. The DC ammeter, an instrument for measuring direct current, often consists of a calibrated galvanometer through which the current to be measured is made to pass. Since heavy currents would damage the galvanometer, a bypass, or shunt, is provided so that only a certain known percentage of the current passes through the galvanometer. By measuring the known percentage of the current, one arrives at the total current. The DC voltmeter, which can measure direct voltage, consists of a calibrated galvanometer connected in series with a high resistance. To measure the voltage between two points, one connects the voltmeter between them. The current through the galvanometer is then proportional to the voltage as is indicated through the pointer reading. Ohm s Law is the principle that ties these two concepts together. Ohm s Law states that although current is directly proportional to the voltage, the flow through the system also experiences some resistance from the materials carrying the electricity. Just as internal friction (viscosity) affects fluid flow, the internal resistance of materials affects the flow of electrons (Wilson 523). The phenomenon occurring here leaves the current in a position of inverse proportionality to the resistance of the materials. This is the simplified form of Ohm s Law. This law can be demonstrated in equation form as I= V/R or V= IR. An ammeter uses a slightly derived version of the Ohm s Law equation. Since the series is in parallel, the galvanometer and the shunt resistor are exposed to a constant value for voltage. The equation form of this is RgIg = RsIs. The term Rg is the known resistance in the galvanometer and Ig is the current required to produce full deflection of the current. The terms Rs and Is are the resistance and current in the shunt, respectively. Since the current in the shunt is unknown, the current in the galvanometer must be subtracted from the current in the system. This is represented, in terms of Rs, by the equation: Rs= Rg (Ig/ I- Is). For voltmeters, the interpretation of Ohm s Law is a little different from the equations derived for ammeters. In this case, a multiplier resistance is placed in series with the galvanometer. This requires that the galvanometer resistance (Rg) be added to the resistance of the multiplier (Rm). This sum is then multiplied to the current in the galvanometer that produces a full deflection. In terms of voltage (V), the equation is stated as: V= Ig (Rm + Rg). With these simple equations, the ammeter and voltmeter can be used to effectively monitor current and potential difference in a circuit.Procedure: The instrument to be used is the Sargent Welch S- 30305 Galvanometer. It contains a galvanometer meter movement and keys to select either of two resistors in series with the galvanometer movement. The resistance of the coil, Rg, is 35d with 10% error. As a galvanometer, a current of 500mA will result in a full-scale deflection (Ig). These are the two values that need to be known in order to construct any ammeter or voltmeter you desire. Galvanometer: To use this meter as a galvanometer, connect it in series with the current source to be measured. A 500 mA current will result in a full-scale deflection. Figure 1 is a representation of the component key arrangement in the meter. The conducting st