Circuit Globe Electrical Measurement Galvanometer Galvanometer Definition: The galvanometer is the device used for detecting the presence of small current and voltage or for measuring their magnitude. The galvanometer is mainly used in the bridges and potentiometer where they indicate the null deflection or zero current. Principle of Galvanometer The potentiometer is based on the premise that the current sustaining coil is kept between the magnetic field experiences a torque. Construction of the Galvanometer The construction of the potentiometer is shown in the figure below. The moving coil, suspension, and permanent magnet are the main parts of the galvanometer.
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Other Apps Ballistic Galvanometer Ballistic galvanometers are the measuring instruments which are used for measuring the quantity of electric charges obtained from magnetic flux.
Its construction is similar to the moving coil galvanometer and it consists of two additional properties. It consists of extremely small electromagnetic damping. It consists of undamped oscillations. So, the coil starts moving. When the charge flows through the coil, it gives rise to a current due to the torque produced in the coil. This torque acts for a short time.
The product of the torque and the time period provides a force to the coil and the coil starts rotating. Working Principle of Ballistic Galvanometer The working principle of ballistic galvanometer is that the charge measured by the ballistic galvanometer must be passed through the coil.
When the initial kinetic energy of the coil is completely used in doing work, the coil starts moving back to its original position. Thus, the coil oscillates in the magnetic field and the deflection is noted from which charge can be calculated. Construction of Ballistic Galvanometer In the construction of ballistic galvanometer, a ballistic galvanometer consists of a circular or a rectangular coil of a copper wire of almost 10 to 15 turns.
This coil is suspended in a radial field between the concave pole pieces of a strong magnet. To minimize the electromagnetic damping the coil should be wound on a wooden frame and the whole suspension is enclosed in a metal case provided with glass faces. Theory of Ballistic Galvanometer The torque developed by the coil at any point of time is: Where L is the length, W is the width, n is the number of turns of the coil and B is the air gap flux density.
The torque of acceleration is: Where J is the moment of inertia of the coil and w is the angular velocity. If the coil is closed to its zero point then the discharge takes place and the torque of suspension is zero. The value of the driving torque is equal if the damping torque is neglected. During the short discharge period: By integrating: Where the subscript zero refers the conditions at the end of the discharge time.
The integral form of the Eq. Therefore: The above equation indicates the velocity of the coil acquires from the pulse is proportional to the quantity of charge that passed through it. During the actual motion, the deflection torque is zero and the equation of motion is: Where D is the damping constant, S is the control constant and is the deflection in radians.
Thus, Where A and B are constant m1 and m2 are imaginary, Then the initial conditions are: Under this condition the solution may be written as: Where The deflections in Eq.
The ratio of successive swings is: The natural logarithm of this ratio is: The third swing in the same manner is as follows: In case of critical damping, Eq.
Substitute this value in Eq. The series resistor provides critical damping and it is used to control the sensitivity of the magnetic flux. This sensitivity is controlled by adjusting the number of turns in a coil. When the magnet is suddenly withdrawn from the coil, an impulse is produced in a coil for few seconds and the deflection of the galvanometer is taken as a measure of the flux. The induced voltage in the coil are: Where flux is measured in webers and N is the number of turns in a coil.
If R is the total resistance of the circuit including series resistor and a galvanometer then the current flowing to the circuit is: or The quantity of charge passed through the galvanometer is: Deflection of the galvanometer is: or Where K2 is the sensitivity factor and it must be properly evaluated for the resistance used in the test measurements.
Calibration of Ballistic Galvanometer The calibration of ballistic galvanometer can be done in so many different ways. Some methods of calibration are as follows: By Capacitor: In this method, a capacitor is charged through the voltage and is discharged by the galvanometer.
The resistor and a switch S2 is used to bring the galvanometer to its zero position quickly after a deflection. The capacitor is charged through the upper position of the switch S1 and is discharged by the contacts of this switch S1 in the lower position. The discharged quantity of electricity and the capacitance of the capacitor is calculated so the constant K2 is divided by the observed deflection.
This is the undamped sensitivity because of the infinite resistance of the galvanometer. A shunt is added in the parallel to the series resistor and a galvanometer. This shunt provides damping and if the shunt is in critical value then the action is sluggish and the damping conditions are improved with the combination of shunt and series resistances.
This method is not used commonly because it is difficult to measure the exact amount of capacitance of the capacitor and the damping of the galvanometer is different during the operation of test. By Standard Solenoid: This method is mostly used for calibration purposes. In this method, a standard solenoid of a long coil is wound on a cylinder of a nonmagnetic material.
The length of a solenoid is at least 1 meter and the diameter is of 10cm. The calibration is done by means of a known flux. This arrangement creates a flux change twice, so by substitution: Where N2 are the turns of the coil, R is resistance of the coil and galvanometer circuit. The calibration for flux measurements is in convenient form, once sensitivity factor K2 is evaluated. If the galvanometer is used for the measurement of unknown flux, then it will be written as: where is the unknown flux change, is the deflection in millimeters and is the number of turns in the coil.
By Mutual Inductance: This method is used to measure the large range of calibration. It consists of a mutual inductor and is very small in size as compared to a solenoid.
By changing primary current: Let R be the total resistance of the galvanometer circuit then the galvanometer current is: By integration.
Other Apps Ballistic Galvanometer Ballistic galvanometers are the measuring instruments which are used for measuring the quantity of electric charges obtained from magnetic flux. Its construction is similar to the moving coil galvanometer and it consists of two additional properties. It consists of extremely small electromagnetic damping. It consists of undamped oscillations.
The coil is attached to a thin pointer that traverses a calibrated scale. A tiny torsion spring pulls the coil and pointer to the zero position. When a direct current DC flows through the coil, the coil generates a magnetic field. This field acts against the permanent magnet. The coil twists, pushing against the spring, and moves the pointer. The hand points at a scale indicating the electric current. Careful design of the pole pieces ensures that the magnetic field is uniform, so that the angular deflection of the pointer is proportional to the current.