Voltmeters draw some extra current, whereas ammeters reduce current flow. Null measurements balance voltages, so there is no current flowing through the measuring device and the circuit is unaltered.
Null measurements are generally more accurate but more complex than standard voltmeters and ammeters. Their precision is still limited. When measuring the EMF of a battery and connecting the battery directly to a standard voltmeter, as shown in, the actual quantity measured is the terminal voltage V. Voltmeter Connected to Battery : An analog voltmeter attached to a battery draws a small but nonzero current and measures a terminal voltage that differs from the EMF of the battery.
Note that the script capital E symbolizes electromotive force, or EMF. Since the internal resistance of the battery is not known precisely, it is not possible to calculate the EMF precisely. The EMF could be accurately calculated if r were known, which is rare. However, standard voltmeters need a current to operate. A potentiometer is a null measurement device for measuring potentials voltages. A voltage source is connected to resistor R, passing a constant current through it.
There is a steady drop in potential IR drop along the wire, so a variable potential is obtained through contact along the wire. An unknown emf x represented by script E x connected in series with a galvanometer is shown in. Note that emf x opposes the other voltage source. The location of the contact point is adjusted until the galvanometer reads zero.
Since no current flows through the galvanometer, none flows through the unknown EMF, and emf x is sensed. Potentiometer : The potentiometer is a null measurement device. A voltage source connected to a long wire resistor passes a constant current I through it. An unknown EMF labeled script Ex is connected as shown, and the point of contact along R is adjusted until the galvanometer reads zero. The unknown EMF is thus proportional to the resistance of the wire segment.
In both cases, no current passes through the galvanometer. The current I through the long wire is identical. The three quantities on the right-hand side of the equation are now known or measured, and emf x can be calculated. There is often less uncertainty in this calculation than when using a voltmeter directly, but it is not zero. Furthermore, it is not possible to tell when the galvanometer reads exactly zero, which introduces error into both R x and R s , and may also affect the current I.
Only AC Instruments The instruments which utilize the electromagnetic induced current for producing deflection are used for ac measurement only. We know that electromagnetic induction is not possible with DC current. Therefore, induction type instruments Voltmeter, ammeter and wattmeter are used only for AC measurements. How Wattmeter works? Please leave your comments below That will be highly appreciated. November 9, December 1, December 9, Your email address will not be published.
Notify me of follow-up comments by email. Notify me of new posts by email. Example: Ampere-hour meter, Energy meter, kilovolt ampere hour meter. Get Started for Free Download App. More Indicating Instruments Questions Q1. Which of the following is NOT the advantage of gravity control instruments? Which of the following principles is utilised in an electrodynamometer type instrument? Which of the following methods utilises a vane mounted on the spindle of the moving system? The vane is of thin aluminium sheet and moves in a closed sector-shaped box?
Which of the following types of damping is used in a permanent magnet moving coil instrument? The permanent magnet moving coil type instrument are best suited for. Voltmeters may have an accuracy of a few percent of full scale, and are used with voltages from a fraction of a volt to several thousand volts. Two common voltage measurements are direct current DC and alternating current AC. Although voltage measurements are the simplest of the different types of analog measurements, they present unique challenges due to noise considerations.
Analog voltmeters move a pointer across a scale in proportion to the voltage of the circuit; digital voltmeters give a numerical display of voltage by use of a converter. Different types of voltmeters are.
Analog voltmeter includes deflecting type of indicating voltmeters. These are of like moving iron, moving coil, electrostatic types of voltmeters.
Moving coil instruments are of two types namely permanent magnet and dynamo meter types. Moving-coil instruments with a permanent-magnet field respond only to direct current. Moving coil instruments consists of permanent magnet to produce magnetic field, and a coil which is wounded on soft iron piece and rotates about its own vertical axis. When the current starts flowing through this coil, deflecting torque is produced according to Lorenz force equation. This torque is directly proportional to the voltage across that particular circuit.
A DC voltmeter is constructed by connecting the resistor in series this instrument and also a very high resistor in parallel with the circuit where we want to measure the voltage. Dynamo meter type Moving coil instrument consists of two coils one is fixed and other is rotating.
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