Ans: –Accuracy is the closeness with which the instrument reading approaches the true value of the variable under measurement.
Ans: – The instruments which are used to measure electrical quantities (such as current, voltage, power, frequency, etc) are called electrical instruments.
Ans: – The instruments which give the value of the quantity to be measured in terms of the constant of instrument are called absolute instrument.
Ans: –The deflection of such instruments gives the magnitude of electrical quantity to be measured directly.
Ans: –The instruments which indicate the magnitude of electrical quantity at the time when it is being measured.
Ans: – The instruments which give a continuous record of the variations of the electrical quantity to be measured are called recording instruments.
Ans: –The instruments which measure the total quantity of electrical energy in a given time are called integrating instruments.
Ans:- The torque due to which moving system moves from its zero position in an indicating instrument is called deflecting torque.
Ans:- The torque which brings the pointer to zero position when instrument is disconnected from the circuit or the torque which allows the pointer to deflect in accordance to the magnitude of electrical quantity is called controlling torque.
Ans:- The instrument which use the permanent magnet for creating the stationary magnetic flux between which the coil moves is called PMMC instrument .
Ans:- The basic principle of PMMC instrument is that when a current carrying conductor is placed in a magnetic field, a mechanical force is experienced by the conductor.
Ans: –Shunt is used to extend the range of DC Ammeter.
Ans: – Moving iron instrument.
Ans: – Air friction damping is used in moving iron and dynamometer type instruments where the working magnetic field is weak and is likely to get distorted with introduction of permanent magnet.
Ans: –An ammeter is an electrical instrument which is used for current in the circuit. It is thus connected in series with the circuit under test so that circuit to be measured or a fraction of it passes through the instrument itself. Its resistance must therefore be small as small as possible; (i) since power losses in the instrument is given by I2m Rm where is the meter current and Rm is the meter resistance. (ii) so that is does not increase the resistance of the circuit into which it is inserted.
Ans: – A voltmeter is a instrument which is used to measure potential difference between two points of a circuit. Voltmeter always connected in parallel. A voltmeter must have high resistance so that : (i) it is not injured by the current that flows through it. (ii) power wasted is small as the same is given by V2/R.
Ans: –If current in the coil is reversed the direction of magnetic field produced by the coil will reverse, also the magnetism produced in the soft iron piece will reverse. Hence, the direction of deflecting torque remains unchanged. Thus, these instruments can be used on d.c. as well as a.c. system.
Ans: – If current in the coil is reversed, the direction of deflecting torque will be reversed because field produced by the permanent magnets does not change. This will give a wrong direction of rotation, thus, the instrument cannot be used on a.c., it can only be used on dc.
Ans: – Since deflection θ = I2, therefore, the scale of such an instrument is non-uniform; being crowded in the beginning.
Ans: –As deflection θ is proportional to the power to be measured which further depends upon the product of I and V , therefore, dynamometer type wattmeters have non-uniform scale being crowded in the beginning.
Ans: –An ammeter is a low resistance indicating instrument while the voltmeter is a high resistance indicating instrument.
Ans: -(a) Attraction type moving iron instrument (b) Repulsion type moving iron instrument.
Ans: –If an ammeter is connected across the circuit like a voltmeter, then a very high current will pass the instrument coil (because of low resistance of an ammeter). This heavy current will produce such as excessive heat, which may damage the insulation of the coil or may destroy the coil. Thus instrument get damaged.
Ans: – In permanent moving coil instruments, controlling torque is provided by two phosphor hair spring either helical or spiral coiled in opposite direction, of equal strength.
Ans: – Moving coil instruments are used only for d.c. measurement. Moving coil instruments have linear uniform scale. Moving coil instruments have comparatively less power consumption. Moving iron instruments are used for a.c. as well as d.c. measurement. Moving iron instruments also have non-uniform scale. Moving iron instruments are comparatively robust due to simple in construction and also cheaper than moving coil instruments.