Effects of temperature changes in Ammeters:

This post provides an information about effects of temperature changes in ammeters.

Errors due to temperature changes in ammeters can be eliminated by using the same material for both shunt and moving coil and kept at the same temperature. But in practice this method is not suitable because temperature of both parts (shunt and moving coil) does not changes at the same rate. If we use same material like copper, there is one more disadvantaging of copper that they are likely to be bulky as the resistivity of the copper is small.

So to avoid these difficulties, there is one another method in which we use swamping resistance. The arrangement of this method is as shown in the figure. In this method we use a resistance of material having negligible temperature coefficient like mangnin and this resistance is called as ‘swamping resistance’. Its resistance is equal to 20 to 30 times the resistance of the coil used in ammeter. The swamping resistance is connected in series with the coil and shunt of mangnin is connected across this combination as shown in figure. Since copper forms a small fraction of the series combination, the proportion in which the currents would divide between the meter and the shunt would not change appreciably with the change in the temperature.

In this manner the effects of change in temperature on ammeters can be minimized.

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Ammeter Voltmeter method:

This is very popular method for measurement of medium resistances since instruments required for this method are usually available in laboratory. The two types of connections employed for ammeter voltmeter method are shown in figure. In both the methods if readings of ammeter and voltmeter are taken then we can measure value of resistance by using formula: The measured value of resistance Rm, would be equal to the true value R, if the ammeter resistance is zero and the voltmeter resistance is infinite, so that the conditions in the circuit are not disturbed. But in actual practice this is not possible and hence both methods give inaccurate results.

Consider circuit of figure (a): voltmeter ammeter method

In this method ammeter measures the true value of current flowing through resistance but voltmeter does not measures the true value of the voltage across the resistance. the voltmeter indicates the sum of the voltage across resistance and ammeter.

Let Ra be the resistance of the ammeter. It is clear from the above equation that the error will be small if the value of the measuring resistance is large as compare to the internal resistance of the ammeter .therefore circuit should be used when measuring resistances are high.

Consider circuit of figure (b): voltmeter ammeter method

In this circuit the voltmeter measures the true value of the voltage across the measuring resistance but the ammeter does not measures the true value of the current flowing through the resistance .the current through the ammeter is the sum of the current through the voltmeter and resistance.

Let Rv be the resistance of the voltmeter. From the above equation it is clear the true value of the resistance will be equal to the measured value only when the voltmeter resistance is equal to the infinite. However, if the resistance of the voltmeter is very large as compared to the resistance under measurement:  It is clear from the above equation that the relative error will be small if the resistance under measurement is very small as compared to the resistance of the voltmeter .hence the circuit should be used when the measuring values of resistances are low.

Operating principle of ammeters and voltmeters

In our day today life, many times we require to measure different electrical quantities like current, voltage, resistance, etc. While doing experiment, there is necessity of multimeter. As we have already discussed about multimeter, how it measures different electrical quantities like electrical current, voltage, resistance, etc. But the basic instruments for the measurement of electric current and voltage are ammeters and voltmeters respectively.

Let us discuss these instruments one by one, operating principle or working principle of ammeters and voltmeters, finally major differences between ammeters and voltmeters.

Operating Principle:

Analog ammeters and voltmeters are classed together as there are no fundamental differences in their operating principles. The action of all ammeters and voltmeters, with the exception of electrostatic type of instruments, depends upon a deflecting torque produced by an electric current. In an ammeter this torque is produced by a current to be measured or by a fraction of it. In a voltmeter this torque is produced by a current which is proportional to the voltage to be measured. Thus all analog voltmeters and ammeters are essentially current measuring devices.

See also: Study & use of Digital Multimeter

The essential requirement of measuring instruments are (i) that its introduction into the circuit, where measurements are to be made, does not alter the circuit conditions ;(ii)the power consumed by them for their operation is small.

Working principle of Ammeters: Ammeter

Ammeters are connected in the series with the circuit whose current is to be measured. The power loss in an ammeter is (I^2.Ra) where I is the current to be measured Ra is the resistance of the ammeter therefore ammeter should have low electrical resistance so that they cause a small voltage drop and consequently absorb small power.

Working principle Voltmeters: Voltmeter

Voltmeters are connected in parallel with the circuit whose voltage is to be measured .the power loss in voltmeter is (V^2/Rv), where V is the voltage to be measured and Rv is the resistance of the voltmeter. Therefore voltmeters should have a high electrical resistance, in order that the current drawn by them is small and consequently the power consumed is small.

Difference between Ammeters and voltmeters:

 Parameters Ammeter Voltmeter Connection It is to be connected in series mode It is to be connected in parallel mode Resistance It has comparatively low resistance It has high resistance Uses It is used to find the amount of current flowing in the circuit It is used to find the potential difference in the circuit Circuit Circuit must be disconnected in order to attach the ammeter Circuit does not need to be disconnected Accuracy Considered as less accurate Considered as more accurate compared to ammeter