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Electrical Instruments: In the topic of electric current, there are various parameters which you might have studied like current, voltage, resistance, electric power, and energy, etc. Ever wondered how do we measure these parameters in the practical circuit? If yes, then this article will help you to understand some of the measurement methods using various electrical measuring instruments. Electrical instruments are devices that are used to measure various electrical parameters like voltage, current, resistance, power, etc. They use the mechanical movements of electromagnetic meters to measure these quantities.
Reading this article, you will get to know about different electrical measuring instruments, their working, and their applications.
So, measurement means to compare to a fixed/standard value. Measurement can include measuring some of the physical quantities like length, mass, time, etc., which we use in our daily lives. Different measuring devices and scales are used to measure these quantities to make various instruments or for various scientific applications and research work.
Electrical measurement would then simply mean to measure electrical quantities or electrical parameters. Terms like current, electricity, voltage, frequency, energy, power, etc., are a part of electrical parameters, and we make use of electrical measuring instruments to measure it. There are different standards of measurement for various applications, so each electrical instrument is selected based on their application about what kind of quantity is to be measured at which place. Things like small electronic circuits/home appliances which doesn’t use much electricity can be measured with a lower scale measuring instrument. But places like power plants/industries which use a very high amount of electricity need a measuring instrument that can measure parameters up to a very high value.
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2. Digital instruments: These instruments are widely used nowadays, taking account of the ease of their use. They can directly measure the physical quantity in digital form according to the number of decimals it is calibrated. It can have a higher range and precise values as compared to analog instruments, and there is no need to continuously change the line of vision, so the problem of personal error decreases. The picture shows a digital multimeter.
2. Recording instruments: These instruments record a set of data of the physical quantity to be measured. These instruments can’t directly indicate the specified value. Instead, it records values over a fixed interval of time. These instruments continuously read the data and record it on a chart using a stylus. They are generally useful for weather forecasting, market analysis, etc., where we need a set of data from the past to predict the outcomes for the future. The picture shows a strip chart recorder with a stylus and a chart.
3. Integrating instruments: These are the type of instruments that neither directly indicates the values nor records them. Instead, they sum up or integrate the physical quantities which are being measured using the instruments. The best example for it is the watt-hour meter which integrates power and time in the units of watt and hour respectively, integrates it and convert it into units. The picture shown is a simple watt-hour meter.
So, this was the classification of electrical instruments. Further, we will see construction and working of three of the very basic instruments – Galvanometer, Ammeter, and Voltmeter (Analog type)
A moving coil galvanometer is an electromagnetic device that is used to detect the direction of the current. Not only this, but it can also measure a small amount of current. Its pointer on the galvanometer deflects due to the magnetic field according to the direction of current flowing in the circuit. Let us look into the details of its construction and work, along with some important equations. The picture shows a simple moving coil galvanometer for your reference.
As you can see, the pointer can deflect in the direction according to the current in the circuit or the part of the circuit where the current is to be detected.
The diagram shows a moving coil galvanometer as viewed from the top. As per the diagram, you can see a conducting coil wound around a soft iron core. This core is placed between two poles of a permanent magnet and is attached to a pointer. This pointer will help to indicate readings on the scale.
As we know, that when a current-carrying loop is placed in a magnetic field, a torque acts upon it, which is given by the equation
\({\rm{\vec \tau }} = Ni{\rm{\vec A}} \times {\rm{\vec B}}\)
Here, \(N\) is the number of turns in the coil, \(i\) is the amount of current flowing through the coil, \(A\) is the cross-sectional area of the coil, and \(B\) is the magnetic field around the coil.
The same thing happens here with the moving coil galvanometer too. The permanent magnet around it generates a uniform magnetic field in the direction of the North pole to the South pole. When electric current passes through the coil, due to the effect of the magnetic field around it, a torque gets generated in it, and a small amount of deflection occurs in the coil. Due to this small amount of deflection, the pointer also gets deflected along with the coil.
If the direction of current is reversed in this case, the direction of the torque is also reversed, and hence the direction of deflection is also changed. Thus, according to the direction of the current, the coil gets deflected in opposite directions.
We can write for the magnitude of torque in this coil as;
\(\tau = NiAB\sin \theta \)
Here, you can see from the diagram that the angle between the area vector and magnetic field of the coil is \(90^\circ .\) Hence, the magnitude of torque will be:
\(\tau = NiAB\)
If the coil deflects by an angle \(\varphi \) then we can write:
\(c\varphi = NiAB\)
Here, \(c\) is the torsion constant of this coil. Rearranging the equation a bit, we get
\(\frac{\varphi }{i} = \frac{{NAB}}{c}\)
The ratio \(\frac{\varphi }{i}\) is the angle by which the coil deflects per unit current which passes through it. Hence, this ratio is known as the current sensitivity of the galvanometer.
Since a galvanometer can only measure a small amount of current, we need to use an ammeter to detect a relatively higher amount of current. But ammeter cannot indicate the direction of the current; it can only detect the value of current flowing in the circuit or the part of the circuit. We will look into its construction and working further. A picture of an analog ammeter is shown here for your reference. Since its range is in \({\rm{mA}},\) we can also call it a milliammeter.
An ammeter can be constructed using a galvanometer. Normally, an ammeter is connected in series with the part of the circuit where the current is to be measured. Since we need the same amount of current as that in the circuit, the resistance of the ammeter should be as much low as possible, and ideally, its resistance should be zero. The diagram shows an ammeter constructed using a galvanometer
Shunt resistance is connected parallel to a galvanometer so that the maximum amount of current flowing through the circuit may pass through it, and we get an accurate reading.
The current entering the ammeter will be divided into two different parts according to \({\rm{KCL}}\) in galvanometer and ammeter. Since the resistance of the shunt is very low, most of the current entering ammeter will pass through it, and a very less amount of current will pass through the galvanometer.
Since the resistance of the ammeter should be as low as possible, a shunt resistance is connected to it in parallel. The value of shunt resistance \(\left( S \right)\) must be as low as possible as compared to the resistance of the galvanometer \(\left( G \right)\) Hence, the net resistance of the ammeter will be
\({R_A} = \frac{{GS}}{{G + S}}\)
This resistance will be even lower and, in this way, we can make the resistance of the ammeter less. Here, \({i_g}\) is the current flowing through a galvanometer, \(i\) is the total current entering the ammeter and hence, \(i – {i_g}\) will be the current through the shunt resistor. Since galvanometer and shunt are connected in parallel, the potential difference across them must be the same
\({i_g}G = \left( {i – {i_g}} \right)S\)
\(\therefore S = \frac{{{i_g}G}}{{\left( {i – {i_g}} \right)}}\)
The above equations help us to find the necessary shunt resistance that needs to be connected across the galvanometer in order to convert it into an ammeter.
For an ammeter, we can define its current sensitivity in two different ways \( – \left( 1 \right)\) Based on a number of divisions covered and \(\left( 2 \right)\) Based on the angle by which the needle is deflected. Hence, we can have two formulas for current sensitivity:
\({\rm{Current\;Sensitivity\;}}\left( {CS} \right) = \frac{{{\rm{Number\;of\;divisions\;covered\;by\;the\;needle}}}}{{{\rm{Current}}}}\)
\({\rm{Current\;Sensitivity\;}}\left( {CS} \right) = \frac{{{\rm{Angle\;by\;which\;needle\;is\;deflected}}}}{{{\rm{Current}}}}\)
A voltmeter is a device that is used to measure the potential difference across a part of the circuit. Voltmeter can be made for different ranges according to its use. Since the potential difference in parallel connection is the same, voltmeters are usually connected in parallel to the part where the potential difference is to be measured. The picture shows an analog voltmeter for your reference
A voltmeter can be constructed using a moving coil galvanometer. Since it is connected in parallel to the circuit, its resistance should be as much high as possible in order to avoid any current flowing through it, and ideally, its resistance should be infinite. To make the resistance high, a resistor with a very high resistance is connected in series with the ammeter. The diagram is of a voltmeter constructed using a galvanometer
The net potential difference across \(A\) and \(B\,\left( V \right)\) will be divided into two parts – potential difference across high resistance and potential difference across galvanometers. The resistance of the resistor connected in series with the galvanometer is very high; hence the net resistance of the voltmeter will be
\({R_V} = R + G\)
Since the resistance connected to the galvanometer in series is very high, ideally, there should not be any current flowing through the voltmeter. The overall resistance of the voltmeter will be even higher. The current through both resistor and galvanometer will be the same. Therefore, we can write for the total potential difference across points \(A\) and \(B\) as;
\(V = {i_g}R + {i_g}G\)
Hence, the resistance of the resistor that should be connected in series with the galvanometer will be;
\(\therefore R = \frac{V}{{{i_g}}} – G\)
The above equation will help us to find the resistance of the resistor which should be connected to the galvanometer in order to convert it to a voltmeter.
Just like the ammeter, the voltage sensitivity of the voltmeter will also be defined as per the number of divisions covered and the angle by which the needle gets deflected. Hence, we can have the formulas for voltage sensitivity:
\({\rm{Voltage\;Sensitivity\;}}\left( {VS} \right) = \frac{{{\rm{Number\;of\;divisions\;covered\;by\;the\;needle}}}}{{{\rm{Voltage}}}}\)
\({\rm{Voltage\;Sensitivity\;}}\left( {VS} \right) = \frac{{{\rm{Angle\;by\;which\;the\;needle\;is\;deflected}}}}{{{\rm{Voltage}}}}\)
Electrical measurements simply mean the measurement of electrical quantities or electrical parameters. From this article, we got to know about electrical measurement, why it is necessary for our daily lives, how does it affect the appliances around us, etc. We also came to know about the various classification of electrical instruments. Then we studied some basic instruments like moving coil galvanometer and how to convert it into ammeter and voltmeter and some important equations related to it.
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Q.1. What are absolute instruments?
Ans: Absolute instruments are those instruments that can directly measure the desired quantity without any previous calibration required.
Q.2. How many types of electrical instruments are there?
Ans: Electrical instruments may have different classifications based on their functions. One of them is based on how they measure the value and give us output – analog and digital. Another is their operation – indicating, recording, and integrating. There are other methods of classification too. These are just a few of them.
Q.3. What are the examples of electrical instruments?
Ans: Some of the examples of basic electrical instruments are ammeter, voltmeter, galvanometer, energy-meter, wattmeter, etc.
Q.4. What are indicating instruments?
Ans: Indicating instruments are those instruments that can directly indicate the quantity to be measured. If analog, then the pointer will point out the value, and if digital, it will directly show the value on its screen.
Q.5. What is the difference between analog and digital instruments?
Ans: The instruments in which the readings of physical quantity vary continuously with respect to time are known as analog instruments and the instruments in which the readings of physical quantities vary discretely are known as digital instruments.
We hope this detailed article on electrical instruments helped you in your studies. If you have any doubts, queries or suggestions regarding this article, feel to ask us in the comment section and we will be more than happy to assist you. Happy learning!
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