Hello, students, welcome to the beautiful chapter of current electricity. Students, from this module onwards we will discuss some instruments and our first instrument is galvanometer.

So, let’s start with the galvanometer. My dear friends, it is a device which is used for detection and measurement of small currents like micro ampere flowing in a circuit. I hope that you have already seen this instrument in your school laboratories. If you want to measure the current when the current is passing through this galvanometer there is some deflection in the needle that will give you the actual reading of the current flowing in the circuit. So, yes, let’s understand this instrument with some deeper concepts. My dear friends, it consists of a coil which rotates when current is passed through it. Students, we have a coil having some resistance G that is also called as the resistance of galvanometer. We have a needle attached with this coil which is pointing towards a printed scale having some lower value to higher value, minimum to maximum. The beauty is when I pass the current into this coil the needle rotates. How it rotates that is none of our business. We will learn this thing in our chapter of the magnetic field. So, now the mechanism is simple. I pass the current, the needle rotates and the angle rotated by the needle is directly proportional to the current passing through the coil. If I will pass more current the deflection shown by the needle will be more. Now, my dear friends, focus on the value of current, if I increase the value of current that deflection in the needle also increases and the current, when the deflection in the needle is maximum is called as the full scale deflection current and that is represented by the variable Ig. Students, till now that was the basic theory of galvanometer.

Now, we will use this theory to make a new instrument called as ammeter and before that galvanometer is represented by simple symbol G.

Now, let’s move towards the ammeter. Students, it is a device which is used for the detection and the measurement of large currents like milli ampere or ampere flowing in a circuit. And it is just a simple arrangement, it consists of a small resistance connected in the parallel with the galvanometer. Let’s understand first, we have this circuit of galvanometer while resistance G needle pointing on a printed scale with minimum value towards the maximum value. Now, if I pass the current Ig that is the full scale deflection current then needle will point towards the maximum value. Now, students I will put a small resistance with parallel with galvanometer. This resistance is also called as the shunt resistance. Now, students, the IG will not enter the coil, in fact it will be divided into I dash and Ig minus I dash. Now, I dash is the current that is entering the coil and it should be noted that I dash is less than Ig. Obviously Ig is divided into I dash and I dash minus Ig. So, I dash is small. If the current entering the coil is small the deflection of needle should decrease. Now, students, observe this picture. The same Ig is measured at a lesser deflection, it means I have more capability of passing the current or in a way my range of measuring the current increases and that is the beauty and the working of ammeter, it measures the large currents. Now, students, till how much time I can pass the current into this circuit. I can pass till the current entering the coil becomes Ig and the current outside is I max. So, that is the maximum ability of ammeter. And I max is that current when we have the full scale deflection in the ammeter and that is also called as the range of ammeter.

So, students, let’s have some mathematical analysis. You can see that Ig is the current going in G and I max minus Ig is the current going in R and G and R are in parallel. So, their potential differences should be equal to each other. And from this equation I can say R is Ig into capital G upon I max minus Ig. So, students, that is a very, very important result in the working of ammeter. So, kindly remember that and we have some important points. The first point is, the resistance of ammeter is small r into G upon small r plus capital G, obviously are r and G are in parallel, equilateral distances R1, R2 upon R1 plus R2. Second point the resistance of ideal ammeter should be equal to zero.

Students, in our upcoming module I will have the mathematical value of the resistance of ammeter. And you will see that it will come out to be very, very small and for ideal ammeter it should be equal to zero.

Now, the next point, ammeter is represented by the symbol capital A in a circle, remember that, that will be used in the numericals. The next point is ammeter is always connected in series with the circuit element. Obviously, if I want to measure the current, the ammeter has to be put in series, so that, that current enters the ammeter. Now, the last point, a practical ammeter always reads a lesser value than the true value of current in the circuit. Students, I will explain this point in my upcoming module with some example. So, kindly remember this point that ammeter will not give me a true reading, it will in fact give me a lesser reading than the true reading. I will show it to you with the help of some example.

Students, I hope you have understood this module. We will meet in the next module which has more concepts on ammeter. Till that time have a good day, thank you.

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