Ammeter
Introduction to ammeter:
Ammeter is a very important instrument used to measure the current flowing in a circuit. It has variety of uses in science experiments because most often many quantities are related to direct measurement of current. In this article we shall discuss ammeter and its construction. I like to share this Ammeter Definition with you all through my article.
A moving coil galvanometer designed to measure the current is called an ammeter. To measure the current in any circuit, we have to connect the galvanometer in series with that circuit. As the galvanometer coil has some resistance, its introduction in the circuit increases the total resistance of the circuit, which reduces the value of the current to some extent. This lower value of current is measured by the galvanometer when used as an ammeter. To minimise this error, the galvanometer resistance should be made very small.
Secondly, the galvanometer gives a full scale deflection even when a very small current flows through it. Hence some arrangement must be made if large currents are to be measured.
Finally, the galvanometer coil is very delicate and easily gets damaged when a larger current flows through it. Therefore it is necessary to protect it from such damage.
Construction of Ammeter from Galvanometer
All the above purposes are served, if a low resistance is connected in parallel with the galvanometer coil. Such a low resistance is called the shunt. Thus a galvanometer can be converted into an ammeter by connecting a low resistance (shunt) in parallel with its coil (Fig. 15.7).
Let G be the resistance of a galvanometer coil which gives a full scale deflection when a current / flows through it. To use the galvanometer for measuring a large current /, a shunt of low resistance S is connected across the galvanometer coil. The shunt resistance should be such that, when the current in the external circuit is /, only a part / should flow through the galvanometer coil and the remaining part should flow through the shunt. If I is the current flowing through the shunt, Please express your views of this topic physics vector problems by commenting on blog
This relation gives the part of the total current flowing through the galvanometer coil. Also it enables us to determine the proper value of the shunt, when a galvanometer, through which a maximum current / can pass, is used to measure currentsupto I.
When a proper shunt is connected across the galvanometer coil and the scale is calibrated in terms of amperes, the galvanometer is converted into an ammeter.
Example for Convertion to Ammeter
A moving coil galvanometer has a resistance of 20 Ohm and it gives full scale deflection when a current of 50 mA passes through it. How will you convert it into an ammeter to measure currents up to 20 A?
Solution : Given
G = 20 Ohm,
i = 50 mA = 50 * 10 -3 A = 0.05 A and I = 20 A
Shunt S = G * i / (I-i)
S = 20 * 50 * 10^(-3) / (20 - 0.05)
S = 20 * 0.05 / 20
S = 20 Ohm
So, a shunt of approximately 20 ohm has to be connected in parallel to convert the above galvanometer into an ammeter to measure a current of 20 A.
Ammeter is a very important instrument used to measure the current flowing in a circuit. It has variety of uses in science experiments because most often many quantities are related to direct measurement of current. In this article we shall discuss ammeter and its construction. I like to share this Ammeter Definition with you all through my article.
A moving coil galvanometer designed to measure the current is called an ammeter. To measure the current in any circuit, we have to connect the galvanometer in series with that circuit. As the galvanometer coil has some resistance, its introduction in the circuit increases the total resistance of the circuit, which reduces the value of the current to some extent. This lower value of current is measured by the galvanometer when used as an ammeter. To minimise this error, the galvanometer resistance should be made very small.
Secondly, the galvanometer gives a full scale deflection even when a very small current flows through it. Hence some arrangement must be made if large currents are to be measured.
Finally, the galvanometer coil is very delicate and easily gets damaged when a larger current flows through it. Therefore it is necessary to protect it from such damage.
Construction of Ammeter from Galvanometer
All the above purposes are served, if a low resistance is connected in parallel with the galvanometer coil. Such a low resistance is called the shunt. Thus a galvanometer can be converted into an ammeter by connecting a low resistance (shunt) in parallel with its coil (Fig. 15.7).
Let G be the resistance of a galvanometer coil which gives a full scale deflection when a current / flows through it. To use the galvanometer for measuring a large current /, a shunt of low resistance S is connected across the galvanometer coil. The shunt resistance should be such that, when the current in the external circuit is /, only a part / should flow through the galvanometer coil and the remaining part should flow through the shunt. If I is the current flowing through the shunt, Please express your views of this topic physics vector problems by commenting on blog
This relation gives the part of the total current flowing through the galvanometer coil. Also it enables us to determine the proper value of the shunt, when a galvanometer, through which a maximum current / can pass, is used to measure currentsupto I.
When a proper shunt is connected across the galvanometer coil and the scale is calibrated in terms of amperes, the galvanometer is converted into an ammeter.
Example for Convertion to Ammeter
A moving coil galvanometer has a resistance of 20 Ohm and it gives full scale deflection when a current of 50 mA passes through it. How will you convert it into an ammeter to measure currents up to 20 A?
Solution : Given
G = 20 Ohm,
i = 50 mA = 50 * 10 -3 A = 0.05 A and I = 20 A
Shunt S = G * i / (I-i)
S = 20 * 50 * 10^(-3) / (20 - 0.05)
S = 20 * 0.05 / 20
S = 20 Ohm
So, a shunt of approximately 20 ohm has to be connected in parallel to convert the above galvanometer into an ammeter to measure a current of 20 A.
