An ammeter is an instrument used for measuring electric current in units of amperes. An ammeter must be connected in series with the path of the current being measured. Send Your Questions to Experts. Invalid OTP! Resend OTP? Suppose you want 10 V to produce a full-scale deflection of a voltmeter containing a galvanometer with a sensitivity. Then 10 V applied to the meter must produce a current of. The total resistance must be.
This voltmeter would not be useful for voltages less than about half a volt, because the meter deflection would be small and difficult to read accurately. For other voltage ranges, other resistances are placed in series with the galvanometer. Many meters have a choice of scales. That choice involves switching an appropriate resistance into series with the galvanometer. Galvanometer as Ammeter The same galvanometer can also be made into an ammeter by placing it in parallel with a small resistance , often called the shunt resistance , as shown in Figure.
Since the shunt resistance is small, most of the current passes through it, allowing an ammeter to measure currents much greater than those producing a full-scale deflection of the galvanometer.
Suppose, for example, an ammeter is needed that gives a full-scale deflection for 1. Since and are in parallel, the voltage across them is the same. These drops are so that. Solving for , and noting that is and is 0. Taking Measurements Alters the Circuit When you use a voltmeter or ammeter, you are connecting another resistor to an existing circuit and, thus, altering the circuit.
Ideally, voltmeters and ammeters do not appreciably affect the circuit, but it is instructive to examine the circumstances under which they do or do not interfere. First, consider the voltmeter, which is always placed in parallel with the device being measured. Very little current flows through the voltmeter if its resistance is a few orders of magnitude greater than the device, and so the circuit is not appreciably affected.
See Figure a. A large resistance in parallel with a small one has a combined resistance essentially equal to the small one. See Figure b. The voltage across the device is not the same as when the voltmeter is out of the circuit.
An ammeter is placed in series in the branch of the circuit being measured, so that its resistance adds to that branch. However, if very small load resistances are involved, or if the ammeter is not as low in resistance as it should be, then the total series resistance is significantly greater, and the current in the branch being measured is reduced. A practical problem can occur if the ammeter is connected incorrectly.
If it was put in parallel with the resistor to measure the current in it, you could possibly damage the meter; the low resistance of the ammeter would allow most of the current in the circuit to go through the galvanometer, and this current would be larger since the effective resistance is smaller.
One solution to the problem of voltmeters and ammeters interfering with the circuits being measured is to use galvanometers with greater sensitivity. This allows construction of voltmeters with greater resistance and ammeters with smaller resistance than when less sensitive galvanometers are used. There are practical limits to galvanometer sensitivity, but it is possible to get analog meters that make measurements accurate to a few percent.
Note that the inaccuracy comes from altering the circuit, not from a fault in the meter. Making a measurement alters the system being measured in a manner that produces uncertainty in the measurement.
For macroscopic systems, such as the circuits discussed in this module, the alteration can usually be made negligibly small, but it cannot be eliminated entirely. The variable resistance R3 is adjusted until the galvanometer reads zero with the switch closed. This simplifies the circuit, allowing Rx to be calculated based on the IR drops. The potential difference between points b and d is then zero, meaning that b and d are at the same potential.
With no current running through the galvanometer, it has no effect on the rest of the circuit. So the branches abc and adc are in parallel, and each branch has the full voltage of the source.
Since b and d are at the same potential, the IR drop along ad must equal the IR drop along ab. Again, since b and d are at the same potential, the IR drop along dc must equal the IR drop along bc.
This equation is used to calculate the unknown resistance when current through the galvanometer is zero. This method can be very accurate, but it is limited by two factors.
First, it is not possible for the current through the galvanometer to be exactly zero. Second, there are always uncertainties in R 1 , R 2 , and R 3 , which contribute to the uncertainty in R x. Privacy Policy. Skip to main content.
Circuits and Direct Currents. Search for:. Voltmeters and Ammeters. Voltmeters and Ammeters Voltmeters and ammeters are used to measure voltage and current, respectively. Learning Objectives Compare circuit connection of an ammeter and a voltmeter. Key Takeaways Key Points A voltmeter is an instrument used for measuring electrical potential difference between two points in an electric circuit.
An ammeter is a measuring device used to measure the electric current in a circuit. A voltmeter is connected in parallel with a device to measure its voltage, while an ammeter is connected in series with a device to measure its current.
At the heart of most analog meters is a galvanometer, an instrument that measures current flow using the movement, or deflection, of a needle. The needle deflection is produced by a magnetic force acting on a current-carrying wire.
Key Terms shunt resistance : a small resistance R placed in parallel with a galvanometer G to produce an ammeter; the larger the current to be measured, the smaller R must be; most of the current flowing through the meter is shunted through R to protect the galvanometer galvanometer : An analog measuring device, denoted by G, that measures current flow using a needle deflection caused by a magnetic field force acting upon a current-carrying wire.
Voltmeter : Demonstration voltmeter from a physics class. Null Measurements Null measurements balance voltages so there is no current flowing through the measuring devices that would interfere with the measurement. What Do Ammeters Measure? An Ammeter is a measuring device that is used to measure the flow of electricity in the form of current in a circuit. Share it:. Write for This Channel. Related Articles 1.
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