How easy it is to measure electrical quantities? An electric quantity is any physical or chemical property which depends on the value of an electric charge. Electric quantities are measured by using instruments called ammeters, voltmeters, ohmmeters and galvanometers. These four instruments are used for measuring different quantities. Let’s find out how these instruments work. Ammeter: An ammeter measures current, which is the rate at which charge flows through a circuit. It also tells us how much charge flows in one second when one ampere current flows for one second (1A = 1C). Voltmeter: A voltmeter measures voltage. It is an instrument for measuring the potential energy (the electric energy) in every point of a circuit. Ohmmeter: An ohmmeter measures resistance. It is a very important instrument for measuring the electric current through a conductor in a circuit. It helps us to measure how much resistance there is in a current path or what percentage of the total current has been lost in the resistance of the circuit. Galvanometer: A galvanometer measures electric field strength which is also called electromotive force (EMF). It is one of the instruments used to measure the voltage in a circuit. To know more about these, read on. When you move a piece of wire inside a magnetic field, the wire experiences an induced emf which is proportional to current flowing through it. The induced emf depends on many things like nature of wire, number of turns in the coil, strength of the magnetic field etc. When you connect this coil to a galvanometer (a sensitive instrument for measuring electric current), it shows deflection when current flows through it. The galvanometer can be considered as part of an ammeter when then when there is no external electromotive force (ohm’s law). However when there is external EMF (i. e. voltage is applied across it), then galvanometer acts as a voltmeter. A simple circuit to find out the relationship between resistance and current is shown in figure. When you apply an EMF across a series R-L circuit, the current flow through the circuit starts increasing initially but then reaches at a constant value called as terminal value of current. At this stage, there are two important points to note: However, there are some other points which must be noted while using ohmmeter or ammeter to measure resistances or quantities respectively also it should be carefully connected with the circuit. The most important of them is the voltage source should be having a steady value of EMF. Otherwise, the values measured by these instruments will not be correct or meaningful. Therefore, all electrical measurements must be done with a constant voltage source. The second point is that these instruments should be properly connected to the circuit being measured. Incorrect connection can give false results which may even harm the instrument itself. Now we have come to know how they work and how to connect them with a circuit using ohmmeter, ammeter and galvanometer. In the next reading you will learn about their working in detail through experiments so you can understand electrical measurements better.
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