Measurement of Current

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Measurement of Current

The most common method of measuring electric current is to connect an ammeter in series with the circuit that’s electric current to be measured. This is so because; the entire electric current flowing through the circuit must also flow through the ammeter also. The ideal internal resistance or impedance of an ammeter is zero. Hence, ideally there is no voltage drop across the ammeter connected in the circuit. A conventional analog ammeter consists of a electric current coil. Whenever electric current flows through this coil, it deflects from its position depending upon the amount of electric current flowing through it. A pointer is attached to the coil assembly; hence it points the electric current reading on the dial of the ammeter. For measuring alternating current, clip on meter or tong tester can also be used instead of conventional ammeter. In this ammeter a current transformer core is attached to the meter which can easily be clipped on the live electric current carrying conductor. Due to this arrangement, electric current in the circuit transforms to the secondary of the CT and this secondary electric current then measured on the dial of clip on meter without disturbing the continuity of the electric current unlike conventional ammeter.

In the early days, it was thought that the electric current is, flow of positive charge and hence electric current always comes out from the positive terminal of the battery, passing through the external circuit and enters in the negative terminal of the battery. This is called conventional flow of current. On the basis of this conception, all the theories of electricity, formulas, and symbols were developed. After the development of atomic nature of matter, we have come to know, that actual cause of electric current in a conductor is due to movement of free electrons and electrons have negative change. Due to negative charge, electrons move from the negative terminal to the positive terminal of the battery through the external circuit. So the conventional flow of current is always in the opposite direction of electrons flow. But it was impossible to change all the previously discovered subsequent rules, conventions, theories and formulas according to the direction of electrons flow in the conductor. Thus the concept of conventional electric current flow was adopted. The true electron flow is used only when it is necessary to explain certain effects (as in semiconductor devices such as diodes and transistors). Whenever we consider the basic electrical circuits and devices, we use conventional flow of electric current i.e. electric current flowing around the circuit from the positive terminal to the negative terminal.