measuring current on certain troubleshooting jobsis required. You will need to know how much value of current is flowing in part of a circuit. Measuring the current establishes part of the basis for analyzing the circuit which can lead you to the cause of the problem. To measure the current, you use an ammeter.
Two basic kinds of meters are in common use. The simplest kind to read is the modern digital multimeter. Digital multimeter displays values as numerals. This instrument can measure current, resistance, and potential difference, you select the kind of measurement and the range by means of controls on the front of the instrument.
Current measurement can only be done you must select knob on Ampere (A). Measuring current of any electrical component requires the probes of meter to connect across it.
D’Arsonval Movement Construction
The older kind of meter has an electro-mechanical movement, called a D’Arsonval movement, that displays values. This movement works by making use of the relationship between electricity and magnetism.
The basic construction of the D’Arsonval movement.
The movement is designed so that a small current flow through a coil of wire suspended in a strong magnetic field. The magnetic field exerts force on the electrons as they move through the coil. The force is in opposite directions in the two sides of the coil. Thus, the two forces, acting on opposite sides of the coil, produce the same twisting effect on the coil.
The coil is suspended so that it can rotate in response to the twisting effect of the magnetic field. But in order to rotate, the coil must partly wind up a small spring. The spring exerts another twisting effect on the coil. This effect opposes the twisting effect of the magnetic field. The more the coil rotates, the more it must wind up the spring, increasing the twisting effect of the spring.
The coil reaches a position where the twisting effect of the magnetic field exactly balances the twisting effect of the spring. This position serves as a measure of the amount of current flowing through the coil. A pointer attached to the coil shows how far the coil has rotated in relation to a fixed scale. The scale can indicate current, resistance, on potential difference, depending on how the meter is connected to the circuit and to other devices inside. The scale can be calibrated in amperes, ohms, or volts
D’Arsonval ammeter for measuring current
In an ammeter, most of the current flowing through the meter goes around the meter movement. It goes through a special resistor, called the shunt. The resistance of the shunt is quite low. Yet it is precisely set, so that a known fraction of the current flows through the D’Arsonval movement. The accuracy of the shunt resistor determines the accuracy of the readings on the ammeter.
The resistance of the shunt determines the sensitivity and the range of the ammeter. The higher resistance of the shunt, the greater the fraction of the current that flows through the meter movement. Increasing this fraction increases the sensitivity of the meter and reduces the amount of current required to produce a full-scale deflection of the meter movement. In most ammeters, you can turn a rotary switch to select any one of several shunts. In this way, you can choose the sensitivity and the range of the meter.