Electrical measuring units and instruments

Any instrument which measures electrical values is called a meter. An ammeter measures the current in amperes. The unit is named after Andre Marie Ampere, a French scientist, who discovered a great number of facts about electricity over a hundred years ago. The abbrevia­tion for the ampere is amp. A voltmeter measures the voltage and the potential difference in volts. The volt is named after Alessandro Volta, an Italian scientist.

The current in a conductor is determined by two things, the voltage across the conductor and the resistance of the conductor. Every material object offers some resistance to the flow of an electron current through it. Good conductors like the me­tals, copper, silver and aluminium offer very little resistance, while nonconductors such as glass, Wood and paper

Fig. П. Circuit diagram showing the offer a very high resistance connections for an ammeter and voltmeter.

The unit by which meter resistance is measured is called the ohm. The resistance in practice is measured with the ohmmeter. A wattmeter measures electrical power in watts. Very delicate ammeters are often used for measuring very small currents. A meter whose scale is calibrated to read a thousandth of an ampere is called a milliammeter. One whose scale is calibrated in millionth of an ampere is called a microammeter or galvanometer. Whenever an ammeter or voltmeter is connected to a circuit to measure electric current or potential difference the ammeter must be connected in series and the volt­meter in parallel.

As illustrated in Fig. 11, the ammeter is so connected that all of the electron current passes through it. To prevent a change in the electron current when such an insertion is made, all ammeters must have a low resistance. Most ammeters therefore have a low resistance wire, called a shunt, connected across the armature coil. A voltmeter, on the other hand, is connected across that part of the circuit for which a measurement of the potential difference is required. If the potential difference between the ends of the resistance R is wanted, the voltmeter is connected as shown.