Electricity of Class 9

Earthing means to connect the metal case of electrical appliance to the earth (at zero potential) by means of a metal wire called "earth wire". In household circuits, we have three wires, the live wire, the neutral wire and the earth wire. One end of the earth wire is buried in the earth. We connect the earth wire to the metal case of the electrical appliance by using a three-pinplug. The metal casing of the appliance will now always remain at the zero potential of the earth. We say that the appliance has been earthed or grounded.

If, by chance, the live wire touches the metal case of the electric iron (or any other appliance) which has been earthed, then the current passes directly to the earth through the earth wire. It does not need our body to pass the current and therefore, we do not get an electric shock. Actually, a very heavy current flows through the earth wire and the fuse of household wiring blows out or melts. And it cuts off the power supply. In this way, earthing also saves the electrical appliance from damage due to excessive current.


An electric appliance is provided with a three-core flexible cable. The insulation on the three wires is of different colours. The old convention is red for live, black for neutral and green for earth. The new 

International convention is brown for live, light blue for neutral and green (or yellow) for earth.

Function Wire Color
Line Brown or Coax
Ground Black (Violet in SIE)
+300 Volts (Downhole Power) Red
-300 volts (Downhole Power) Blue
+12 Volts Orange
-12 Volts Yellow
+24 Volts Red
-24 Volts Violet
120 Vac Line Voltage, "Hot" Grey
120 Vac Line Voltage, "Neutral" White
Less Standardized Wire Color
Plate Leads, FET Drain Blue
Transistor Base, Diodes Green
Transistor Base 2 Orange
Emitters, Cathode Yellow
Bias Supply White


A galvanometer is an instrument that can detect the presence of current in a circuit. The pointer remains at zero (the centre of the scale) for zero current flowing through it. It can deflect either to the left or to the right of the zero-mark depending on the direction of current.


Galvanometers are of two types:

  • Moving coil galvanometer
  • Moving magnet galvanometer 

It is used to make ammeter and voltmeter as follows:

(A) Ammeter:

Ammeter is an electrical instrument which measures the strength of current in 'ampere' in a circuit which is always connected in series in circuit so that total current (to be measured) may pass through it. The resistance of an ideal ammeter is zero (practically it should be minimum).


Conversion of Galvanometer into Ammeter

(B) Voltmeter:

It is an electrical instrument which measures the potential difference in 'volt' between two points of electric circuit. The only difference between ammeter and voltmeter is that ammeter has a negligible (approximately zero) resistance so that it may measure current of circuit passing through it more accurately giving the deflection accordingly, while the voltmeter passes negligible current through itself so that potential difference developed due to maximum current passing through circuit may be measured.

Voltmeter has a very high resistance and the resistance of an ideal voltmeter is infinite. A voltmeter is always connected in parallel.


earthing revision

Further Reading 

  1. Ohm's Law
  2. Electric Current
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