In filling the gasoline tank of an aeroplane, the metal nozzle of hose from the gasoline truck is always carefully connected to the metal of the aeroplane by a wire, before the nozzle is inserted in the tank. Explain, why?

Explanation

Since, the aeroplane and the gasoline truck usually have wheels with rubber tyres, they are insulated from the ground. Further, the service ramps are usually made of concrete and are not necessarily good conductors to the earth. Therefore, inspite of grounding metallic ropes, the aeroplane and the truck could remain charged. A spark may jump and ignite the explosive gasoline, when the metal nozzle is brought near the aeroplane. The connection of metal of the acroplane and the nozzle of the hose with a wire avoids any unbalance of charge and hence the risk of gasoline explosion reduces.