Ambulances have chains attached beneath them to ensure the vehicles are constantly grounded. The air contains many electrons that move from areas of high density to areas of low density. As the ambulance comes into contact with these electrons, static-electricity risk increases.
By suspending chains under the vehicle, the charged electrons move to the less-charged ground below. This effectively protects the vehicle from static electricity. Other vehicles that have these chains are fire engines and large trucks that carry tanks containing petroleum products. Some people have erroneously believed that the chains are meant for towing or for warning people and animals to clear the way.