The flash is practically instantaneous. The direction taken seems to depend upon the conditions of the surrounding air. It has generally a zigzag motion, and is then called _chain lightning_. The air in the path of the electricity becomes intensely heated; it is this effect, and not the electricity which we see. In hot weather flashes are often seen which light up whole clouds, no thunder being heard. This is called _heat lightning_, and is generally considered to be due to distant discharges, the light of which is reflected by the clouds.
The _potential_ of the lightning spark is beyond all calculation. The spark jumps through miles of air, which is, really, an insulator. This spark represents billions and billions of volts.
_=219. Thunder=_ is caused by the violent disturbances produced in the atmosphere by lightning. The nature of the sound depends, among other things, upon the distance of the observer from the discharge, and upon the length and shape of the path taken. Clouds, hills, and other objects produce echoes, which also modify the original sound. It takes nearly five seconds for the sound to travel one mile.
_=220. Lightning-Rods=_, when well constructed, often prevent violent discharges of atmospheric electricity. They have pointed prongs at the top, which allow the negative of the earth (which is being attracted by the cloud when it is positively charged) to pass quietly into the air above; this neutralizes the cloud. In case of a discharge, the rods aid in conducting the electricity to the earth.
_=221. Causes of Atmospheric Electricity.=_ There are several theories about this. Some think that it is due to the rotation of the earth, different parts being acted upon differently by the heat of the sun.
Some claim that the evaporation of the water in the ocean produces it, while others say that the electrification is produced in the clouds by the friction of their particles upon each other. The matter has not been settled.
_=222. St. Elmo's Fire=_ Electrification from the earth is often drawn up through the masts of ships to neutralize that in the clouds (see § 220), and, as it escapes from the points of the masts, light is produced. This may be clearly shown by repeating Exp. 71 in the dark; the head of the pin (Fig. 39) will represent the end of a mast, and the charged electrophorus cover will be the charged cloud.