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. _=223. Aurora Borealis=_, also called Northern Lights, are luminous effects often seen in the north. They often occur at the same time with magnetic storms, at which times telegraph and telephone work may be disturbed. The exact cause of this light is not known, but it is thought by many to be due to disturbances in the earth's magnetism, caused by the action of the sun. CURRENT ELECTRICITY. PART III.--CURRENT ELECTRICITY. CHAPTER XIV. CONSTRUCTION AND USE OF APPARATUS. =Note.=--Before taking up the study of cells and the electric current, let us perform a few experiments in order to understand the construction and use of some of the apparatus needed for such study. A dry cell will be used as the source of the electricity for these first experiments, because it is convenient. You will understand its action later. Use this cell only as directed; improper use of it might spoil it. [Illustration: Fig. 61.] =EXPERIMENT 101. To study the effect of the electric current upon the magnetic needle.=