HOW LIGHTNING WORKS

Hokitika Guardian, 19 October 1931, Page 8

HOW LIGHTNING WORKS

FRANKLIN’S EXPERIMENT. KITE AND IRON KEY. Less than two hundred years ago nobody knew that lightning was an electric discharge, writes "Hotspot” in "Amateur Wireless.” We even read that Benjamin Franklin, American pioneer of electricity, had the queer idea that lightning was' caused by “the inflammable breath of Pyrities:” But in 1749, several years after this remarkable .pronouncement, Franklin drew many significant comparisons between IgTi tiling and electricity. He had already experimented with electricity, having foutld that electricity is easily discharged by pointed conductors. Naturally he wanted to know whether lightning behaved likewise, so he prepared the famous kite experiment. During a thunderstorm he Hew « silk kite, with an iron key tied to the anchored end of the string. As the thundercloud passed over he touched the key with the knuckle of his handand off came a fat spark! 'Having thus proved that lightning is indeed an electrical phenomenon, Franklin applied this knowledge of electricity to lightning conductors. He knew that electricity passed more readily at points ; he had proved that lightning was electricity; so he suggested that high buildings should be protected by lightning rods—as we commonly use to-day. * Even in those days America was ahead of us, for, whereas the first lightning conductor in England was fixed up in 1762, America had protected buildings by this means as from 1754! To-day we know that in addition to making a lightning conductor pointed it should have a large sur-face-area and as few bonds and twists as possible. To understand the action of a lightning discharge one must know something of the electric field. Let me explain. Suppose we have two metal plates placed parallel with each other, a fraction of an inch apart; and suppose we connect a battery to these plates ; the negative pole of the battery to one plate, the positive pole of the battery to the other prate; the result of this connection is one positively charged plate and an equally negatively charged plate, the two opposite charges being evenly distributed over the surfaces of the plates. Something'invisible is created by tb s arrangement—lines of force, stretching from one plate to the other. Between the plates an electric strain is set up. For the intervening matter is composed of atoms, each of which consists of a positive centre or microns and an encircling group of negative charges called electrons. The positive centres of atoms—nuclei —are attracted to the negative plate and the electrons of the atom, being negative, are attracted to the positive plate.

, So far so good; if the intervening material between the plates lias what is known as a high dielectric strength, the charging of the two plates will

simply set up an electric strain among the atoms of the material—will polarise them in fact—but that is all. li the difference of potential is increased it may so strain the atoms that the molecular structure of the material containing the atoms may be broken down.

When that happens there is a clash between the constituents oi the atom which, being positively charged, rush across in opposite directions. The clash is so great that heat, and sometimes fight, is generated. So we get the familiar electric spark from a condenser. For the two plates, with the intervening dielectric, comprise a condenser.

What has' all this to do with lightning? Everything, for Nature performs a similar sparking process when \\o see lightning. A cloud may become highly charger], either positively or negatively, and acts like one of the piates of a condenser. The earth acts as the other plate!