GALVANISM. 



effects also are tremendous. A single jar, of 

 moderate size, is sufficient to give a shock to a 

 whole regiment. The men have simply to join 

 hands and form a ring, the first man holding a jar, 

 and the last man touching its knob. All will feel 

 the shock at the same instant, so rapid is the rate 

 of discharge. Indeed, the velocity of electricity, 

 or of electric discharge, is almost incredible. 

 Wheatstone, by a very ingenious method, has 

 calculated that it can be no less than 288,000 

 miles a second that is, it might flash round our 

 earth in one-tenth of a second. More recent 

 experiments and calculations, however, bring the 

 velocity much nearer to that of light, which is 

 about 190,000 miles a second. 



ATMOSPHERIC ELECTRICITY. 



Lightning. Franklin was the first to identify 

 the lightning flash with the electric spark. By his 

 famous kite experiment, he found that the thunder- 

 cloud acts just like the charged conductor of an 

 electric machine. The same disruptive, burning, 

 and luminous effects are common to both, though 

 in vastly different degrees. There are several 

 kinds of flashes. First, there is the zigzag flash, 

 or forked-lightning, which appears as a broken 

 line of light, bright, thin, and sharp at the edges. 

 It may pass between clouds, or it. may dart from 

 the clouds to the earth. Often it splits up into 

 branches as it approaches the earth, and they may 

 be sometimes several thousand feet apart. Its 

 zigzag form is owing to the resistance of the air. 

 Second, there is the large indefinite blaze of sheet- 

 lightning. It seems to spread over a large space, 

 and is not so intense as forked-lightning. It is 

 ascribed to an electric discharge within the clouds 

 themselves, which illuminates their mass for a 

 moment. When it occurs at a great height or 

 distance, no thunder is heard, and a vague flash 

 passes across our field of view. Such is common 

 in summer evenings. A third and less frequent 

 form is that of ball-lightning, which is perhaps 

 rather a meteor than an electric phenomenon. It 

 is said to occur in this way. After a violent 

 explosion of lightning, a ball is seen bounding like 

 a bomb to the earth. When it reaches the ground, 

 it either splits up at once and disappears, or it 

 rebounds like an elastic ball before doing so. It 

 may last as long as ten seconds, and is thus very 

 different from the flash, which has been proved to 

 last less than one ten-thousandth of a second. It 

 is very dangerous, and readily sets fire to any 

 building in its way. 



Thunder. The snap of the electric spark is 

 heard on a grand scale in the thunder-clap, which 

 accompanies the lightning. Whether it is due to 

 the disruptive scattering of the air particles in the 

 line of discharge, or to the rushing in of air to fill 

 up the vacuum produced, is not quite clear. Nor 

 can the prolonged rolling, the strange rising and 

 falling of the peal, be properly accounted for. 

 When the thunder is near, it is short and sharp, 

 though the same report may have been heard as 

 a long roll at a distance. The echoes sent between 

 the clouds and the earth may account for this 

 rising and falling to some extent, but not fully. 

 Some assign the cause to the zigzag path of the 

 flash, forming a series of centres of sound, not all 

 in a line. The waves of sound may alternately 

 swell into one roar, or meet and weaken each 



other. Thunder is not so loud as we are apt to 

 fancy it. It has never been heard more than 14 

 miles from the flash; The cannonading at Water- 

 loo was heard at Creil, in the north of France, 

 about 115 miles from the field. 



Lightning-conductors. Electricity, like any 

 other force, always takes the easiest route. We 

 have seen that, if we hold a pointed rod of metal 

 to the prime conductor of a machine, its electricity 

 will be all drawn silently off to the ground, as fast 

 as it is produced. In the same way, if we present 

 a pointed rod near to a charged cloud, its elec- 

 tricity will pass silently and harmlessly to the 

 earth. This is the principle of the lightning- 

 conductor. It is a pointed rod of copper or 

 galvanised iron, reaching from 8 to 30 feet above 

 a building, and carried down to the earth, in 

 which it is carefully buried. The part above the 

 building is called the rod, and the rest the con- 

 ductor. They must be very carefully constructed, 

 otherwise their presence is the opposite of pro- 

 tective. It is led down two feet or so into the 

 ground, and then turned away into a well or water, 

 if possible, or else into a drain filled with charcoal 

 for 12 or 1 6 feet. In large buildings, there are 

 several rods all connected to one common con- 

 ductor. In ships, there is one on each mast, 

 leading to the metal of the keel When properly 

 made, they are, beyond all doubt, sufficient pro- 

 tection from the ravages of lightning. 



Ordinary Electricity of the Atmosphere. In all 

 kinds of weather, clear as well as cloudy, the air 

 is more or less electrified. With a clear sky, it 

 is always positive, and more or less at different 

 times during the day. It is most strongly electri- 

 cal shortly after sunrise, and again shortly after 

 sunset. It is least when the heat of the day is 

 greatest, and also just before sunrise. These 

 changes are explained by the effect of the sun's 

 rays on the moisture present in the air. In cloudy 

 weather, it is sometimes positively, sometimes 

 negatively, electrical. Rain, snow, hail, &c. when 

 caught on an electroscope, are found sometimes 

 positive, and sometimes negative. The cause of 

 this electricity of the air is not satisfactorily ex- 

 plained. Some ascribe it to evaporation and 

 vegetation. Some ascribe it to induction by a 

 permanent negative electricity, resident in the 

 earth. 



GALVANISM. 



The discovery of galvanism was the beginning 

 of a new era in the history of electrical science. 

 No results of great practical value had followed 

 the study of the electricity of friction. Its nature 

 was so capricious. A constant tendency to escape 

 made it difficult to excite, and yet more difficult to 

 control ; and it found a place merely in the labor- 

 atory of the philosopher, or among the curiosities 

 of the drawing-room. But the revelation of 

 another and an easier mode of producing the ex- 

 citement soon drew the attention of the whole 

 scientific world. This new field of investigation 

 displayed an entirely novel class of phenomena ; 

 and the short period of half a century saw marvel- 

 lous and unexpected results. Frictional electricity 

 thus became of subordinate importance, while 

 \ galvanism or voltaic electricity became of the 

 highest practical interest 



