814 



"LESSONS tff ELECTRICITY. 



Well - pointed lightning conductors, 

 when acted on by a thunder cloud, dis- 

 charge their induced electricity against 

 the cloud. Franklin raw this with great 

 clearness, and illustrated it with great in- 

 genuity. The under side of a thunder 

 cloud, when viewed horizontally, he 

 observed to be inched, composed, in 

 fact, of fragments one below the other, 

 sometimes reaching near the eaith. 

 These he regarded is so many stepping- 

 stones Y/hich assist in conducting the 

 stroke of the cloud. To represent those 

 by experiment he took two or three locks 

 of fine loose cotton, tied them ir, a row, 

 and hiing them from his pr.' n .ic con- 

 ductor. When this was excited the locks 

 stretched downward toward tho earth ; 

 but by presenting a sharp point erect 

 under the lowest bunch of cotton, it 

 shrunk upward to that above it, nor did 

 the shrinking cease till all the locks had 

 retreated to the prime conductor itself. 

 " May not," SMVS Franklin, "the small 

 ek'Ctrified cloud, whose equilibrium with 

 the earth is so soon restored by the point, 

 rise up to the main body, and by that 

 means occasion so large a vacancy that 

 the grand cloud cannot strike in that 

 placet" 



19. History of the Ley den Jar. The 

 Ley den Battery. 



The next discovery which we have to 

 master throws all former ones into the 

 shade. It was first announced in a letter ad- 

 dressed on the 4th of November, 1V45, 

 to Dr. Liebcrkuhn, of Berlin, by Kleist, 

 a clergyman of Cammin, in Fomerania. 

 By means of a cork, c, fig. 34, he fixed a 

 r.ail, N, in a phial, G, into which he had 

 poured a little mercury, spirits, or water, 

 w. On electrifying tho nail he was ablo 

 to pass from one room into another wit: 

 the phial in his hand nd to ignite spirits 

 of wine with it. " If," said he, * 4 while 

 it is electrifying I put my finger, or a 

 piece of gold which I hold in my hand, 

 to the nail, I receive a shock which stuns 

 my arms and shoulders." 



In the following year Cunaeus of Ley- 

 den made substantially the ssme discov- 

 ery. It caused great wonder #nd dread, 

 which arose chietly from the excited im- 

 agination. Musschcnbroekfclt the shock, 

 and declared in a letter to a friend that 



lie would not take a second one for th 

 crown of France. Bleeding at the nose, 

 ardent fever, a heaviness of head which 

 endured for days, were all ascribed to the 

 shock. Boze wished that he might die 

 of it, so that he might enjoy the honor 

 of having his death chronicled in the 

 Paris " Academy of Sciences." Kleist 

 missed the explanation of the phenome- 

 non ; while the Leyden philosophers cor- 

 rectly stated the conditions necessary to 

 the success of the experiment. Hence 

 tlio paial received the name of the Ley- 

 den phial, or Leyden jar. 



Tho discovery of Kleist and Cunasus 

 excited the most profound interest, and 

 the subject was explored in all directions. 

 Wilson in 174G filled a phial partially 

 with water, and plunged it into water, so 

 as to bring the water surfaces, within 

 and without the phial, to the same level. 

 On charging such a phial the strength of 

 the shock was found greater than had 

 oeen observed before. 



Two years subsequently Dr. Watson 

 And Dr. Bevis noticed how the charge 

 grew stronger as the area of the conduct- 

 or in contact with the outer surface of 

 the phial increased. They substituted 

 shot for water inside the jar, and ob- 

 tained substantially the same effect. Dr. 

 Bevis then coated a plate of glass on both 

 sides with silver foil, to within about an 

 inch of the edge, and obtained from it 

 discharges as strong as those obtained 

 from a phial containing half a pint of 

 water. Finally Dr. Watson coated his 

 phial inside and out with silver foil. By 

 these steps the Leyden jar reached the 

 form which it possesses to-day. 



It is easy to repeat the experiment of 

 Dr. Bevis. Procure a glass plate nine 

 inches square ; cover it 0:1 both sides, as 

 he did, with tin-foil seven inches square, 

 leaving the rirn uncovered. Connect one 

 side with tho earth, and the other with 

 tho machine. Charge and discharge : 

 you obtain a brilliant spark. 



In our experiment with the Golden 

 Fish. (fig. 33), we employed a common 

 form of the Leyden j:',r, only with the 

 difference that to get to a cufiicient dis- 

 tance from tho glass, so as to avoid the 

 attraction of the fish by the jaritsolf, the 

 knob was placed higher than usual. But 

 with a good flint-glass tumbler, a piece of 

 tin-foil, akd a bit of stout wire, you can 



