LESSONS IN ELECTRICITY. 



31$ 



lower one, is sometimes called ** conden- 

 sation. ' ' The electricity on one plate or 

 sheet was figured ai squeezed together, 

 ^r condensed, by the attraction of the 

 other. A special instrument called a 

 condenser is constructed by instrument 

 makers to illustrate the action here ex- 

 plained. 



You may readily make a condenser for 

 yourself. Take two circles, p p', fig. 41, 

 of tin or of sheet zinc, and support the one, 

 p', by a stick of sealing-wax or glass, G : 

 the other, p, by a metal stem, connected 

 with the earth. The insulated plate, p', 

 is called the collecting plate ; the unin- 

 sulated one, P, the condensing plate. 

 Connect the collecting plate with your 

 electroscope by the wire w, and bring tha 

 condensing plate near it, leaving, how 

 ever, a thin space of air between them. 

 Charge the collector, p', or the wire, w, 

 with your glass rod, until tho leaves of 

 the electroscope begin to diverge. 

 Withdraw the condensing plate, the 

 leaves fly asunder ; bring tho condensing 

 plate near, the leaves again collapse. 



Or vary your constraction, Rnd make 

 your condenser thus. Employing the 

 table, or a sheet of foolscap if the table 

 V>e an insulator, as one plate of the con- 

 denser, sproad upon it the sheet of india- 



rubber, P, fig. 42, and lay upon the 

 rubber the sheet of block-tin, A B. Con- 

 nect the tin by the wire, w, with the 

 electroscope, T. Impart electricity to 

 the little weight, A, till the leaves, L, W- 

 gin to diverge ; then lift the tin plate by 

 ks two silk loops ; tho leaves at once fly 

 csander. 



Finally, show your complete knowledge 

 of the Ley den jar, ami your freedom 

 from the routine of the instrument makers, 

 by making a "jar" in the following 

 novel way. Stand upon a board sup- 

 ported by warm tumblera. Hold in your 

 right hand a sheet of vulcanized india- 

 rubber, and clasp, with it between you, 

 tho left hand of a friend in connection 

 with the earth. Place your left hand on 

 tho conductor of the machine, and let it 

 bo worked. You and your friend sooa 

 feel a crackling and a tickling of the 

 hands, due to the heightening attraction 

 of the opposite electricities across the in- 

 dia-rubber. The " hand- jar" is then 

 charged. To discharge it you have onJy 

 to bring your other hands together : ths 

 shock of the Leyden jar is then felt and 

 its spark seen and heard. 



By the discharge of the hand-jar you 

 can tire gunpowder. But this will be re- 

 ferred to more particukirly further OB. 

 (See 25.) 



23. Seat of Charge in the Ley den Jar. 



Franklin sought to determine how tho 

 charge was hidden in the Leyden jiir. 

 He charged with electricity a bottle half 

 filled with water and coated on the out- 

 side with tin-foil ; dipping the finger oJ 

 one hand into the water, and touching 

 the outside coating with the other, he 

 received a shock. He was thus led to 

 inquire, Is the electricity in the water I 

 He poured the water into a second bot- 

 tle, examined it, and found that it had 

 carried no electricity along with it. 



His conclusion wa3 '* that the electric 

 fire must either have been lost in tho d&- 

 canting, or must have remained in the 

 bottle. The latter he found to be true ; 

 for, filling the charged bottle with fresii 

 water, he obtained the shock, and vrra 

 therefore satisfied that the power of giv- 

 ing it resided in the glass itself."* 



* Priestley's " History of Electricity," 

 Sa edition, p. 149 



