492 ELECTRICITY. 



ductor. This tumbler ia now inverted over a number of 

 pith balls lying upon the table, and the balls by an alter- 

 nate attraction and repulsion, similar to that explained at 

 length under the head of dancing images, leap up and 

 down until they have conveyed away all the electricity 

 from the glass to the table and thus to the earth. 



9. The electrical spider. Two Leyden jars are elec- 

 trified with opposite electricities, and a little image of an 

 insect, the form of the spider is generally chosen, is 

 suspended between them. The spider is attracted to 

 one, then repelled, because at the moment of contact, it 

 becomes similarly electrified ; it is then immediately at- 

 tracted to the other, to touch it, and to fly off again at 

 the instant of contact. This flying from one to the 

 other continues until the jars are discharged. 



10. The electrical bells. An apparatus is made by 

 which two bells hanging side by side, are connected, the 

 one with the prime conductor, and the other with the 

 earth, a little brass ball is suspended between them 

 which, precisely on the principle above described, 

 swings from one to the other, giving each a slight stroke 

 sufficient to emit a very distinct sound. As the clapper 

 flies across with great rapidity, and in ordinary cases 

 there are three bells, two of which are connected with 

 the conductor, the apparatus keeps up a very merry ring- 

 ing, sometimes for half an hour. 



It is said that Franklin had an apparatus of this kind 

 attached to his lightning rod by the ringing of which, he 

 was notified of the approach of thunder. 



11. Electrical powders. Perhaps the most beautiful of 

 all the experiments illustrating electrical attraction and 

 repulsion, are those in which various powders are attract- 

 ed to electrified surfaces. One of the simplest modes by 

 which the experiment can be performed is as follows. 

 Pour upon a small board with raised edges, a quantity of 

 resin. When it is cooled, it forms a smooth and level 

 surface. Touch this surface in several points with the 

 knob of a jar electrified vitreously or positively, and in 

 several other points with the knob when it is electrified 

 resinously or negatively. The knobs will communicate 

 there respective electricities to a little region around the 



