204 



SCIENTIFIC RECREATIONS. 



which are good and bad conductors are good and bad conductors of electricity. 

 Water is a good conductor, air is a bad one ; were it otherwise, electricity 

 would escape from the ground into the air ; as it is, the air manages in some 

 degree to retain the electricity at the surface of bodies, for it is on the surface 

 that we find the electric " fluid." 



We have mentioned electrical induction in a former experiment with 

 the tea-tray. We will now explain it more fully, as a consideration of it will 

 bring us to the electric spark, or lightning, with the account of the discovery 

 of the Conductor and the Electrical Machine. 



Let us look at the illustration next below. A B is a cylinder 

 supported on a glass rod, and at each extremity is a small pith ball, a and b. 

 The cylinder is in a neutral condition, as is evidenced at first by the pellets 

 being in a vertical position. But suppose we bring a ball, c, towards the 



Fig. 207. Electrical induction. 



cylinder, c is charged with positive electricity, which attracts the negative 

 to itself, and so repels the positive away at the opposite side. So the pellet 

 at one side will be attracted to c, and the other will fly in an opposite 

 direction. 



Let us take another illustration. Here we have a horizontal metal rod, 

 c c\ insulated on a glass stand. Two balls of 

 cork are attached at both ends of the rod by 

 metallic wires. Hold a rod of resin, r, which 

 has been made negatively electrical, and apply 

 it -to one pair of the cork balls. The positive 

 electricity will be attracted at d, and the negative 

 repulsed, and fly away at c. If we remove the 

 resin the equilibrium will be again established, 

 and the balls will fall to a vertical position. 

 We can also by drawing off the negative electricity by the finger at c,, 

 while the resin rod is still held to the other side, c, fill the whole of 

 the metal rod with the positive electricity when the finger and the resin 

 have been removed respectively first and last. The balls will then fly in 

 opposite directions again, in consequence of the repulsion exercised by the 

 positive poles. 



Fig. 208. Induction. 



