28 ELEMENTARY LESSONS ON [CHAP. i. 



instantly drop down straight. There now remains only 

 the - electricity on the knob, " bound " so long as the 

 + electricity of the glass rod is near to attract it. But 

 if, finally, the glass rod be taken right away, the - 

 electricity is no longer " bound " on the knob, but is 

 "free" to flow into the leaves, which once more diverge 

 but this time with negative electricity. 



26. The " Return-Shock." It is sometimes noticed 

 that, when a charged conductor is suddenly discharged, 

 a discharge is felt by persons standing near, or may 

 even effect electroscopes, or yield sparks. This action, 

 known as the " return-shock," is due to induction. For 

 in the presence of a charged conductor a charge of 

 opposite sign will be induced in neighbouring bodies, 

 and on the discharge of the conductor these neighbour- 

 ing bodies may also suddenly discharge their induced 

 charge into the earth, or into other conducting bodies. 

 A " return-shock " is sometimes felt by persons standing 

 on the ground at the moment when a flash of lightning 

 has struck an object some distance away. 



LESSON IV. Conditction and Distribution of Electricity. 



27. Conduction. Toward the close of Lesson I. 

 we explained how certain bodies, such as the metals, 

 conduct electricity, while others are non-conductors or 

 insulators. This discovery is due to Stephen Gray ; 

 who, in 1729, found that a cork, inserted into the end 

 of a rubbed glass tube, and even a rod of wood stuck 

 into the cork, possessed the power of attracting light 

 bodies. He found, similarly, that metallic wire and pack- 

 thread conducted electricity, while silk did not. 



We may repeat these experiments by taking (as in 

 Fig. 17) a glass rod, fitted with a cork and a piece of 

 wood. If a bullet or a brass knob be hung to the end of 

 this by a linen thread or a wire, it is found that when the 



