LESSONS IN ELECTRICITY. 



897 



We now use the term positive or plus 

 electricity to denote that developed on 

 glass by the friction of silk ; and negative 

 or minus electricity to denote that devel- 

 oped on sealing-wax by the friction of 

 flannel. These terms are adopted purely 

 for the sake of convenience. There is 

 no reason in nature why the resinous 

 electricity should not be called positive, 

 and the vitreous electricity negative. 

 Once agreed, however, to apply the terms 

 as here fixed, we must adhere to this 

 agreement throughout. 



10. Fundamental Law of Electric 

 Action. 



In all the experiments which we have 

 hitherto made, one of the substances op- 

 erated on has been electrified b friction, 

 and the other not. But once engaged in 

 inquiries of this description, questions 

 incessantly occur to the mind, the an- 

 swering of which extends our knowledge 

 and suggests other questions. Suppose, 

 instead of exciting only one of the bod- 

 ies presented to each other, we were to 

 vxcite both of them, what would occur ? 

 This is the question which was asked and 

 answered by Du Fay, and which wo must 

 now answer for ourselves. 



Here your wire loop, fig. 1, comes 

 again into play. Place an unrubbed 

 gutta-percha tube, or a stick of sealing- 

 wax, in the loop, and be sure that it is 

 unrubbed that no electricity adheres to 

 it from former experiments. If it fail to 

 attract light bodies, it is unexcited ; if it 

 attract them, pass your hand over it sev- 

 eral times, or, better still, pass it over 

 or through the flame of a spirit lamp. 

 This will remove every trace of electric- 

 ity. Satisfy yourself that the unrubbed 

 gutta-percha tube is attracted by a rubbed 

 one. 



Remove the unrubbed tube from the 

 loop, and excite it with its flannel rub- 

 ber. One end of the tube is held in your 

 hand and is therefore unexcited. Return 

 ihe tube to the loop, keeping your eye 

 upon the excited end. Bring a second 

 rubbed tube near the excited end of the 

 suspended one : strong repulsion is the 

 consequence. Drive the suspended tube 

 round and round by this force of repul- 

 sion 



Bring a rubbed glass tube near the ex- 

 cited end of the gutta-percha tube : 

 strong attraction is the result. 



Repeat this experiment step by step 

 with two glass tubes. Prove that the 

 rubbed glass tube attracts the unrubbed 

 one. Remove the unrubbed tube from 

 the loop, excite it by its rubber, return it 

 to the loop, and establish the repulsion 

 of glass by glass. Bring rubbed gutta- 

 percha or sealing-wax near the rubbed 

 glass : strong attraction is the conse- 

 quence. 



These experiments lead you directly to 

 the fundamental law of electric action, 

 which is this : Bodies charged with the 

 same electricity repel each other, while 

 bodies charged with opposite electricities 

 attract each other. Positive repels posi' 

 five, and attracts negative. Negative 

 repels negative and attracts positive.^ 



Devise experiments which shall still 

 further illustrate this law. Repeat, for 

 example, Otto von Guericke's experi- 

 ment. Hang a feather by a silk thread 

 and bring your rubbed glass tube near it : 

 the feather is attracted, touches the 

 tube, charges itself with the electricity of 

 the tube, and is then repelled. Cause it 

 to retreat from the tube in various direc- 

 tions. Du Fay's experiment with the 

 gold-leaf will be repeated and explained 

 further on. See 18. 



Hang your feather by a common 

 thread ; if no insulating substance inter- 

 venes between the feather and the earth, 

 you can get no repulsion. Why ? Ob- 

 viously because the charge of positive 

 electricity communicated by the rod is 

 not retained by the feather, but passes 

 away to the earth. Hence, you have not 

 positive acting against positive at all. 

 Why the neutral body is attracted by 

 the electrified one, will, as already 

 stated, appear by and by. 



PIG. 11. 



