LESSOXS IN ELECTRICITY. 163 



lated, will answer. Let a small weight suspended from a silk string 

 rest on one end of the conductor, and hold your rubbed glass rod 

 near the other end. You can predict beforehand what will occur 

 when you remove the weight. It carries away with it electricity, 

 which repels rubbed glass, and which attracts your balanced lath. 



Stand on an insulating stool: make one, if necessary, by placing 

 a board on four warm tumblers. Present the knuckles of your right 

 hand to the end of the balanced lath, and stretch forth your left arm. 

 There is no attraction. But let a friend or an assistant bring the 

 rubbed glass tube over the left arm ; the lath immediately follows 

 the right hand. 



While matters continue thus, touch the lath, which I suppose to 

 be uninsulated ; the " attractive virtue," as it was called by Gray, dis- 

 appears. After this, as long as the excited tube is held over the arm 

 there is no attraction. But when the tube is removed the attractive 

 power of the hand is restored. Here, you will at once comprehend, 

 the first attraction was by positive electricity driven to the right hand 

 from the left, and the second attraction by negative electricity, liber- 

 ated by the removal of the glass rod. 



Stand on an insulating stool, and place your right hand on the 

 electroscope : there is no action. Stretch forth the left arm and per- 

 mit an assistant alternately to bring near, and to withdraw, an excited 

 glass tube. The gold-leaves open and collapse in similar alternation. 

 At every approach, positive electricity is driven over the gold-leaves ; 

 at every withdrawal, the equilibrium is restored. 



I will now ask you to charge your Dutch gold electroscope posi- 

 tively by rubbed gutta-percha, and to charge it negatively by rubbed 

 glass. A moment's reflection will enable you to do it. You bring 

 your excited body near : the same electricity as that of the excited 

 body is driven over the leaves, and they diverge by repulsion. Touch 

 the electroscope, the leaves collapse. Withdraw your finger, and 

 withdraw afterward the excited body : the leaves then diverge with 

 the opposite electricity. 



The simplest way of testing the quality of electricity is to charge 

 the electroscope with electricity of a known kind. If, on the approach 

 of the body to be tested, the leaves diverge still wider, the leaves and 

 the body are similarly electrified. The reason is obvious. 



The wealth of knowledge, and of interest, which these experiments 

 involve, may be placed within any boy's reach by the wise expendi- 

 ture of half a crown. 



Once firmly possessed of the principle of induction and versed in 

 its application, the difficulties of our subject will melt away before us. 

 In fact, our subsequent work will consist mainly in unraveling phe- 

 nomena by the aid of this principle. 



Without a knowledge of this principle we could render no account 



