126 



ELEMENTARY GENERAL SCIENCE 



CHAP. 



scope of our subject. The following experiment will show that 

 we are right in regarding electrification as a manifestation of 

 energy : 



EXPT. 119. Suspend a pith-ball by a silk thread to a bent 

 wire, as is shown in Fig. 52. Rub a rod of sealing-wax with 

 fur and touch the pith -ball with the rod. Notice that after 

 the contact it is impossible to make the ball come to the rod. 

 They repel one another. 



EXPT. 120. Having touched the pith-ball with the rod of 

 sealing-wax, which has been rubbed with fur as in the last 

 experiment, bring near to it a rod of glass Avhich has been 

 rubbed with dry silk. Notice that the ball is attracted towards 

 the glass rod. 



What is the significance of these experiments ? In both 

 cases the pith-ball moves through a certain distance under the 



influence of a force, in one case 

 of repulsion, in the other of at- 

 traction, and in consequence work 

 is done. Under certain circum- 

 stances, as in the discharge of 

 a Leyden jar, the energy of elec- 

 trification becomes manifest in 

 the form of a vivid spark and a 

 slight explosive sound. 



Electricity in motion consti- 

 tutes what is known as the elec- 

 tric current, and of its capability 

 of doing work the student has 

 abundant evidence in the heat 

 and light of an incandescent elec- 

 tric lamp, where the passage of 

 the current through a wire, which 



offers considerable resistance to its passage, causes the wire to 

 become sufficiently hot to be utilised as a source of light. 



It will be very instructive to consider briefly the case where 

 the electric current is formed as the result of chemical action in 

 a battery, and thence passed by wires to a lamp of the kind 

 mentioned. This is the ordinary condition of things as already 

 described ; but imagine the lamp left out and the battery made 



FIG. 52. Electrical Attraction. 



