in Energy, the Poiver of Nature 47 



great, electricity will overcome the greatest resistance of 

 glass or air. The energy expended by electricity in over- 

 coming resistance is changed directly into heat or light 

 vibrations, as in the case of an electric glow-lamp. 



78. Disruptive Discharge. When the amount of 

 electricity on the surface of a small body increases, the 

 potential rapidly rises, and a transference of electricity 

 takes place along the path that offers least resistance. 

 With high potential, electricity can force its way across an 

 interval of air, and as the resistance of air is very great 

 much of the electrical energy is transformed into heat in 

 the process, and the particles of air are set in such violent 

 vibration that they become luminous. Such a transfer- 

 ence is called a disruptive discharge, or when it occurs in 

 Nature a flash of lightning. 



79. Magnetism. An oxide of iron which exists natur- 

 ally in considerable quantities has the power of attracting 

 to itself pieces of iron, this attractive force being much 

 more powerful than gravitation. When a bar of this 

 mineral is cut, and so uniformly shaped that no difference 

 in appearance can be found between its two ends, the ends 

 still differ, much as the right hand differs from the left. If 

 the bar be balanced on a pivot it will turn and come to 

 rest with one end pointing toward the north. On this 

 account the mineral is called the lodes tone. If two similar 

 bars are balanced in this- way the north-seeking end of 

 each can be found and marked. The effect of one such 

 lodestone on another emphasises the difference between 

 the two ends. If the north-seeking end of a lodestone 

 is brought near the south-seeking end of another which is 

 balanced the latter is strongly attracted,* but if brought 

 near the north-seeking end of the balanced lodestone there 

 is as strong repulsion. The property of two-endedness in 

 bodies outwardly similar is called polarity, and the ends 

 are termed poles. The rule of magnetic attraction and 

 repulsion is very simple Unlike poles attract, like poles 

 repel. The lodestone imparts all its properties to steel 

 when rubbed upon a bar of that metal, and such steel bars 

 are then termed magnets. 



