THE CONQUEST OF NATURE 



where can be made manifest. In so doing we shall 

 discover that there are varying types of electricity, yet 

 that these have a singular uniformity as to their essen- 

 tial properties. As usually divided and the classifica- 

 tion answers particularly well from the standpoint of the 

 worker electricity is spoken of as either statical or 

 dynamical. The words themselves are suggestive of 

 the essential difference between the two types. Statical 

 electricity produces very striking manifestations. We 

 have already spoken of it as theoretically due to the 

 conditions of the electrons at rest. It must be under- 

 stood, however, that the statical electricity will, if given 

 opportunity, seek to escape from any given location to 

 another location, under certain conditions, somewhat 

 as water which is stored up in a reservoir will, when 

 opportunity offers, flow down to a lower level. The pent- 

 up static electricity has, like the water in the reservoir, 

 a store of potential energy. The physicist speaks of it as 

 having high tension. In passing to a condition of lower 

 tension, the statical electricity may give up a large por- 

 tion of its energy. 



If, for example, on a winter day in a cold climate, you 

 walk briskly along a wool carpet, the friction of your 

 feet with the carpet generates a store of statical electric- 

 ity, which immediately passes over the entire surface of 

 your body. If now you touch another person or a 

 metal conductor, such as a steam radiator or a gas pipe, 

 a brilliant spark jumps from your finger, and you ex- 

 perience what is spoken of as an electrical shock. 

 If the day is very cold, and the air consequently very 

 dry, and if you will take pains to rub your feet vigor- 



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