LESSONS' IN PHYSICS. 89 



machine and sometimes separate. A common form of condenser 

 is the Ley den jar, which is a wide-mouthed jar covered inside and 

 out with tin foil for about two-thirds its height. Through the 

 wooden stopper passes a brass rod, terminating at the upper 

 extremity in a ball and at the other in a chain, which rests on 

 the bottom of the jar. The condenser in this case consists of two 

 good conductors separated by glass, a poor conductor, or insula- 

 tor. The jar is charged by connecting the inner coating with the 

 electrical machine, the other with the earth, the electricity inside 

 the jar holding or developing the opposite kind on the outside of 

 the jar by induction. The jar may be discharged by connecting 

 the two coatings of tin foil by holding the jar in one hand and 

 touching the ball with the other, which maybe done with safety if 

 the jar is a small one, or by connecting them by a wire attached 

 to a glass handle. 



It has been found that smooth bodies without angles or 

 points retain electricity best. It is usually impossible to charge 

 the condenser of an electrical machine if it has points or angles, 

 as electricity passes away so quickly through them, but in 

 such cases it passes away very quietly, while in the case of 

 a smooth body the electricity is retained until the accumula- 

 tion is so great that disastrous results are liable to follow its 

 discharge. 



The electricity of the atmosphere is of the same nature as 

 the frictional electricity. The air is usually positively elec- 

 trified, while the earth is negatively electrified. It is stronger 

 in summer than in winter, and during the day than in the 

 night. During cloudy weather, and on the approach of thun- 

 der storms the electrical condition of the air changes frequently 

 and rapidly. Clouds separated by dry air become Leyden jars 

 or condensers, and the electricity bursting across through the 

 non-conducting air causes lightning and thunder and often rain ; 

 again the clouds and the earth separated by dry air form a con- 

 denser, and while electricity is streaming toward the clouds 

 from every angle and point, from every tree and shrub, in some 

 places the accumulation becomes so great that it bursts through 



