130 FIRST YEAR COURSE IN GENERAL SCIENCE 



in an electrical machine, it finally discharges itself that 

 is, it passes through the air to the nearest body. When 

 the discharge takes place, a sound is heard, varying from 

 a slight snap to a loud cracking 'sound. If a sheet of 

 paper is held between the conductors, a hole like a needle 

 prick shows where the electricity tore its way through the 

 paper. 



A flash of lightning is a great electric spark. Elec- 

 tricity, having accumulated upon cloud particles, passes 



FIG. 64. AN ELECTRICAL MACHINE 



Electricity, which is on the surface of revolving glass plates, passes by 

 metallic conductors to knobs at the front of the machine. If these knobs 

 are near together, a succession of sparks crosses the space between them so 

 rapidly as to seem continuous. If they are several inches apart, the dis- 

 charges are less frequent and more violent. They are like flashes of light- 

 ning in miniature. 



from cloud to cloud or from clouds to the earth. As it 

 tears its way through the air, it causes a noise whose echoes 

 reverberate, making a succession of crashes or rolling sounds 

 which we call thunder. The flash of the electric discharge 



