212 Common Science 



flows in through one wire and out through the other, the 

 next instant flowing the opposite way, then the first 

 way again, and so on. This kind of current is called 

 alternating current (a.c.), because the current alternates, 

 coming in the upper wire and out of the lower for a 

 fraction of a second ; then coming in the lower and out 

 of the upper for the next fraction of a second ; then com- 

 ing in the upper again and out of the lower for a fraction 

 of a second ; and so. on, back and forth, all the time. 

 For heating and lighting, this alternating current is 

 just as good as the direct current, and it is probably 

 what you have in your own home. For charging storage 

 (batteries and making electromagnets, separating water 

 into two gases, and for running certain kinds of motors, 

 however, the direct current is necessary. Find out 

 whether the current in your laboratory is direct or 

 alternating. 



Application 49. Explain why we need fuel or water to 

 generate large currents of electricity ; how we can get small 

 amounts of electricity to flow without using dynamos ; why 

 automobiles must be cranked unless they have batteries to 

 start them. 



Inference Exercise 

 Explain the following : 



301. Mexican water jars are made of porous clay ; the water that 



seeps through keeps the water inside cool. 



302. When you crank an automobile, electricity is generated. 



303. Potatoes will not cook any more quickly in water that is 



boiling violently than in water that is boiling gently. 



304. When you brush your hair on a winter morning, it some- 



times stands up and flies apart more and more as you con- 

 tinue to brush it. 



305. You cannot see a person clearly through a ground-glass 



window, although it lets most of the light through. 



