Electricity 225 



323. You should face forward when alighting from a street car. 



324. There are always at least two electric wires going into a 



building that is wired. 



325. Woolen sweaters keep you warm. 



326. Steel rails are not riveted to railroad ties but the spikes are 



driven close to each rail so that the heads hook over the 

 edge and hold the rail down without absolutely preventing 

 its movement forward and backward. Why should rails 

 be laid in this way? 



327. The earth keeps whirling around the sun without falling into 



it, although the pull from the sun is very great. 



328. Electricity is brought down from power houses in the moun- 



tains by means of cables. 



329. White clothes are cooler than black when the person wearing 



them is out in the sun. 



330. All the street cars along one line are stopped when a trolley 



wire breaks. 



SECTION 36. Grounded circuits. 



Why can a bird sit on a live wire without getting a shock, 

 while a man would get a shock if he reached up and took 

 hold of the same wire ? 



We have just been laying emphasis on the fact that 

 for electricity to flow out of a dynamo or battery, it 

 must have a complete circuit back to the battery or 

 dynamo. Yet only one wire is needed in order to tele- 

 graph between two stations. Likewise, a single wire 

 could be made to carry into a building the current for 

 electric lights. This is because the ground can carry 

 electricity. 



If you make all connections from a battery or dynamo 

 just as for any complete circuit, but use the earth for 

 one wire, the electricity will flow perfectly well (Fig. 

 127). To connect an electric wire with the earth, the 

 wire must go down deep into the ground and be well 



