PROJECTS 639 



of water. The electric stream may vary in size as does a stream 

 of water. We speak of a stream of water as running so many gallons 

 per second. The size of the electric current we measure in amperes. 

 For example, only a small stream of one-half ampere is required 

 to run an ordinary incandescent lamp of 16-candle power, but a 

 large stream of five amperes is necessary to run an electric iron. 



It is in connection with the size of the stream of electricity in a 

 house that fuses serve the purpose of safety devices. For example, 

 suppose your electric company has a 15-ampere fuse on your din- 

 ing room circuit. Now suppose you are operating on this circuit 

 two 16-candle power incandescent lamps, each requiring one-half 

 ampere ; and a toaster and a chafing-dish, each requiring 5 amperes. 

 This makes a total of 11 amperes. If now you add a percolator, re- 

 quiring 5 amperes, all the devices on the circuit together would de- 

 mand a current of 16 amperes, and the overstrain would blow the 

 15-ampere fuse on that circuit. 



The remedy is to put in a new 15-ampere fuse, and not to use so 

 many devices on the circuit at the same time. Or it may be that 

 the company will allow you a 20-ampere fuse on that circuit, so 

 that you may use all the devices at the same time. But do not use 

 fuses of larger amperage without the consent of your electric company, 

 because your wiring may not safely carry a larger stream. If the fuse 

 should be of larger amperage than the wiring would carry, an over- 

 load would burn out the wiring instead of the fuse. There must 

 always be a safe margin between the size of stream your wiring will 

 carry and the size of stream your fuses will withstand. 



Water at the faucet is under a certain number of pounds of 

 pressure (p. 201). This pressure has nothing to do with the size of 

 the stream. For example, you may open the faucet only slightly 

 and get a very small stream of water or you may open it wide 

 and get a full stream. The pressure behind both streams is the 

 same. What corresponds to pressure in a stream of electricity is 

 measured in volts. The most common "pressure" or voltage for 

 a lighting circuit is 110 to 120 volts. 



The power of a stream of water flowing from a faucet depends 

 on the size of the stream and the pressure behind it. The power 



