260 



'ELECTRICITY 



long wire offers more resistance than a short one of 

 the same size just as a long water pipe offers more 

 resistance to a water current than a short one. 



EQUAL 

 SIZES 



SHORT PIPE 

 LOW RESISTANCE 



EQUAL 

 SIZES 



SHORT WIRE 

 LOW RESISTANCE 



LONG WIRE 

 HIGH RESISTANCE 



Courtesy National Carbon Company 



FIG. 419. COMPARISON OF WATER CURRENTS IN 

 PIPES AND ELECTRIC CURRENTS IN WIRES 



Turning on an electric light, iron, or motor by snap- 

 ping a switch is similar to turning on a faucet in a 

 water system. It makes possible the flow of electric 

 current in one case and of water current in the other. 



LARGE CABLC 

 LOW RESISTANCE 



Courtesy National Carbon Company 



FIG. 420. SMALL PIPES AND WIRES HAVE HIGH RE- 

 SISTANCE; LARGE PIPES AND WIRES HAVE LOW 

 RESISTANCE 



If one wishes to measure the rate at which a water 

 current is flowing from a faucet, he can use a gallon 

 measure and watch and easily determine how many 

 gallons a minute are coming out. In measuring the 

 rate of flow of electric current through a lamp, how- 

 ever, one would measure it in amperes by use of a me- 

 ter called an ammeter. In the same way that a water 

 faucet only partly open will not permit as many gal- 

 lons a minute to flow, one electric light or other device 

 may provide a smaller opening in the circuit than an- 

 other and allow fewer amperes to flow. 



Pressure in a water system can be measured by a 

 pressure gauge in pounds per square inch while elec- 

 trical pressure is measured in volts. An ordinary dry 

 cell will give about 

 one and a half volts 

 while our house light- 

 ing circuits operate at 

 a pressure of one hun- 

 dred and ten volts. 

 Voltage is measured 

 with a voltmeter. 



In water currents it 

 is of no practical value 

 to have a unit for 

 measuring the resist- 

 ance offered to their 

 flow by different kinds 

 and sizes of pipe. 

 In electrical currents, 

 however, resistance is 

 very important and is 

 measured in a unit 

 called the ohm. This 

 unit is named in hon- 

 or of George S. Ohm, 

 a German scientist, 

 while the ampere is 

 named for A. M. Am- 

 pere, a French scien- 

 tist, and the volt for 

 Alessandro Volta, an 

 Italian scientist. 



The following table 

 will summarize the 

 things which you have 

 learned about water 

 and electric currents. 



AMMETER 



VOLTMETER 



FIG. 421. COMPARISON OF MEASURE- 

 MENT OF CURRENT AND PRESSURE, 

 WATER AND ELECTRICITY 



Exercise. Carefully examine the connection of elec- 

 tric light bulbs where the wiring is exposed as in the 

 basement or in the attic. Would you infer that the lights 

 are connected in parallel or in series? Give reasons. 



Exercise. When one bulb in a string of Christmas tree 

 lights goes out, all others in the same string go out. Would 

 you infer from this fact that these lights are connected 

 in series or in parallel? Give reasons. 



