HEAT AND LIGHT FROM ELECTRIC CURRENTS 205 



three incandescent lamps connected in parallel use more cur- 

 rent than two lamps, but the voltage is unchanged. Part C 

 shows that no change in voltage occurs when the number of 

 cells connected in parallel is increased. Before discussing 

 the last column of the table, we must understand certain 

 units of measurement. 



218. Units used in measuring electricity. Air and water 

 pressures are usually measured in pounds per square inch. 

 When the pressure is high the water faucet or the com- 

 pressed-air tube delivers a greater quantity than when the 

 pressure is low. The unit of electrical pressure (electro- 

 motive force) is called the volt in honor of Count Alessandro 

 Volta (1745-1827), a great Italian physicist. The value of 

 the volt is slightly less than that produced by an ordinary 

 dry cell. Ordinary storage batteries may operate at 4, 6, or 

 12 volts; incandescent lamps at 10, 55, 110, and 220 volts; 

 and street-car motors at 400 to 600 volts. 



The flow of water may be measured as the number of 

 gallons passing through a pipe per second. Similarly, when 

 a certain quantity of electricity passes through a wire in a 

 second, the current is said to have a value of 1 ampere. 

 This term is selected in honor of a great French physicist, 

 Andre Marie Ampere (1775-1836). 



An idea of the amount of current which an ampere repre- 

 sents may be obtained from the following illustrations : The 

 old style 16 candle-power incandescent lamp (carbon filament) 

 required about .5 ampere, and the common 60-watt tungsten 

 lamp uses about .5 ampere. An average electric bell uses 

 ^ ampere. A dry cell when short-circuited for an instant 

 will give from 15 to 30 amperes. Many of the fuses in 

 residences are designed to carry 6 amperes safely; others 

 are rated to carry from 1 to 25 amperes; special types carry 

 large currents. 



It is well to remember also that there may be a great 

 pressure and no current. If the valve in the water main is 



