210 APPLIED SCIENCE 



Power is the rate of doing work and the electrical unit is 

 the number of joules per second. It is expressed as watts. 



257. Size of Wire. In distributing electricity there is, 

 as previously stated, more or less resistance to its passage 

 through wires. In overcoming this resistance heat is devel- 

 oped and energy is lost by the friction caused by the electric- 

 ity moving through the conductor. The resistance offered 

 to an electrical current depends upon the material through 

 which it passes, the length and sectional area of the circuit 

 wires, and the surrounding conditions. 



To illustrate: If 900 ft. of a certain wire offers a resistance of 2 

 ohms, the resistance of 450 ft. of the same wire is 1 ohm. If the 

 diameter of the wire were one-half, the area would be one-quarter 

 and the resistance four times as great, or 4 ohms. This is often 

 expressed in mathematical language by stating: Resistance varies 

 directly as the length and inversely as the square of the diameter 

 of wire. 



Since watts are the product of electromotive force and current, 

 the question of furnishing 15,000 watts to a certain point from a 

 power station might be settled by having either an electromotive 

 force of 1500 volts and a current of 10 amperes, or 150 volts and 

 100 amperes. The loss due to heat increases with the strength of 

 the current. 



The size of wire necessary to transmit a given current is deter- 

 mined by the drop in voltage allowed between the generator and 

 the point of application of the current, and the increase in tempera- 

 ture due to the current. 



High voltages are used in long-distance transmissions to increase 

 the carrying power of a given size of wire, in other words, to decrease 

 the cost of line necessary to transmit a given amount of energy. 



258. Kilowatt and Kilowatt-Hour. Many people con- 

 fuse kilowatt (kw.) and kilowatt-hour (kw.-hr.). Kilowatts 

 (watts divided by 1000) represent the number of units of 



