224 VELOCITY OF ELECTRICITY. 



building in order to offer the electricity the shortest path 

 to the ground. It is important to have each projecting 

 part of the building, as chimneys, towers and gables, pro- 

 tected by a separate rod. The rod should afford an un- 

 broken connection ; the joints, if there be any, should be 

 carefully made. The rod should terminate below in water, 

 or in earth that is always moist. A rod having a blunted 

 tip, a broken joint, or terminating in dry earth, is more 

 dangerous than no rod at all. 



(a.) The greatest value of a lightning-rod is due to its quiet work 

 in the prevention of the lightning stroke. Bring the point of a knife- 

 blade near the conductor of an electric machine in operation, and 

 notice the instant cessation of sparks. The quiet passage of elec- 

 tricity from the earth neutralizes the charge of the conductor and 

 restores the electric equilibrium. In the same way, a lightning-rod 

 tends to restore the electric equilibrium of the cloud, a -id prevent 

 the dangerous discharge. For this quiet but very valuable service, 

 few persons ever give the rod any credit. Every leaf of the forest 

 and every blade of grass is a pointed conductor acting in the same 

 way. (371 [9].) 



364. Velocity of Electricity and Duration 



of the Spark. Experiment has shown that the 

 velocity of electricity along an insulated copper wire is 

 about 288,000 miles per second, and that the duration of 

 the electric spark is not more than -^Q-Q of a second. 

 The danger from any lightning stroke has passed when we 

 hear the crash. ( 425.) 



365. Thermal Effects. When an electric current 

 has to overcome a resistance in its passage, heat is pro- 

 duced. By passing a strong current over a small wire, the 

 wire may be heated to fusion. Metals have even been 

 vaporized by electricity. The worse conductor a wire is, 

 the more it is heated. See 371, [19]-[23]. By means 

 of a Leyden battery and a universal discharger, remark- 



