242 



ELECTRICITY 



cloud that may be at a lower pressure or voltage. 



When lightning strikes it usually hits a tree, tele- 

 phone pole, or building that is somewhat higher than 

 other objects in the immediate vicinity. It has been 

 proved, though many persons doubt it, that lightning 

 may strike in the same place time after time. 



Many facts about the behavior of lightning have 

 been learned by observation and careful study. If one 

 is out in a thunderstorm it is unwise to seek shelter 

 under a tree that stands by itself away from other 

 trees, and one should stay well away from telephone 

 and telegraph poles. If you are indoors during an elec- 

 tric storm, keep away from objects that may have a 

 connection to the ground, such as the telephone, radio, 

 radiators, sinks, and the like. 



Lightning rods are used to protect buildings from 

 being struck by lightning. The lightning rods are con- 

 nected to metal plates in the ground by heavy con- 

 ducting cables. If lightning should strike a house 

 equipped with rods, the electricity most likely would 

 be conducted to the ground by the cables and would 

 do no harm to the building. 



Exercise. Would a tall chimney made of brick or one 

 made of iron be more likely to be struck by lightning? 

 Explain. 



Exercise. What inference can be drawn from the fact 

 that the cables from lightning rods on buildings are at- 

 tached to plates buried deep in the earth -where the soil 

 is moist, rather than near the surface where the soil 

 is more likely to be dry? 



Taylor Instrument Company 

 FIG. 380. LIGHTNINr. FLASH 



Courtesy Smithsonian Institution 

 I'lG. 381. EARLY VOLTAIC PILE 



How is electrical energy obtained from chemical 

 energy? About 1785 Luigi Galvani, an Italian scien- 

 tist whose wife was in poor health, secured some frogs 

 from which frog-leg soup was to be prepared for her. 



While he was skinning them his knife happened to 

 touch a metal clamp with which they were being held 

 and he was very much surprised to notice a twitching 

 in the legs. He repeated the experiment many times 

 and finally wrote a paper on what he called "animal 

 electricity." 



About ten years later Alessandro Volta, another 

 Italian, after reading of Galvani's experiments thought 

 that the electricity had come from the unlike metals 

 touching the moist flesh of the frog. He set up an 

 experiment in which, he had plates of zinc and silver 

 separated by cloth pads moistened in salt water and 

 was able to get electricity from them. This was the 

 first electric battery and was described by Volta in a 

 paper which he wrote. Figure 381 shows a photograph 

 of an early voltaic pile, as it was called. 



Volta discovered that he could get electrical energy 

 from chemical energy whenever two unlike metals 

 were placed in a solution, provided one of the metals 

 was acted upon more rapidly by the solution. This has 

 become one of the most important modern methods 

 of securing electricity and has resulted in many dif- 

 ferent types of electric batteries, among which are 

 the wet cell, the dry cell, and the storage battery. 



The wet cell or ammonium 

 chloride cell is shown in Figure 

 382. It has a cylinder of carbon 

 A and a rod of zinc B dipping 

 into a solution of ammonium 

 chloride, commonly called sal 

 ammoniac. In this cell the zinc 

 rod is acted upon and eaten 

 away by the solution and is 

 therefore the negative plate, 

 while the carbon cylinder is the 

 positive plate. These cells are 

 not now used to any great ex- 

 tent, but formerly found wide 

 use for ringing doorbells. We 

 study this cell because it is in 

 many ways like the dry cell which is commonly used. 



The dry cell has come to be one of the most com- 

 mon sources of electricity because it is easy to handle 

 and takes up a very small space. The dry cell is really 

 dry only from a point of view of handling it, for it 

 supplies electricity because it has two unlike plates 

 touching a chemical paste which acts upon one of the 

 plates more than upon the other. The cross-section 

 drawing in Figure 383 shows the construction of the 

 dry cell. The zinc can A acts both as a container to 

 hold the rest of the cell and as a negative plate. Inside 

 the zinc can is placed a layer of blotting paper D 

 which has been soaked in sal ammoniac. The carbon 

 rod B extends down through the center of the cell 

 almost to the bottom. Between the carbon rod and 



FIG. 382. AMMONIUM 

 CHLORIDE CELL 



