132 Master Minds of Modern Science 



method is too costly for ordinary use, though it might 

 be applied to a powder magazine or dynamite factory. 



Sir Oliver pointed out that the danger lies not so much 

 in the opposing charges of electricity in cloud and earth, 

 but in the vast store of energy in the stratum or layer 

 between the two (what is called the dielectric). To 

 dissipate such a volume of energy suddenly by means of 

 a thick rod of copper is not the safest way, for an electric 

 discharge is very likely to overshoot itself and not to be 

 exhausted in a single swing. Sir Oliver says : 



The hastily discharged cloud, at first supposed positive, over- 

 discharges itself and becomes negative ; then again discharges 

 and over-discharges till it is positive as at first, and so on with 

 gradually diminishing amplitude of swing, all executed in an 

 extraordinarily minute fraction of a second, but with a vigour 

 and wave-producing energy which are astonishing. 



It was usual formerly to use a thick and costly copper 

 rod, copper because it is the best conductor of electricity, 

 but Sir Oliver showed that a thin iron wire may actually 

 be better. Its extra resistance dissipates some of the 

 energy and tends to damp out vibration sooner. A side 

 flash is less likely to occur from thin iron than from stout 

 copper. He proved also that metal tape is electrically 

 better than a round rod, but that four detached and 

 well-separated wires are better than either. 



Sir Oliver's experiments have been of very great help 

 to the British Post Office, and to telegraph, telephone, 

 and cable companies, in showing them how to protect 

 their wires and cables from the effects of lightning. 

 Besides proving that iron is better than copper for this 

 purpose, he demonstrated that several points are prefer- 

 able to a single point, that conductors should be con- 

 tinuous and all unavoidable joints soldered; that high 

 lightning rods are not of special value ; that greater sur- 

 face should be given to earth connexions ; and that both 



