Popular Science Monthly 



365 



to objects in the foreground, such as trees, 

 buildings, etc., and, the distance of these 

 objects from the cameras being known, the 

 distance of the flash can be determined. 



Lightning Sometimes 

 Shifts to One Side 



struck and comparing these quantities with 

 those similarly obtained in the laboratory. 

 From the strength of the magnetic field 

 produced in the rock by the lightning, he 

 estimated the maxi- 



A brick stack cut in two by a stroke 

 of lightning during a thunder storm 



The ionized path 

 of a multiple flash 

 is not always station- 

 ary, but is sometimes 

 shifted a considerable 

 distance by the wind. 

 In the case of a 

 photograph taken by 

 Riimcker with a sta- 

 tionary camera, when 

 the place where the 

 lightning struck — 

 and hence the dis- 

 tance of the flash 

 from the observer — 

 was accurately known, 

 the flash shifted 

 laterally a distance of 

 thirty-six feet during 

 visibility. This phe- 

 nomenon appears to 

 explain certain cases 

 in which well-install- 

 ed lightning-rods are 

 ineffective. The in- 

 itial discharge probably takes to the rod 

 and is carried off harmlessly, but the dis- 

 charges following keep to the ionized path 

 as it is swept aside by the wind and strike 

 a projecting corner of the building or a 

 neighboring tree. Thus we have what ap- 

 pear to be "divided strokes;" but these 

 are really successive strokes in different 

 places at very small intervals of time. 



Several at- 

 tempts have 

 been made to 

 estimate the 

 strength of cur- 

 rent in a stroke 

 of lightning. 

 Pockels, in Ger- 

 many, adopted 

 the ingenious 

 method of 

 measuring the 

 residual mag- 

 netism in a 

 mass of basalt 

 rock near a 



Mace Where A good example of what is commonly designated as 



lightning had forked lightning, photographed with a hand camera 



mum strength of cur- 

 rent in the latter to 

 amount, in some 

 cases, to as much as 

 20,000 amperes. 

 Humphreys, in this 

 country, has recently 

 examined a hollow 

 copper lightning-rod, 

 crushed by lightning 

 (shown in the pho- 

 tograph on page 

 368), and has esti- 

 mated that the 

 strength of current 

 necessary to produce 

 such an effect may 

 have been as great 

 as 90,000 amperes. 

 Both estimates are 

 very rough, since 

 they depend upon as- 

 sumptions that can- 

 not be verified, but 

 they prove beyond 

 a doubt that the 

 currents in lightning 

 flashes must be reckoned in thousands of 

 amperes. Steinmetz estimated from the 

 intensity of illumination due to a lightning 

 stroke that the amount of energy involved 

 was of the order of 10,000 kilowatt seconds, 

 or 13,400 horsepower seconds. If we 

 assume the duration of the flash to have 

 have been .01 seconds, this would represent 

 a delivery of energy at the rate of 1,340,000 

 horsepower. 

 But all this is 

 little better 

 than guess- 

 work. 



What Causes 

 Lightning? 



The origin of 

 thunderstorm 

 e lectric ity , 

 after having 

 been the sub- 

 ject of endless 

 discussion for 

 generat ions, 

 appears to have 

 been satisfac- 



