Page:Popular Science Monthly Volume 91.djvu/381

 Popular Science Monthly

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��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-

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