94 ATMOSPHERIC ELECTRICITY. 



Schleswig-Holstein, 1 for while 6.3 churches per thousand were struck, 

 and 8.5 windmills, the number per thousand of factory chimneys was 

 only 0.3. 



Franklin was acquainted with the action of flames. He also discov- 

 ered that no charge can be given to a red-hot iron ball, a fact which 

 seems to have been forgotten until rediscovered in our own times by 

 Guthrie. Franklin also tried the action of sunlight, but obtained no 

 result. Had he performed the experiment with carefully- cleaned zinc, 

 he would have anticipated one of the most striking of Hertz's discov- 

 eries. We now know that a negatively-charged surface will discharge 

 into air when illuminated by strong violet Jight, and sunlight will be 

 sufficient with specially sensitive materials. This action has been 

 investigated in detail by Elster and Greitel, who have not, however, 

 succeeded in obtaining results with sunlight acting on such bodies as 

 we know the earth's crust to be made of. So far, then, we have no 

 experimental evidence to include light as an active agent in the phe- 

 nomenon of atmospheric electricity. 



We possess in the electric discharge itself a very powerful and prob- 

 ably very generally active means of breaking down the insulating power 

 of" air. Some of the experiments (Proc. Roy. Soc, Vol. XLII) which 

 I described some years ago to prove this were objected to on the ground 

 that it might not be the discharge itself, but the ultra-violet light sent 

 out by the luminosity of the discharge, which was active. The follow- 

 ing form of the experiment conclusively shows that the discharge acts 

 independently of light. 



In Fig. 2, Plate I, R represents a Khumkorff coil entirely surrounded 

 by a metallic box B, which is connected to earth. The terminals of the 

 coil lead to two electrodes inside a metallic tube T, which is also kept 

 at zero potential. This tube is arranged so that a current of air can 

 be blown through it. The air, on escaping through the tube, is made 

 either to impinge on or to pass near a metallic plate connected to a 

 charged electroscope. Under these circumstances the electroscope is 

 not discharged either by a current of air alone or by the coil alone ; 

 but as soon as the air is blown through the apparatus while the sparks 

 are passing and then made to impinge on the plate C, the electroscope 

 is instantaneously discharged. The experiment succeeds when a plug 

 of cotton- wool is inserted at W to stop the action of the dust; but a 

 plug of cotton-wool at the other end diminishes the action so much that 

 I am doubtful whether the effect then really exists there. I am, so far, 

 not inclined to believe that the action is due to dust, but rather that 

 the cotton- wool acts in increasing very considerably the interval which 

 elapses between the time at which the spark acts and the time at which 

 the sparked air passes out of the tube. The effect may be observed 



1 "Veroffentl. des kgl. preuss. stat. Bureaus," 1886, p. 177, (quoted by Bebber, 

 "Meteorologie," p. 245. 



