1905.] on a Pertinacious Current. 83 



Electricity and Mist. 



As to the hind of electricity which it is best to discharge into a 

 fog, it is a familiar experience that negative electricity escapes from 

 points rather more easily than positive does. Hence that is one 

 advantage in using negative ; but there is a further advantage. A 

 fog or mist is itself usually more or less electrified, and the sign of 

 the electricity with which it is charged is generally positive. Electro- 

 scopes at meteorological observatories indicate this general positive 

 electrification of mist, and they further show that when the mist 

 begins to drizzle and clear away, or turn into rain, the sign of the 

 electrification is frequently reversed and becomes negative ; in fact, 

 negative electrification is generally associated with rain. 



Now it is difficult to say which is cause and which is effect in 

 such a case, but if two things are accustomed to go together, then 

 the artificial production of one may bring about the other. Certainly 

 the easiest way of dispersing a cloud or mist or fog is to bring it 

 down as rain ; and if it is artificially supplied with negative electricity, 

 that is what is very likely to happen, for that is what certainly 

 happens on a small scale in the laboratory. In the laboratory, how- 

 ever, it must be admitted that either positive or negative electricity 

 will serve the purpose ; though, on the whole, negative electricity 

 does rather the better, perhaps l3ecause it escapes more easily ; and 

 large-scale experiments in open air, involving a considerable amount 

 of capital expenditure, are still wanting. But there is every reason 

 to suppose that the natural positive electrification of a fog will assist 

 the discharge of negative electricity from points immersed in it, and 

 that the electrification thus suppHed will result in its condensation 

 and dissipation. 



The whole of the arrangement, if it is to be applied on a large 

 scale with currents derived from engineering mechanism, interrupted 

 automatically and transformed to high-tension intermittent currents 

 at each post or discharging-station, depends on the efficiency of the 

 electric valves employed. 



These are of many kinds : any air-gap is liable to act to some 

 extent in that direction, inasmuch as it can transmit electric pres- 

 sures above a certain magnitude and must obstruct those below that 

 critical magnitude : hence if it is interposed in the path of an 

 alternating current of which the opposite pulses are propelled by 

 electromotive-forces of different intensity, half of such pulses may go, 

 and the other haK may be stopped. That is the case, for instance, in 

 ordinary experience with the usual spark gap in the secondary of the 

 common Kuhmkorff coil : it acts, in fact, as a rectifier or valve when 

 it is of sufficient length to check the current at " make," and yet is 

 short enough to yield to the force of the current at " break." If 

 its terminals, instead of being similar, have different surfaces — as, for 

 instance, if one is a point and the other a plate — then its rectifying 



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