ATMOSPHERIC AND OTHER ELECTRICITY 119 



seat of the phenomena in real actions going on in the medium ; they were satisfied that they had found it in a 

 power of action at a distance impressed on the electric fluids." The theory of centres of force attracting at a 

 distance was no doubt founded on the effects known to be produced by the heavenly bodies on each other. It, 

 however, does not apply to molecular action. 



For some fifty years after Faraday's brilliant discoverj'^ of magneto-induction, attention was almost exclusively 

 directed to the production of electric currents in one direction instead of interrupted or to-and-fro currents, such 

 as are obtained by the rapid alternate thrusting and withdrawing of the north pole of a magnet into a spool of 

 insulated wire. The interrupted currents are, however, the more varied and powerful, especially when the hnes 

 of magnetic force are made to quiver at high speeds. They can by means of a transformer convert a comparatively 

 low electric pressure or voltage into an enormously high one. They can produce electric hght, heat wires, and 

 accomplish most, if not all, the work performed by the constant currents. The interrupted currents are daily 

 becoming more important, from the great variety of work they are capable of performing. 



In order to convert interrupted or to-and-fro electric currents into constant currents flowing in one direction, 

 a commutator is necessary. This is to the electric circuit what the eccentric is to the valves of the steam-engine 

 when it lets on and cuts ofE the steam which gives continuity of movement to the piston and fly-wheel. The com- 

 mutator is briefly a mechanical arrangement by the aid of which the free ends or terminals (electrodes) of wires 

 conveying interrupted or alternating currents of electricity are transposed or crossed, with the result that the 

 interrupted currents are converted into constant currents. 



Electric currents are pecuhar in this : if the currents in two neighbouring wires are running in the same direc- 

 tion, the wires attract each other ; if in opposite directions, they repel each other. In the case of magnetic poles, 

 like poles repel and unUke poles attract. 



The production of electricity is not less interesting than its mysterious manifestations. It occurs as a natural 

 product in the lodestone, the lightning, and in certain fishes ; notably the electric eel of tropical America, the 

 electric rays, and the electric cat or sheath-fish. 



In the fishes, electricity is manufactured by special glands, and discharged in self-defence or attack. The 

 discharge results in a shock hke that obtained from ordinary electricity or from lightning.^ In the case of 

 the electric eel, according to Humboldt, the discharge is sufficiently strong to destroy a horse or other large 

 animal. 



ATMOSPHERIC AND OTHER ELECTRICITY 



The mode of production of atmospheric electricity is still svb judice. It has been ascertained not to be due, 

 as was supposed, to the evaporation of water, although the friction of the particles of water against material 

 substances can produce it. Thus, in Armstrong's electrical machine, where jets of spray are forced through nozzles, 

 a high state of electrification is obtained. The friction of dust particles is also a potent cause of electrification ; 

 the tops of the Egyptian pyramids during sand-storms being strongly charged. 



Electricity is produced artificially in many ways — for example, by the action of certain fluids on different metals, 

 as in galvanic batteries — that is, when two dissimilar metals are connected by a wire and immersed in a 

 liquid that acts upon one of them ; by heat, when two dissimilar metals are mechanically joined and heat is applied 

 to the junction ; by friction, when a warm glass rod is rubbed with dry silk. The most common form of artificial 

 electricity is that produced by the aid of the dynamo machine, which is driven by steam-engines, petrol-engines, 

 or water running at high velocities. The modern dynamo consists of a number of wire coils arranged on a 

 rapidly revolving shaft surrounded by fixed pieces of iron, around which the currents formed in the revolving coils 

 are made to circulate. The speed of the shaft, in some cases, exceeds one thousand revolutions per minute. 

 " Currents of electricity are excited in a coil of wire passing rapidly near the poles of a magnet ; a current being 

 excited in the coil in one direction by movement near a south pole, and in the opposite direction by movement 

 near a north pole ; a commutator can be made to direct these opposite currents in the same direction through 

 another coil." The electricity generated by the dynamo is carried away by wires, and used up or stored as desired. 

 The electricity flows so long as the dynamo is at work, and is delivered at the extremities or electrodes of the wire. 



' According to Trowbridge the bolt of lightning is not one continuous discharge, but is an interrupted, alternating current which pulsates 

 to and fro ten or twelve times, or even more, in a millionth of a second. Photographs of powerful electric sparks lead one to conclude that a 

 discharge of lightning makes way for its oscillations by first breaking down the resistance of the air by means of a disruptive pilot spark ; through 

 the hole thus made in the air the subsequent surgings or oscillations take place. In lightning discharges high electro-motive force and great 

 quantity are frequently combined in a very short interval of time. The study of the disruptive or oscillatory discharge of lightning is closely 

 related to that of the bnish discharge and the phenomenon of the aurora borealis. 



