ELECTRIC DISCHARGE 



2842 



ELECTRICITY 



Neither the development nor the 

 dissipation of an electric charge 

 makes any measurable difference 

 to the weight of the body. The phe- 

 nomena are doubtless bound up 

 with the elemental constitution of 

 matter, and as our knowledge of 

 that constitution increases the ex- 

 planation of the electric charge and 

 its double character will duly ap- 

 pear. See Electron. 



Electric Discharge. Act of 

 neutralisation of an electrical 

 charge. Electricity which resides 

 chiefly on the surface of bodies may 

 disappear in any one of several 

 ways. It may be carried away con- 

 tinuously through a wire or metal 

 rod, and is then said to disappear 

 by conductive discharge ; it may 

 disappear suddenly as in a spark or 

 a flash of lightning, which is said to 

 be a disruptive discharge ; or it 

 may go gradually by being com- 

 municated to particles of air in its 

 neighbourhood, which then fly off 

 by repulsion, when it is said to dis- 

 appear by convective discharge. 

 Convective discharge may take 

 place in a liquid as well as in air. 



The electricity which disappears 

 in any of these ways reappears in 

 some other form of energy. Thus 

 the energy which goes out in a con- 

 ductive discharge may present it- 

 self in the light of an electric lamp 

 or in some chemical action ; that 

 of a disruptive discharge presents 

 itself in the form of sound or light 

 or heat, or all three, or in some 

 mechanical effects, as when a sheet 

 of glass is shattered or pierced by 

 the discharge ; that of a convec- 

 tive discharge may be found in the 

 movements of the particles of air 

 which may be set flying in all direc- 

 tions with increased temperature. 



The discharge in a resisting 

 medium, as in air, is always ac- 

 companied by the development of 

 heat ; an electric spark will light 

 a gas jet ; the discharge in certain 

 forms of electric furnaces will melt 

 the most refractory metals. The 

 discharge is now applied in indus- 

 try in the manufacture of nitrates 

 and nitric acid from the air, and in 

 detonating high explosives as in 

 shells and mining cartridges. Very 

 beautiful luminous effects are pro- 

 duced by the discharge through 

 vacuum tubes, that is in rarefied 

 air or gas ; while the physical effects 

 produced in the gas or air are very 

 remarkable, the molecules of the 

 gas being broken up and the atoms 

 interchanged, thus producing what 

 is called ionisation. The Rtfntgen 

 or X-rays (q.v. ) are indirectly pro- 

 duced by such electric discharges. 

 Electric Fish. Genus of fishes 

 possessing the power of adminis- 

 tering an electric shock. There are 

 three known fishes which possess 



such a power, these being, in the 

 order of their electrical strength, 

 the electric eel (q.v.), the African 

 catfish, and the torpedo, a species 

 of ray fish. 



The electrical catfish (Malap- 

 terurus ekctricus) is found hi all the 

 larger rivers of Africa, the finest 

 and most powerful occurring in the 

 Nile. It frequents the darker and 

 more sluggish portions of the 

 streams, where it kills or stuns 

 other fish which it eats. It is found 

 up to four feet in length. In some 

 catfish the electrical power seems 

 present all over the body, in others 

 just under the skin at each side. 



The torpedo or electrical ray is 

 the most numerous of these fish. A 

 considerable number of species oc- 

 cur in the warmer seas of the world, 

 and at least two are found near the 

 southern shores of the United 

 States. The best known is the 

 Torpedo marmorata of the southern 

 shores of Europe and of the Medi- 

 terranean ; large specimens may 

 weigh as much as 80 Ib. The elec- 

 tricforce resides in the powerful tail. 



The muscles and the nerves 

 which are concerned in the exercise 

 of the electrical power of these fish 



are known ; the direction in which 

 the current of electricity flows 

 through the body of the fish in each 

 case is also known, this direction 

 being from tail to head in the elec- 

 tric eel, from head to tail in the cat- 

 fish, and from underneath up in the 

 rays ; but the source of the power 

 and how the organisms become 

 charged with it is not known. Its 

 exercise evidently calls for much 

 nervous force, as after a particu- 

 larly powerful shock or a series of 

 shocks has been given the fish is 

 exhausted, and must have rest. 



Electrician. In the general 

 sense, one who is skilled in the 

 science of electricity, or who is en- 

 gaged in the art of applying elec- 

 tricity to practical ends, that is to 

 say, a worker in electricity. Hence 

 it covers in the broadest view the 

 university professor, the student, 

 the man who " wires " a house or 

 tests a faulty telephone line, and 

 the telegraph, the " wireless," and, 

 equally, the telephone operator. 

 One who designs or constructs 

 electrical works or industrial elec- 

 trical machinery, or who operates 

 such, is called anelectrical engineer. 

 See Engineering. 



ELECTRICITY: GENERAL INTRODUCTION 



James Rice, M.A., Lecturer in Physics, Liverpool University 



This article forms an introduction to the group of articles on electrical 



subjects which follows it, and also to others scattered throughout the 



work. It is followed by articles dealing with special forms of 



electricity, e.g. Atmospheric, Medicinal, etc. 



The scientific study of electricity 

 began in the 16th century. The 

 ancient Greeks were acquainted 

 with some isolated facts concern- 

 ing the electrification of a few sub- 

 stances by friction, the epithet 

 " electric " being, in fact, coined 

 from the Greek word elektron, the 

 name for amber, which was one of 

 these substances. This term, 

 among others, was introduced by 

 William Gilbert, of Colchester, who 

 made the first detailed study of 

 the property of attracting light 

 materials, which bodies acquire 

 after being rubbed with textiles 

 such as silk or flannel. 



Gilbert, who might be called the 

 father of electricity, published in 

 1600 his great work On the Magnet, 

 Magnetic Bodies and the Great 

 Magnet the Earth, paving the way 

 for the systematic and scientific 

 experiments on electrical pheno- 

 mena which culminated with those 

 of Faraday. Until Gilbert pub- 

 lished his results nothing was 

 known about electricity, save that 

 certain substances as amber, jet, 

 etc., attracted light objects such 

 as leaves, feathers, etc. 



The researches of Boyle, New- 

 ton, and Gray in England, of Von 

 Guericke in Germany, and Du Fay 



in France, had, by the first half of 

 the 18th century, established the 

 fact that all materials could ac- 

 quire this property, i.e. be elec- 

 trified, by friction. In the case of 

 some substances such as very dry 

 glass, sulphur, wax, ebonite, and 

 mica, the property is confined to 

 the portion of the surface which 

 has been rubbed ; but in general 

 it tends to be diffused over the sur- 

 face, no matter where friction has 

 been applied. This power of " con- 

 duction " is manifested most 

 notably in the case of the metals, 

 but is also possessed by most of the 

 materials which constitute the 

 earth's crust, also by animal tissue 

 and any damp surface. It is for 

 this reason that such materials 

 cannot retain the electrification 

 unless suspended or supported by 

 insulating strings or rods of silk, 

 sulphur, glass, etc., and thus the 

 earliest experimenters, notably Gil- 

 bert, were misled into the belief 

 that these were " non-electric." 



Du Fay (and also Kinnersley 

 and Franklin in America) dis- 

 covered the dual quality of electri- 

 fication, and ultimately Franklin's 

 terminology was adopted, which re- 

 ferred to bodies as "positively" or 

 "negatively" electrified, according 



