ELECTRON 



of the instrument is again, in effect, 

 a Leyden jar A, the lower portion of 

 which is filled with sulphuric acid, 

 which serves four purposes it acts 

 as the inner coating of the jar, 

 keeps the interior dry, and provides 

 means by which the movement of 

 the needle may be steadied, and by 

 which the magnetism of the needle 

 is permanently maintained. The 

 outside of the jar is coated with 

 foil in the usual way and the foil 

 connected with the earth. 



The jar is a powerful condenser 

 of electricity, and exists to pro- 

 vide what may be called an electric 

 atmosphere so powerful and steady 

 that it will be unaffected by any 

 electricity that may enter the in- 

 strument during any test. B, C, are 

 two of the quadrants, the other two 

 being removed in the upper left- 

 hand Fig. so that the needle may be 

 seen. They are secured to the frame 

 of the instrument. The quad- 

 rants and the needle are shown 

 on an enlarged scale in the Fig. 

 They are small brass boxes with 

 two open sides and are four in 

 number. Opposite quadrants are 

 joined by a wire, thus making them 

 one electrically. The " needle " 

 is a thin vane of aluminium the 

 lightest suitable material with 

 rounded ends as shown, anything 

 in the way of a point which might 

 tend to discharge the electricity of 

 the vane being avoided. The 

 needle is attached to a stiff platinum 

 wire which is suspended by a silk 

 fibre secured at the top of the in- 

 strument so that it hangs verti- 

 cally with the needle floating 

 horizontally as shown. The wire 

 continues below the needle and 

 ends in a vertical vane dipping in 

 the sulphuric acid which serves to 

 steady or " damp " the movement 

 of the needle. M is a small concave 

 mirror attached to the wire, so 

 that it revolves with the needle. 



One pair of quadrants is connected 

 with the terminal, T, and the other 

 pair with T 1 ; these are the chief 

 electrodes, and are connected to 

 the two bodies, the potential differ- 

 ence of which is to be determined, 

 one of the bodies being usually the 

 earth. P is the charging electrode 

 by which the jar is charged. When 

 the circuit is completed the needle 

 will turn horizontally in one direc- 

 tion or another, according as to 

 which pair of quadrants carries the 

 higher potential A ray of light fall- 

 ing on the mirror will be reflected 

 through a narrow slit on to a scale 

 some few feet away as the mirror 

 turns in sympathy with the needle, 

 the intensity of the potential being 

 shown by the degrees over which 

 the reflected ray travels on the 

 scale. Various modified forms of 

 this beautiful instrument are now 



2856 



in use. The torsion balance is 

 also a form of electrometer. See 

 Electroscope ; Meter, Electric. 



Electron. Name given by 

 Johnstone Stoney in 1891 to the 

 " atom " of electricity, that is to 

 say the smallest quantity that can 

 exist by itself or can be trans- 

 ferred from one atom of matter 

 to another. Electricity is supposed 

 to consist of enormous quantities 

 of such atomic particles aU of equal 

 dimensions. The electron is nega- 

 tive ; and a charge of negative 

 electricity on a body means that 

 there is no accumulation there of 

 electrons. An electron may exist 

 by itself or in association with 

 atoms or molecules of matter ; but 

 if there are such things as atoms of 

 positive electricity they are always 

 combined with atoms of matter. 



The origin of the conception of 

 the electron may be traced to a lec- 

 ture delivered by Von Helmholtz in 

 London in 1881 on the electrical 

 theories of Faraday ; but it was 

 more immediately developed upon 

 the discovery of the cathode rays of 

 the late Sir William Crookes. The 

 phenomena of these rays, as shown 

 in a vacuum tube (q.v.) in which a 

 discharge of electricity is taking 

 place, can be best explained by 

 supposing that, from the cathode of 

 the tube, flights of electric particles 

 or corpuscles proceed at tremendous 

 velocities, all being negatively elec- 

 trified with equal charges. These 

 particles are electrons. The single 

 charge of a negative ion in elec- 

 trolysis is believed to be identical 

 with the electron of the cathode 

 rays. The electron has infinitely 

 little weight and dimensions. 



The diameter of a molecule of 

 hydrogen is perhaps 1 /42,000,000th 

 part of an inch, but it is 140,000 

 times as big as an electron ; while 

 the weight of an atom of hydrogen, 

 the lightest substance of which we 

 have any actual knowledge, is from 

 1,800 to'2,800 times that of an elec- 

 tron. It will be seen that the elec- 

 tron is a philosophic conception ; 

 its value and interest lie in its use- 

 fulness in explaining the pheno- 

 mena of electricity and matter. See 

 Electricity ; Ion ; Radio-activity. 



Electrophone. Term first ap- 

 plied by a French scientist, M. C. 

 Ader, to a high tension electric 

 sound-transmitting instrument of 

 his design, intended to be used for 

 the purpose of enabling conversa- 

 tion to be carried on through a sub- 

 marine electric cable. It is now ap- 

 plied to the apparatus used in con- 

 nexion with an ordinary telephone 

 service for the purpose of enabling 

 one to listen to a concert or thea- 

 trical performance or to a public 

 speech which is being carried 

 on or delivered at a considerable 



ELECTROPHORUS 



distance away. By its aid a tele- 

 phone subscriber, in London, for 

 example, who is connected also 

 with the Electrophone Exchange, 

 may sit in his drawing-room in the 

 evening and listen to any one of a 

 number of theatrical performances 

 going on at the theatres, and be 

 switched off from one and put on 

 to another as he may desire. The 

 transmitters are usually fitted in 

 sets of four near the footlights of 

 the stage or platform, or in front of 

 the speaker at a public meeting. 



Electrophorus. The simplest 

 device which has been introduced 

 for the development of electricity. 

 It was invented by the Italian 

 scientist Alessandro Volta in 1775, 

 the general arrangement being 

 shown in the accompanying illus- 

 tration. It consists of a metal dish 

 about a foot in diameter, called the 

 sole-plate; a layer of some good 

 non-conducting substance, glass, 

 indiarubber, ebonite, or pitch may 

 be used, but the usual material is 

 resin ; a metal disk, called the 

 cover, rather smaller than the sole- 

 plate ; and a glass rod attached 

 to the disk and serving the purpose 

 of a handle by which it may be 

 lifted. 



In using the apparatus the 

 parts are usually first warmed to 

 ensure that they are dry, and the 

 resin base is then struck or rubbed 

 with a piece of cat's-skin or other 

 fur, or a piece of dry woollen cloth, 

 and is thus electrically " excited," 

 the charge of electricity developed 

 upon it being negative. The metal 

 upper plate is then placed on the 

 resin base ; it does not, however, 

 receive from the resin any direct 

 charge of electricity, but by induc- 

 tion develops a charge of positive 

 electricity on the surface of the 

 disk where it is in contact with the 

 resin and a charge of negative elec- 

 tricity on the upper surface of the 

 disk, as shown 

 in the sectional 

 illustration. If 

 now the upper 

 surface of the 

 disk be touched 

 by the finger 

 and thus put 

 into electrical 

 contact with 

 the earth, or 

 "earthed," to 

 use the techni- 

 cal expression, 

 the negative 

 charge of the 

 disk will pass 

 to earth leav- 

 ing the disk 

 positively elec- 

 trified through- 

 out,a n difn g ow 

 planation see text 1 1 f t e d a w ay 



