ELECTROSCOPE 



while the use of nickel- 

 plated articles both of 

 ornament and utility is 

 rapidly extending. The 

 process Is precisely the 

 same as that which has 

 just been described in the case 

 of the teapot ; the only differences 

 being in the solutions and in 

 the anode plates employed, which 

 must be prepared according to the 

 metal to be deposited. In some 

 cases a preliminary plating is 

 given to promote the deposition 

 and adhesion of the final plating 

 metal. Thus, when iron or steel 

 articles are to be nickel-plated they 

 are first given a coating of copper. 

 Such nickel-plated articles are now 

 largely used for military weapons, 

 household utensils, and parts of 

 bicycles, motor-cycles, and motor- 

 cars. Other metals occasionally 

 deposited include tin, on iron cast- 

 ings ; and iron, on engraved copper 

 plates or on electrotypes to give a, 

 harder surface so as to permit a 

 greater number of impressions to 

 be taken. See Electrolysis. 



Electroscope. Term given to an 

 instrument universally employed 

 in the study of electricity for the 

 purpose of determining whether a 

 body is electrified or not, and if so, 

 the character of the electrification, 

 whether positive or negative. In its 

 simplest form it consists of two 

 small balls of pith suspended by silk 

 threads from the arm of a metal 

 stand, as shown below. An 

 almost equally simple form is repre- 

 sented by the balanced needle, simi- 

 lar to the needle of a mariner's 

 compass, devised by Dr. Gilbert 

 of Colchester 

 in 1600. 



The usual 

 modernform 



Electroscope. Simple pith ball 

 electroscope 



electroscope consists of a glass bell 

 jar commonly fitted with a brass 

 ring round the bottom or fitted to a 

 wooden base so as to be easily re- 

 movable. The jar has a stopper of 

 ebonite in which is fitted a stout 

 brass wire with a removable plate 

 or ball at the top, and from the 

 bottom of which two strips of gold 

 or aluminium leaf depend. Oppo- 

 site each strip and within reach of 

 its end is a strip of tin foil attached 

 to the interior surface of the jar. 

 To ensure the dryness of the in- 

 terior of the jar when very delicate 

 determinations are to be made, a 

 shallow dish containing sulphuric 



2858 



acid may be placed within. A glass 

 rod electrically excited by having 

 been rubbed, if brought 

 near the instrument, will 

 cause the leaves to 

 repel one another; in 

 favourable conditions 

 so sensitive is the 

 instrument that 

 the leaves will 

 begin to move 

 apart while the 

 glass rod is several 

 feet away. What 

 happens is that 

 the electricity on 

 the rod attracts 

 the opposite kind 

 into the plate or 

 knob of the elec- 

 troscope and re- 



Electroscope. 

 Modern form of 

 gold leaf elec- 

 troscope 



pels the same kind into the leaves, 

 which fly apart in accordance with 

 the well-known law that like elec- 

 tricities repel and unlike attract. 



In determining the particular 

 kind of electricity on an object, the 

 electroscope is first charged by 

 touching the knob or plate with a 

 glass rod that has been rubbed with 

 silk, which causes the leaves to 

 diverge under the influence of posi- 

 tive electricity. If then the body 

 whose character is to be determined 

 be brought near the plate and the 

 leaves diverge still further, the body 

 is electrified positively ; if the 

 leaves close, it has a negative 

 charge. The strips of foil on the 

 sides of the jar are used to prevent 

 the leaves from being damaged by 

 the violence with which they are 

 repelled or from adhering to the 

 sides of the jar. The instant they 

 touch the strips of foil they lose 

 their charge to the earth, and fall 

 back into their normal positions. 

 In Volta's condensing electro- 

 scope the normal plate of an 

 electroscope is enlarged and 

 a second is prepared to rest 

 upon it as shown ; the under 

 surface of the latter is well 

 varnished to insulate it from the 

 lower; the upper surface of the 

 latter may also be varnished. The 

 two plates make a condenser. In 

 using the instrument one or other 

 of the plates is charged by means 

 of the body to be tested while the 



I 



Electroscope. Balanced needle type 

 invented by Dr. Gilbert of Colchester 



other is connected to earth ; they 

 thus receive charges of opposite 

 kind. If now the upper plate be 

 raised the charges become intensi- 

 fied in accordance with the prin- 

 ciple of the electric condenser, 

 the one becoming more strongly 



ELECTROTYPING 



negative and the other more 

 strongly positive, with the result 

 that an extremely weak charge will 

 be detected by the divergence of 

 the leaves. 



By means of this instrument 

 Volta demonstrated that the con- 

 tact of dissimilar metals in air gave 

 rise to opposite kinds of electrifica- 

 tion. The electroscope has rendered 

 valuable service in the study of 

 radio-activity and of the character- 

 istics and properties of atoms. 



Electrostatics. Term applied 

 to that branch of the science of 

 electricity which is concerned with 

 electricity at rest or with electric 

 charges, and is particularly occu- 

 pied with the measurement of such 

 charges. The science deals alike 

 with the most elementary facts of 

 electricity, such as the phenomena 

 exhibited by a rubbed glass rod, 

 and with the profound problems 

 associated with the electrical rela- 

 tions of atomic particles. 



Electrostatic machines are ma- 

 chines for the conversion of me- 

 chanical work into electric energy 

 and are of two kinds, frictional 

 and influence machines. Rams- 

 den's plate electrical machine be- 

 longs to the former class, and 

 Wimshurst's well - known appa- 

 ratus to the latter. 



Electrotyping. Particular form 

 of electro -deposition. It differs 

 from electro-plating in that the 

 metal deposited does not become 

 an intrinsic portion of the article on 

 which it is laid, but is removed 



Electroscope. Volta's condensing 

 electroscope 



from the latter after it has been de- 

 posited ; its object is not to encase 

 or permanently cover one metal 

 with another, but to make a copy 

 of a surface. It is largely used 

 for making reproductions of coins 

 and medals and other works of art, 

 and for preparing " electrotypes " 

 for printing, both from typed 

 matter set up in the usual way and 

 from engraved surfaces in wood or 

 metal. Electrotype is the term 

 given to the product of the process. 

 If a reproduction of a medal is 

 required, and the original is not too 



