ELECTROMETER 



set free by the action, will not be dis- 

 solved, but will settle to the bottom 

 of the vat in the form of slime. 



The action goes on so long as 

 there is any copper left at the anode 

 and the current is continued, or 

 until the bath becomes " sick " 

 with the dissolved impurities, 

 when the current is cut off, the 

 bath allowed to settle so that all 

 the gold and silver may go down, 

 the liquid carefully decanted, and 

 the slimes removed for separate 

 treatment for the recovery of the 

 precious metals. The cathode will 

 have grown in thickness by the 

 deposition of pure copper, and is 

 removed for further treatment 

 because, while it contains only 

 minute traces of foreign sub- 

 stances perhaps 4 or 5 parts in 

 10,000 parts of the metal its 

 physical condition is too spongy to 

 allow it to be sent directly into the 

 market It is, therefore, melted 

 and cast into ingots, and appears 

 in commerce in that form as elec- 

 trolytic copper. The solution, 

 loaded as it is with impurities, is 

 usually thrown away. 



The refining process is slow ; it 

 cannot be hastened beyond a 

 certain limit without detriment to 

 the quality of the finished metal ; 

 the crude copper plates may weigh 

 anything from 200 Ibs. to 600 Ibs. 

 each, and the entire operation may 

 require several weeks' time. A 

 large refinery (there are now estab- 

 lishments turning out 50,000 tons 

 of refined copper a year) contains 

 a considerable number of cells 

 which are formed into batteries, 

 the cells in each being connected 

 in series. In addition to the simple 

 type of cell shown in the diagram, 

 a more complex form is also used, 

 in which a number of plates are 

 suspended side by side, the crude 

 and the pure plates alternating. 

 The process is of great importance 

 not only because it is a highly con- 

 venient method of obtaining cop- 

 per, and economical especially 

 where cheap water power is avail- 

 able for the development of elec- 

 tricity, but because of the remark- 

 able purity which may be given to 

 the finished metal, which makes it 

 peculiarly suitable for electrical 

 work. 



" Base bullion," i.e, gold or 

 silver collected in a mass of lead, 

 is also refined by this process. The 

 bullion is cast into thin plates, 

 which are enclosed in muslin bags 

 and made the anodes of a cell ; the 

 cathode is a thin sheet of pure lead, 

 and the bath is a solution of lead 

 sulphate in sodium acetate. The 

 lead of the bullion passes into so- 

 lution, and then deposits on the 

 lead cathode, leaving the gold and . 

 silver, and perhaps other metals, 



2855 



ELECTROMETER 



in the muslin bag in a form ready Various forms of electrometers 

 for subsequent treatment. The have been proposed. Interesting 

 process in this instance may be designs are associated with the 

 regarded as a lead refining opera- 

 Zinc is also refined on similar 



tion. 

 lines. 



The Elmore process (q.v-.) is a 

 particular apph'cation of this prin- 

 ciple of electro-metallurgy. The re- 

 covery of gold from a cyanide solu- 

 tion in the Siemens' modification 

 of the McArthur-Forrest process 

 is another example of this type 

 of electro-metallurgical operations, 

 only in this instance it is a case 

 of the extraction of a metal from 

 its ores, and not of refining. It 

 has been proposed to apply this 

 system to the extraction of other 

 metals, particularly copper, from 

 its ores, and while this has not been 

 done on any considerable scale, 

 the trend of metallurgical science 

 is strongly in that direction, and 

 there can be little doubt that this 

 development will come in due 

 course, particularly in regions 

 where cheap water power, as in 

 Tasmania, is available contiguous 

 to the source of the ores. Mount 

 Lyell mine already uses the electro- 

 lytic process in the treatment of 

 its crude copper from the Bessemer 

 converter. 



For the two other types of 

 electro-metallurgical processes, i.e. 

 those in which heat is an active 

 agent either by itself or in combi- 

 nation with elec- 

 trolytic action, 

 what is essentially 

 a furnace, and not 

 merely a cell or 

 battery of cells, is 

 required. See Alu- 

 minium ; Furnace. 



Electrometer. 

 lustrum e n t for 

 measuring electri- 

 city. It is to be 

 distinguished from 

 indicators, such as 

 ampere meters 

 (ammeters) and 

 the volt meter, 

 and also from 

 electric meters, 

 although all these 



names of Dellmann and Peltier, 

 but the first really effective in- 

 struments were introduced by Lord 

 Kelvin, when Sir William Thom- 

 son. The instruments almost uni- 

 versally in use to-day are in all 

 essentials of Kelvin' s design. These 

 are mostly two, the first being what 

 Kelvin called the attracted-disk 

 or trap-door electrometer, and the 

 other the well-known quadrant 

 electrometer. 



The former consists essentially 

 of a Leyden jar (q.v.) containing 

 special provision for keeping its 

 interior dry, in which are placed 

 two parallel disks of metal, one 

 fixed at the bottom of the jar 

 and the other adjustable and sus- 

 pended over the former in such a 

 way that it can be moved closer to 

 or farther away from the fixed 

 disk as required. The fixed disk has 

 near its centre a hole covered with 

 a thin piece of aluminium constitu- 

 ting the trap-door and the indi- 

 cator of the instrument. The trap- 

 door is attached to a fine platinum 

 wire, and prolonged so as to have 

 the form of a lever, the arrange- 

 ment being such that it may be 



Electrometer. Kelvin's quadrant electrometer, showing 



instrument as a whole and enlarged views of quadrants 



and needle. For full explanation see text 



instruments are used to measure 

 electricity. These latter instru- 

 ments, however, are the more per- 

 manent, practical instruments of 

 industrial electricity power de- 

 velopment and commercial supply. 

 The electrometer, while it is con- 

 tinually being used for practical 

 purposes and for certain determina- 

 tions is indispensable, is more an in- 

 strument of research and of special 

 and ' delicate tests and indications. 

 Its purpose is to measure difference 

 of potential, that is to say, electric 

 pressure ; it may be said to be a 

 highly developed electroscope. 



attracted by the adjustable disk 

 above it when the electrical con- 

 nexion is made. The potential 

 difference is determined by the 

 distance apart of the two disks 

 when the trap-door is in a deter- 

 mined position and the current 

 passing. This distance is read on 

 minutely divided scales forming 

 parts of the instrument. 



Kelvin's quadrant electrometer 

 is shown in its essential features in 

 the views appended, showing one 

 form of the instrument as a whole, 

 and enlarged views of the quadrants 

 and the enclosed needle. The body 



