Page:The New International Encyclopædia 1st ed. v. 06.djvu/900

* ELECTROMETER. 784 ELECTROSCOPE. work. The others tigure mostly in experiments in statical electricity and for accurate determina- tions rei|uiring careful manipulation. A descrip- tion of electrometers wil be found in most of the works on electricity and physics, among which Tliompson, Elementari/ /.es.vojis in Electricity and Magnetism {Sew York, 1901), can be recom- mended as a simple treatise. For the advanced student, Miiller-Pouillet-Pfaundler, Lchrbuch der I'hiisik (Brunswick. 18!»8), will be found to con- tain n detailed description of the more important instruments, while the various journals dealing with physics can also be referred to for further information. ELECTRO - MOTIVE FORCE. See Elec- TiticiTV, where, under Eleetiokiueties. the fun- damental ideas of electricity are discussed. See also r.LEcrKo-CiiiMisTHY. (Jenkhal. ELECTRON HYPOTHESIS. See Eu:ctbo- ClIE.MISTRY. (_;em;KAL. E'LECTROPH'ORUS (Neo-Lat.. from Gk. 1j(KTpi'p. i~lel:tioii. iimlicr + -0ofpos. -phoros, bear- ing, from ipiptiv. {lilt rein, to bear). This appa- ratus consists of a circular disk of hard rubber, vulcanite, shellac, or other insulating material on a metallic surface or form, and a movable metal cover with an insulating handle. The disk is first negatively electrified by being struck briskly with a foxtail or eatskin, and then the metal cover is placed on it. The next step is to touch the cover with the finger, which may produce a sliglit spark, and then grasping the insu- lating handle to raise it. keeping it parallel to the sur- face of the disk. If presented to the finger or any body in contact with the ground, a spark will ensue, and this can lie secured for a number of times by simplv placing llie cover on the disk, touching it with the linger, and remov- ing. This jirocpss can lie repeated until the disk loses its charge by leakage into the air. The ac- tion of the clecti'ophorus depends upon induction, and the underlying principle is the same as in the electrical macliine and other electrical api)a- ratus. Wlien the vulcanite nr shellac is rubbed it is negatively charged, the charge residing some- wliat below the surface. Xow, when the cover is put on. the negative charge of the disk acts on the plate inductivelv, attracting the positive elec- tricity to the lower side and rejielling the nega- tive to the upper side. When the finger is lirought into contact with the matal it ail'ords an oppor- tunity for this negative electricity to escape to the ground, and leaves the cover with a positive charge which is held bound, liaising the cover now posit ivel.v charged, and bringing it near some object in connection with the ground, we have a sharp spark as the «(|ui1ibrinni is restored and the plate discharged. The electrophorus can be used to charge bodies and (lerform a number of experiments in frictional electricity. See Elec- tricity. * ELECTRO-PLATING. . process by means of whidi salts of gold, silver, and other precious ELECTEOPHORU6. metals are decomposed by the gahanic current, and a coating of their metals is deposited on the surface of objects made of some cheaper metal. In 1838, Jacobi, of Saint Petersburg. |)ublished a description of a method capable of reprotlucing any line engraved on copper into a relief by gal- vanic process. Soon after, Thomas Spencer, of Liverpool, independently announced his diseovery of a similar process. From these discoveries have grown the extensive electro-plating processes that include the deposition of gold, silver, copper, nickel, and other metals. In order to obtain a reproduction — as, for in.stance, a medal or similar object — a mold or cast is made on jvhich the layer of metal is to be deposited by the current. This is sometimes of an inferior metal, as base or white metal; or. as in the case of ty|K'. of a mold made of some non-conducting material, such as wax or guttapercha, which must be carefully brushed over before inmiersion with very finelv [lowdered graphite. (See Pki^tixg.) In order that the metal to be deposited ma.v adhere perfectly to the object to be plated, it is necessary that it shall lie perfectly clean; it is therefore usuall.v dipped in a cleansing solution, as of an acid or a caustic alkali, and then rinsed in water to remove all traces of the cleansing solution. The current for the electrolytic deposition of metals should be of great constancy. The Hunsen. Daniell, Grove, and Smee batteries are often used to supply the current for electro-plating small objects; but for commercial purposes it is customary to employ a dynamo machine as the source of electricity. The solutions employed in elet'lro-plating are very numerous, but in ordinary commercial practice they consist of the fcdiowing salts dissolved in distilled water: For (lold, a solution of gold cyanide and potassium cyanide; for silver, a solution of silver cyanide and potassium cyanide; for copper, an amnioniacal solution of copper and potassium cyanide; for nickel, a solution of nickel and ammonium sulphate, or of the corre- sponding chloride. The electrolysis is elTected in a convenient bath, which, when small objects are to be plated, may be of glass or iionelain. but in commercial practice is a trough lined with a non-conducting material. Tlie latter may be wood or cement, in the case of silver; zinc, in the case of copper; and asphaltum. in the case of nickel. The object on which the metal is to be deposited is connected by a wire with the nega- tive electrode of the battery, while the positive pole is'oonnccted with an anode of the same metal which is to be deposited on the object, thus clos- ing the circuit. As fast as the sohition is dccom- jiosed the lilicratcd acid dissolves the metallic anode, in conse'ipience of which the solution is kept in about the same state of concentration. Consult: McMillan. Treatise on Electro-Metal- Inrg!) (London, 1800) ; Partridge, .1 Practical Treatise on the Art of Elect rot t/pinfi (Chicago. 1809) ; llrquhart, Electro-Platin(j. .1 I'nictical Handbook on the Deposition of Copper, ffilrcr, Xickel, (lold. Aluminium, Urass. Platinum, etc. (London. 1S04) ; Van Home, Modern Electro- Platinq (Chicago, 1897). See Electro-Chemis- TRY. IN'DISTHIAI.. ELECTROSCOPE (from Gk. oi'. llek- tron, ;imlK'r -f- ctkottuv. sknprin. to lo(ik ). An instrument for the detection of the presence of electricity. Tt depends for its action on the prin- ciple that bodies charged with like electricity re7)el, while those charged with unlike electricity