ELECTRICITY. 762 ELECTRICITY. conductor and the surrounding medium; and it can be proved that the rate at which the ampli- tude of the wave — that is. the maximum value of the current at any one point — decreases owing to absorption of the energy of the waves varies with the wave-length, l^ong waves, that is those for which p is small, die down more slowly than short waves, for which p is large. It is owing to this fact that long waves travel farther than short ones; and therefore on ocean cables very long electrical waves are used. A discovery made in moo by Professor Pupin of Columbia tniver- sity has shown a method by which a special kind of conductor — one in which coils of wire are in- troduced at rcuiilar distances — will convey all waves (within certain limits) to the same dis- tance, and will convey them much farther than will an ordinary wire or cable. H.LL Effect. One of the most important effects of a magnetic field on an electric current is ob- served when a thin sheet of metal, e.g. gold-foil, i.s traversed by an electric current and is held between the poles of an electro-magnet. When there is no magnetic field, the current flows through the sheet in a definite manner; and it is possible to find pairs of points on opposite edges of the sheet which have the same potential: this can be done bj' joining one terminal of a gal- vanoscope to one edge, and then joining the other terminal to such a point on the other edge that there is no current through the instrument. If now the electro-magnet is made to produce a field of magnetic force perpendicular to the sheet, there will be a steady current through the gal- vanoscope. If the magnetic field is reversed, so is this current. This efTect was first observed by E. H. Hall in 1880. and is called the Hall efi'ect. The action is exactly as if, owing to the magnetic field, the line joining the two points on the two edges, which were at the same potential, was rotated slightl.v around an axis parallel to the lines of magnetic force. This action varies great- ly for difl'erent metals, both in direction and amount, and is proportional to the strength of the magnetic field and to the current-strength in the sheet. Far.dat Kffect. Faraday obsei-ved in 1845 that, if a beam of i)lanc polarized light is passed through a hollow iron rod forming the core of an electro-magnet, the plane of polarization (see Light) is rotated when the current is passed around the helix forming the magnet. The rota- tion is in the direction of the current in the helix; that is. it ol)eys the 'right-handed screw law.' The amount of the rotation varies greatly with the material which occupies all or part of the hollow iron core, and is proportional to the intensity of the magnetizing current. Both the Hall and Faraday effects are evidences of the rotatory action around a magnetic field of force. Bini,ior.R.PiiT. While the bibliography of elec- tricity is extensive, the subject will be foimd fully treated in the larger text-books of physics. Of the reference books dealing exclusively with elec- tricity and magnetism the following are recom- mended, and for the convenience of the reader are grouped in tITree classes: Mnthemntwal and Atiranccd Treafixes: Clerk Maxwell. FArdricitji and Maqnetism (2 vols. .Lon- don, 1881); Webster. The Theori/ of EIrrlririly and Magnetism (Xew York, 1807) ; Thomson, Xotes on Itccent licscurvhcs in Electricity and Maynetism (London. KSIIS). Elvmcntary Tcxt-Jiuoks: Thomson, Elements of the Muthemalical Theory of Electricity and Magnetism (London, 1895) ; Gerard, Electricity and Maynetism, translated by Duncan (New York, 1897). Elementary and Dcscriptire Works: Per- kins, Outlines of Electricity and Magnetism (Xew York, 1890) ; Thompson, Elementary Les- sons in Electricity and Magnetisjn (London, 1891) : Gordon, Physical Treatise on Electricity and Magnetism (2 vols., London. 1883). ELECTRICITY, Animal. The property in animal tissues, as nerves and muscles, to produce electrical reactions. It is exhibited especially in various rays, eels, etc. (see Electuic Fishes), and in tlic hind leg of the frog. ELECTRICITY, Medical Uses of. Three forms of electricity are used in medicine: Static, galvanic, and faradic. Static electricity is obtained from a macliine in which glass plates are rai)idly revolved against cliamois rubbers or horsehair brushes. The elec- tricity is produced in a succession of sparks, which may be received in a l.eyden jar. or in the body of a patient, insulated by being placed on a glass-footed chair. Static electricity has but little quantity, but an enormous electro-motive force; the current has high potential and higli frequency but is devoid of apjireciable electro- lytic effects. Its use in medical practice is to modify and regulate functional processes, includ- ing secretion, excretion, sleep, respiration, circula- tion, and nutrition. Galvanic electricity is obtained from a galvanic battery, composed of several cells connected in the usual way. the terminal wires being inserted into metal binding posts, to which electrodes are fastened. The element-s used are zinc and carbon in most wet batteries, and the fluid in which they are immersed is a so- lution of bichromate of potash containing sul- phuric acid, and also a salt of mercury, that the zinc may be continuously amalgamated. A current-controller, or switch, is so arranged that one, two, three, or more cells may be used at a time, according to the strength of current de- sired. The electrodes, or terminals, which are a])plied to the body, are coniposc<l of metal disks, covered with sponge, and insulated so as to be gras])ed by the wooden handles by the physician without drawing off the current. The sponges are wet before applying. Galvanism causes a contraction of the muscle against which the elec- trode is applied, on application and again on withdrawal. During the interval of application there is a sensation of beat (also of pain if the amperage be great) and of tingling. . sensation of light, a nu'tallic taste, or a btizzing sound is cxjicrienced when the current is passed through the retina, the tongue, or the ear respectively, fialvanism is applicable in treating very many conditions and diseases of the nervous system, as well as of the greatest use in diagnosis of dis- ease. Faradism is obtained from a faradic battery. An important ])art of this apparatus is a small induction coil consisting of a core of short, straight, soft iron wires, of larcre diameter, around which is coiled many feet of wire. Thrinigli the wire is run an electric current which
