CONDUCTOR and INSULATOR OF ELECTRICITY. The property of electrical conductivity is possessed in some degree by all !mown substances. There is, however, an enor mous difference between the conductivity of those that are used as conductors and those that are used as insulators, the former having many million times the conductivity of the latter. The substances at the lower end of the scale are therefore of sufficiently low conductivity to serve for covering and supporting wires made of the good conductors, without permitting any serious escape of an electric current when the wires are charged.
In transmitting electricity from point to point, through telegraph or electric-light lines, for ex ample, those substances possessing the greatest conductivity, such as copper and iron, arc selected to form a path for the current, and these are sur rounded with materials which have the least con ductivity or offer the highest resistance, such as air, rubber, and glass, in order to confine the current and compel it to travel to the end of the line. When a conductor is so surrounded or so placed on non-conducting supports that it pre vents the electricity communicated to it from passing into the ground or escaping, it is said to he insulated.
As the difference between conductors and in sulators is merely one of degree, there is, even under the best conditions, a loss in the transmis sion of electricity over a line of wire propo•t-ion ate to the amount of leakage through the insula tion and the resistance to travel offered by the wire itself, since the slight effort required of the current to force its way ahead through the wire causes some of it to escape through the insula tion in the same way that water forced through a long pipe will escape in small quantities at weak points. It is, indeed, extremely necessary to prevent conductors buried in the earth from having any connection with water or dampness, and, if the imsnlation of the conductor is defective in a damp spot, a certain proportion of the cur rent will leave the conductor and travel through the earth. Chemically pure water is a non-con ductor, but as found in the earth, water is an excellent conductor on account of the materials it holds in solution.
The electrical conductivity, and its reverse, the resistance of materials, are, therefore, subjects of great importance to electricians; and the selec tion of materials of high conductivity or low re sistance for wires, and materials of high resist ance for insulators, receives careful attention. The
best conductors are gold, copper, silver, alumi num, brass, iron, and all other metals. The best insulators, or the worst conductors, are dry air, glass, mica, porcelain, paraffin, rubber, silk, gutta-percha. and almost all the hydrocarbons. The intermediate substances are many liquids and damp substances, carbons, minerals, and com pound substances. The conductivity of all sub stances is greatly affected by changes of tempera ture. An increase of temperature increases the resistance of all the metals, while it has the opposite effect in other substances. Glass loses its insulating propeilies at a red heat, and so do wax, sulphur, amber, and shellac when fused.
Glass, if not the most perfect insulator, far ex ceeds others in hardness and durability, and is much employed as insulation for light electrical apparatus. Hard rubber or vulcanite enters into the construction of much electrical apparatus on account of its high resistance. Paraffined paper is also employed for condensers (q.v.), while for the armatures of dynamos and motors mica is used. See ELECTRICITY.
CONE (Lat. conus, Gk. Ki.iroc, !Janos, cone, Slat. iuna, whetstone. from to sharpen). A solid formed by a plane cutting all of the ele ments of a conical surface. conical surface is generated by a line called the generatrix pass ing through a fixed point and tracing a. fixed curve called the directrix. If the line is unlim ited it generates two conical surfaces, on opposite sides of the point (vertex), called the nappes of the cone. A cone whose base is a circle is called a circular cone. If the vertex is on a perpen dicular to the base of a circular cone, through its centre, the cone is called a right circular cone. A right circular cone may be generated by rotat ing a right-angled triangle about one of its per pendicular sides. The line from the vertex to the centre of the base is called the axis. If the axis makea with the base an angle other than 90°, the cone is called oblique. If a plane cuts a cone between the vertex and the base, the cone is said to be truncated ; if the cutting plane is parallel to the base, the lower part is called a frustum of the cone, the section made by the cutting plane being called the upper base. Three curves, called conic sections (q.v.), may be formed by planes cutting a right circular cone at various angles to the base.