118 METHODS OF COMMUNICATION FOR FOREST PROTECTION 



Section 85 Definition of Electrical Terms 



(a) Voltaic Cell. A voltaic cell is an arrangement of two electro-chemically dii> 

 fereiit solids in a liquid which will attack one of them more actively than the other. 

 This liquid is called the electrolyte and is generally an acid. One of the solids is 

 nearly always zinc. Carbon is very commonly the other. Dry voltaic cells usually 

 consist of a zinc cup containing a carbon plate and an absorbent porous substance 

 saturated with the liquid electrolyte. This cup is sealed with wax or asphaltum. Dry 

 cells are now commonly employed to furnish the current for the transmission of speech 

 by telephone. Inert dry cells that contain all the necessary elements except water 

 may be o'btained. They are entirely inactive until water is added and keep indefinitely, 

 while the ordinary type deteriorates rapidly even though not in use. 



(fr) Conductor. Any substance through which electricity flows readily is called 

 a conductor. Most metals are good conductors. In conductivity, copper is surpassed 

 only by silver and is followed by aluminium, zinc, iron, tin, lead, and carbon in the 

 order named. Iron has only about -one-sixth the conductivity of copper. 



(c) Insulator. Any substance through which electricity will not flow readily is 

 termed an insulator. " Non-conductor " or " dielectric " are other terms having the 

 same meaning. Among common insulators are ebonite, glass, resins, paper, paraffin, 

 porcelain, mica, rubber, and dry *air. " Conductor " and " insulator " are, however, 

 relative terms. No substance is an absolute insulator and all substances oppose some 

 resistance to the flow of electricity, so that no substance is a perfect conductor. 



(d) Circuit and Grounded Circuit. The entire path along which electricity flows 

 is called a circuit. It comprises the voltaic cell or cells called a battery or some other 

 device, such as a generator, for producing electrical pressure, and the wire or other 

 conductor connecting the battery plates or the ends of the generator coils. Bringing 

 the two extremities of the wire in contact and separating them are called, respectively, 

 closing and opening, or making and breaking, the circuit. A complete metallic circuit 

 is unnecessary, it being possible to use the earth itself as a return path for the current. 

 It acts more like a great reservoir of electricity than as a conductor. A circuit, part 

 of which consists of the earth, instead of being a complete metallic path, is called a 

 grounded circuit. 



(e) Magnet. A magnet is a piece of iron or steel (generally a bar) which has 

 the property of attracting other pieces of iron or steel. If freely suspended at it? 

 centre it will point north and south. It can impart these properties to another piece 

 of iron or steel without losing any of its own. These powers may also be -imparted to 

 a piece of iron or steel by passing an electric current through an insulated wire wound 

 around the iron or steel. Steel will retain these magnetic properties; very soft iron 

 will retain practically none. An iron magnet thus produced is called an electro- 

 magnet. 



(/) Electro-motive Force The Volt. That force which maintains or tends to 

 maintain a current of electricity through a conductor is called electro-motive force. 

 The abbreviation for this term is E.M.F. Electro-motive force is, in effect, electrical 

 pressure and is analogous to the " head " or pressure which maintains a flow of water 

 through a pipe from one vessel to another at a lower level. Electricity flows in a con- 

 ductor only when there is a difference of such electrical pressure or difference of 

 "potential" between its own ends. Such a difference may be maintained by a voltaic 

 cell or t>y a generator. It is this difference of pressure which sets up a current in the 

 conductor, and as long as it is maintained the current will continue to flow. The 

 unit of E.M.F. is the volt, for practical purposes, about the E.M.F. of a Darnell's cell, 

 i.e., it is about the difference of potential between the zinc and copper plates of this 

 cell. 



