120 METHODS OF COMMUNICATION FOR FOREST PROTECTION 



are 10 turns in the primary and 100 turns in the secondary and a current of 1 ampere 

 at 10 volts pressure is passed through the primary, then the current induced in the 

 secondary will have a pressure of 100 volts but a current strength of only 0-1 of an 

 ampere. This principle is made use of in telephone transmission, in induction tele- 

 graphy, and in wireless. 



(&) Electro-magnetic Generator. The electro-magnetic generator is a device by 

 which induced currents are -produced continuously and made to now through an 

 exterior closed circuit. The dynamo generator consists of a magnet to produce a 

 magnetic field, and a coil of wire wound on an iron core, which latter part is called, 

 the armature. The armature is that part of the circuit in which the induced current is 

 generated. If moved in the magnetic field of the magnet in such a way as to cut its 

 lines of force, a current is produced by induction, and if the motion of the armature 

 is continuous the current that results is continuous also. Electro-magnets are usually 

 employed to produce a magnetic field but in some small generators permanent steel 

 magnets are used. Such generators are called magnetos and being extensively used in 

 certain types of telephones have given rise to the term " magneto telephone." 



(Z) Direct Current and Alternating Current. The electric current among its 

 other properties has direction. This has already been indicated in referring to the 

 production of induced currents as in an induction coil. A current which moves 

 continuously in one direction only is called a direct current. Such a current is that 

 which flows from the carbon pole to the zinc pole of a voltaic cell when these are con- 

 nected by means of a conducting material, such as a piece of copper wire. The current 

 used in ordinary telegraphy is a direct current. As contrasted with this form of 

 current we have that which is constantly changing in direction, flowing first m one 

 direction and then reversing and flowing in the other. This is known as an 

 alternating current and is produced by many forms of dynamo generators and in the 

 secondary of an induction coil. Alternating currents are especially important in tele- 

 phone work as both the talking currents and those which operate the common type of 

 polarized bell are of this character. So also, is the current which actuates the howler 

 used in vibratory signalling. 



(m) Frequency. This term is used to designate the rate of reversal of direction 

 of alternating currents. High-frequency currents are alternating currents that reverse 

 direction very rapidly, while low-frequency currents reverse comparatively slowly. 



(n) Tension. This term as applied to electricity has the same meaning as 

 potential difference or E.M.F. High-tension currents are those of high voltage or 

 E.M.F. 



(o) Electro-static Induction. An insulated conductor, such as a sheet of tin-foil, 

 has the property of receiving a charge of static electricity when subjected to an E.M.F., 

 as, for instance, if it is connected to one pole of a cell, the other pole being connected 

 to the ground. The charge that it receives is of the same polarity as the terminal of 

 the cell to which it is connected and the amount of electricity held depends upon the 

 capacity of the conductor. At the same time it is always found that when a conductor 

 is charged as above described, an equal charge of opposite polarity is induced on the 

 bodies surrounding it. If two sheets of tin-foil, for instance, are insulated from each 

 other by means of a sheet of mica and one of them is charged as above, the other will 

 be found to have an equal charge of opposite polarity. This action by which bodies are 

 charged through an insulating medium is called electro-static induction. 



(p) Condenser. -A condenser is a device for storing static electricity. It consists 

 of two insulated conducting bodies separated from each other by an insulating material. 

 The common form of condenser used in telephone work consists of sheets of tin-foil 

 separated by sheets of paraffin-paper. However, any two insulated bodies, as, for 

 instance;, two parallel wires or a single wire and the surface of the earth, or the 



