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ELECTRICAL ACTl 15
known as a "two-pole, direct current, shunt wound generator or dynamo." (34) N is the north and S the south pole of its permanent magnet: FF its held windir its commutator, DD the brushes, B the armature, H the field rheostat and EE the outside circuit to which current is supplied by the generator. 1, 2, 3 and 4 are resistance coils of the field rheostat, upon either contact button of which the movable arm just under G may be made to rest. As the arm is now it is plain that all these coils are "cut out," there being no resistance in series with field coil FF.
(35) Magnetic force like electromotive force represents pressure. (36) When a wire is coiled around a suitable iron core in close proximity thereto and a. c. is sent through the wire the iron will be magnetized; (37) the magnetizing power exerted will be in exact proportion to the current strength (E.M.F.) and the number of turns the wire makes. (38) The density (pressure) of a magnetic field is usually spoken of and considered as the number of lines of magnetic force per square inch. The greater the magnetic intensity of a field the greater the number of lines of force per unit area.
(39) The capacity of such a machine would, within certain limits, depend upon (1) the strength of field magnet NS, since that controls the number of lines of magnetic force the commutator coils will cut during each revolution of the armature. (2) The number of turns of each armature coil and the number of coils mounted on the armature. (3) Number of rotations of the armature per minute. Each of these items, you will observe, has to do with the number of lines of magnetic force that will be "cut" per second. Other things enter, such as, for instance, the kind of armature core, size and winding of magnets, etc., but our purpose here is only to briefly explain the principles upon which the electric generator operates. With infinite variations in method, these are always the same.
(40) In Fig. 3, N and S is a "permanent magnet" by which is meant that the magnet retains some of its magnetic force when the machine is not in operation, and