If the paper be covered with mucilage the fil ings will maintain a permanent position so that they may be studied at the leisure of the student.
The field magnets may be made of steel, magnetized, or preferably they may be electro magnets made of soft iron over which a coil of wire is wound carrying a current of elec tricity which induces magnetic lines in the iron. It is to be noted that if the ends of the mag net are bent in the form of a horse-shoe, the lines will be intensified by the reduction of the air space between the poles, and as the amount of current induced in the wire depends on the number of lines of force cut, the current induced will be greater, the greater the strength of the field magnets.
Considering first the ideal simple dynamo: This would consist of a single loop of wire mounted on centres, and rotating between the poles of a magnet, placed horizontally, each end of the loop being connected to a collector which in direct current machines is called a commutator, and is mounted on the shaft out side of the poles, and insulated therefrom. If the loop is placed at right angles to lines of magnet force, in a vertical position and re volved through 180 degrees, each side will pass through the whole number of lines of force flowing between the poles which will induce a current in one direction in the loop. If the ro tation is maintained in the same direction dur ing the next 180 degrees, the loop will cut the lines of force in the opposite direction, that is, the lines of force will be passing through it in the opposite direction to that in the first case. This will induce a current which will be in the opposite direction from the current induced through the first half of the revolution; so that the current will be pulsating, first in one direction and then in the other, during each revolution.
If the collector or commutator be cut into two halves parallel with the shaft and the ends of the loop be connected one to each half, and if a pair of brushes be supplied to collect the current, one above and one below the com mutator, then when the loop is vertical the brushes will change contact from one end of the loop to the other, and as no current is then being generated, the change is made with out sparking and current flowing in the same direction continuously can be obtained from the brush terminals. During the moment of chang ing from the one contact to the other, the cir cuit is momentarily opened or interrupted. This would cause sparking at the brush or col lector, were it not that the brushes are placed at a point at which the current is practically zero. This is found in practice to be slightly in advance of the theoretical neutral point on account of lines of force being dragged in the direction of rotation by the conductors.
To advance from the ideal simple dynamo: —the next step is to reduce the air gap between the poles of the field magnet and concentrate the lines of force in the effective space. This is accomplished by placing an iron core on the armature which in the first place reduces the magnetic resistance of the air gap and thus in creases the number of lines of force through the armature conductors, and also serves as a support for them. Other machines were built with shuttle wound armatures, the arm ature consisting of an iron shuttle cut out with grooves longitudinally to take the con ductors. These were usually wound with a number of turns of copper wire, the ends being brought out to a two-part commutator. (See