Electric Bells and All About Them: A Practical Book for Practical Men

by Selimo Romeo Bottone

themselves in a definite position along the lines of greatest attractive force; or, as scientists usually say, the iron filings arrange themselves in the direction of the lines of force. The entire space acted on by the magnet is usually known as its "field." [Fig. 3] gives an idea of the distribution of the iron filings, and also of the general direction of the lines of force. It is found that if a body be moved before the poles of a magnet in such a direction as to cut the lines of force, electricity is excited in that body, and also around the magnet. The ordinary magneto-electric machines of the shops are illustrations of the application of this property of magnets. They consist essentially in a horse-shoe

magnet, in front of which is caused to rotate, by means of appropriate gearing, or wheel and band, an iron bobbin, or pair of bobbins, coiled with wire. The ends of the wire on the bobbins are brought out and fastened to insulated portions of the spindle, and revolve with it. Two springs press against the spindle, and pick up the current generated by the motion of the iron bobbins before the poles of the magnet. It is quite indifferent whether we use permanent steel magnets or electro-magnets to produce this effect. If we use the latter, and more especially if we cause a portion of the current set up to circulate round the electro-magnet to maintain its power, we designate the apparatus by the name of Dynamo.

Fig. 4.
Typical Dynamo, showing essential portions.

§ 16. Our space will not permit of a very extended description of the dynamo, but the following brief outline of its constructive details will be found useful to the student. A mass of soft iron (shape immaterial) is wound with many turns of insulated copper wire, in such a manner that, were an electrical current sent along the wire, the mass of iron would become strongly north at one extremity, and south at the other. As prolongations of the electro-magnet thus produced are affixed two masses of iron facing one another, and so fashioned or bored out as to allow a ring, or cylinder of soft iron, to rotate between them. This cylinder, or ring of iron, is also wound with insulated wire, two or more ends of which are brought out in a line with the spindle on which it rotates, and fastened down to as many insulated sections of brass cylinder placed around the

circumference of the spindle. Two metallic springs, connected to binding screws which form the "terminals" of the machine, serve to collect the electrical wave set up by the rotation of the coiled cylinder (or "armature") before the poles of the electro-magnet. The annexed cut ([Fig. 4]) will assist the student in getting a clear idea of the essential portions in a dynamo:—E is the mass of wrought iron wound with insulated wire, and known as the field-magnet. N and S are cast-iron prolongations of the same, and are usually bolted to the field-magnet. When current is passing these become powerfully magnetic. A is the rotating iron ring, or cylinder, known as the armature, which is also wound with insulated wire, B, the ends of which are brought out and connected to the insulated brass segments

known as the commutator, C. Upon this commutator press the two springs D and D', known as the brushes, which serve to collect the electricity set up by the rotation of the armature. These brushes are in electrical connection with the two terminals of the machine F F', whence the electric current is transmitted where required; the latter being also connected with the wire encircling the field-magnet, E.

When the iron mass stands in the direction of the earth's magnetic meridian, even if it have not previously acquired a little magnetism from the hammering, etc., to which it was subjected during fitting, it becomes weakly magnetic. On causing the armature to rotate by connecting up the pulley at the back of the shaft (not shown in cut) with any source of power, a very small current is set up in the wires of the armature, due to the weak magnetism of the iron mass of the field-magnet. As this current (or a portion of it) is caused to circulate around this iron mass, through the coils of wire surrounding the field-magnet, this latter becomes more powerfully magnetic ([§ 13]), and, being more magnetically active, sets up a more powerful electrical disturbance in the armature.

This increased electrical activity in the armature increases the magnetism of this field-magnet as before, and this again reacts on the armature; and these cumulative effects rapidly increase, until a limit is reached, dependent partly on the speed of rotation, partly on the magnetic saturation of the iron of which the dynamo is built up, and partly on the amount of resistance in the circuit.