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31. If the conducting wire is covered with an insulator (5), such as silk or cotton, so as to compel the current to traverse its entire length, and is wound into a spiral or coil, surrounding a magnetic needle, the needle will be deflected from its natural position, and will tend to take up a position at right angles to the direction of the current. If the current be passed in the opposite direction through the wire, the deflection of the needle will also take place in the opposite direction. The Galvanometer, an extremely useful instrument for the purpose of indicating the presence, direction, and strength of a voltaic current, is constructed upon this principle.
32. If the conduction wire, covered as above, be wound upon a bar of soft iron, the iron becomes magnetic as long as the current continues to flow, and possesses the property of attracting other pieces of iron in its vicinity. This arrangement is called an electromagnet. (34.)
33. If the iron is very soft and pure it loses its magnetism instantly upon the cessation of the current, but if impure, or if hardened by hammering or turning, it retains a certain amount of residuary magnetism, especially after it has been acted upon by a powerful current. It is, therefore, necessary that the iron cores, as they are termed, of electro-magnets, should be annealed with great care.
34. Electro-Magnets are generally made in a `U' form, two bobbins or spools, a a (fig. 8), being filled with covered copper wire, and the soft iron cores, c c, passing through them, fixed upon a connecting bar, b, also of soft iron, as shown in the figure. The two spools, a and a, are virtually continuations of one spool, the direction being apparently reversed by the bend of the `U'. The ends of the cores, c c, opposite to the connecting bar, are called the poles of the magnet, the magnetic force being accumulated at these points. The bar of soft iron, d, upon which the magnet exerts its force, is called the armature.
35. In electro-magnets and galvanometers the magnetic effect of the current is multiplied by the number of convolutions of the wire in the coil, but it is diminished in proportion to the distance of the wire from the core, each layer of wire acting with less power than the one beneath it.
36. Every addition to the length of the conducting wire enfeebles the current, because of the increased resistance (5, 6,) it offers to its passage. In a very long circuit, such as a telegraph line, the action of the current will necessarily be feeble, and the coil is, therefore, made of fine wire, which occupies little space, and allows many layers to be wound on without too greatly increasing the distance from the cores, while its resistance is too small in proportion to the rest of the circuit to reduce the strength of the current materially.
37. When, however, the circuit is very short, coarser wire is employed in the coil. A fine wire would add to the resistance of the circuit more than would be made up by the effect of an increased number of turns, for even a very few layers would double the resistance of the circuit.
The former is frequently called and intensity, and the latter a quantity magnet.
38. Iron does not acquire its full magnetism instantaneously, and the act of demagnetization also requires time, but is effected more rapidly than magnetization. The greater the tension of the battery the more rapidly the iron acquires its magnetism ; therefore, if very rapid action is required, even on a short circuit, a number of cells of battery must be used.
It has also been ascertained by experiment that an electro- magnet with short cores, will acquire and lose its magnetism with much greater rapidity than one with long cores, but in other respects similar.
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