Fig. 229.—Magnetic field about a wire carrying an electric current.
Fig. 230.—Right-hand rule for the magnetic field of a current.
257. Magnetic Field of a Helix.—If a wire be wound about a cylinder to form a cylindrical coil with parallel turns, it forms a helix or solenoid. The shape of the magnetic field about a current depends upon the form of the conductor. If the latter is in the form of a helix its magnetic field resembles that of a straight bar magnet. (See Fig. 231). In fact the helix has the properties of a magnet with north- and south-seeking poles while a current is flowing through it. If such a coil is suspended so as to turn freely, it tends to turn until the field within it is parallel to the earth's magnetic field. Such a suspended helix may therefore be used as a compass. In order to strengthen the magnetic field of a helix or solenoid, the space within its turns is filled with iron, often in the form of small soft-iron wires. This bundle of iron wire is called the core of the helix. The core becomes strongly magnetized by the field of the helix while the current is flowing and quickly loses its magnetic force when the current is stopped. The direction of the current in a helix (Fig. 232) or the polarity of its core may be determined by another right-hand rule. If the helix is grasped with the right hand so that the fingers point in the direction in which the current is flowing, the extended thumb will point in the direction of the north pole of the helix. On the other hand, if the poles of the helix are known, then, when the helix is grasped with the right hand so that the thumb points to the north-seeking pole, the current is flowing in the wires in the direction that the fingers point.
Fig. 231.—The magnetic field of a helix.
Fig. 232.—Right-hand rule for a helix.
258. The Electromagnet.—These "right-hand" rules are applied in many different devices. Among these, perhaps the most important is the electromagnet, which is used in the electric bell, the telegraph, the telephone, the dynamo, the motor, and many other electric contrivances.
The electromagnet is defined as a mass of iron around which is placed a helix for conducting an electric current. On account of its large permeability, the iron core of the helix adds greatly to the effectiveness of the electromagnet, since the magnetism of the iron is added to that of the current in the helix. The magnetism remaining in the iron after the current stops is called the residual magnetism. The residual magnetism is small when the core is made of small wires or thin plates, but is larger when the iron core is solid. Like artificial steel magnets, electromagnets are usually of two forms, bar and horseshoe. (See Figs. 233 and 234.) For most purposes the horseshoe form is the more effective since it permits a complete iron circuit for the magnetic lines of force. (See Fig. 235.) This is the form used in the electric bell, in the telegraph sounder, and in lifting magnets. (See Fig. 236.)
Fig. 233.—A bar electromagnet.
Fig. 234.—A horseshoe electromagnet.
Fig. 235.—A horseshoe electromagnet may have a complete iron circuit for its lines of force.
Fig. 236.—A lifting magnet.