=256. Right-hand Rule for a Conductor.=--To show the presence of the magnetic field about a current, pass a thick copper wire vertically through a sheet of paper, and connect the ends of the wire to a source of current. While the current (this should be as much as 10 amperes if possible) is flowing, sprinkle iron filings upon the paper and tap gently. The filings will arrange themselves in circles about the wire showing the magnetic field. (See Fig. 229.) The needle of a magnetoscope tends to place itself parallel to the lines of force of this field and from this action or tendency the _direction_ of the magnetic lines about a current may be determined. The following rule is helpful and should be memorized: _Grasp the conductor with the right hand with the outstretched thumb in the direction that the current is flowing. The fingers will then encircle the wire in the direction of the lines of force._ This rule may be reversed, for, if the fingers of the right hand grasp the wire so as to point with the magnetic field, then the current flows in the direction in which the thumb points. (See Fig. 230.) [Illustration: FIG. 229.--Magnetic field about a wire carrying an electric current.] [Illustration: 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. [Illustration: FIG. 231.--The magnetic field of a helix.]