=428. Alternating currents= are of interest to us because of their general commercial use. To understand the reason for the extensive application of alternating currents it is necessary to learn the fundamental principles which pertain to them. The production of such currents has already been explained in Arts. 300-304. It should be remembered that the current developed in the armature of a dynamo is alternating. A dynamo may _deliver_ a direct or an alternating current, depending on the method of collecting the current from the armature. If a _commutator_ is used, the machine delivers _direct_ current, if _slip rings_ are employed, an _alternating_ current is delivered. =429. The Magnetic Field of an Alternating Current.=--The magnetic field of a direct current has been considered in Arts. 255-256. It has been shown to be arranged in circles about the conductor, according to the _Right Hand Rule_. (See Figs. 229 and 230.) These facts will help one to understand the following experiment: If a number of magnetic compasses be arranged in a circle about a straight vertical wire carrying a direct current, the compass needles will point out a circle about the wire. (See Fig. 430, _A_.) If now the current be reversed the compass needles will reverse themselves and point in a direction just opposite to that taken at first. (See Fig. 430, _B_.) This will be clear if you imagine yourself walking around the wire in the direction the compass needles pointed at first, and then walking around the wire in the reverse direction. This illustrates what happens in the field of an alternating current. The field reverses each time the current reverses. The magnetic field of an alternating current not only rapidly reverses itself, but also continually _changes in intensity_. At the instant when the current reverses, the force of the magnetic field is zero since the current at that instant is zero. As the current begins flowing and increases to its maximum intensity, the magnetic field appears and increases in intensity; and as the current decreases to zero, the magnetic field changes in a similar manner. The field as it grows in strength extends farther and farther from the wire, as it decreases in strength it contracts or draws closer to the wire. Thus the magnetic field may be said to expand and contract. We may picture the lines of force as continually moving. In a typical a.-c. circuit, the complete series of changes takes place in a small fraction of a second, and is repeated many times over in a second. Contrast this with the magnetic field of a constant direct current. Here the magnetic field has the same direction as long as the current flows and does not change in strength. This comparison is important because most of the differences between direct and alternating currents depend on differences in the action of their magnetic fields.