a. __General__. Alternating current is an electrical current which flows back and forth at regular intervals in a circuit. When the current rises from zero to a maximum, returns to zero, increases to a maximum in the opposite direction, and finally returns to zero again, it is said to have completed one cycle.

(1) A cycle is divided into 360 degrees. Figure 10-19 is a graphical representation of a cycle and is called a sine wave. It is generated by one revolution of a single loop coil armature in a two-pole alternating current generator. The maximum value in one direction is reached at the 90° position, and in the other direction at the 270° position.
(2) The number of times this cycle is repeated in one second is called the frequency, measured in hertz. b. Alternating current for arc welding normally has the same frequency as the line current. The voltage and current in the ac welding arc follow the sine wave and return to zero twice each cycle. The frequency is so fast that the arc appears continuous and steady. The sine wave is the simplest form of alternating current.c. Alternating current and voltage are measured with ac meters. An ac voltmeter measures the value of both the positive and negative parts of the sine wave. It reads the effective, or root-mean-square (RMS) voltage. The effective direct current value of an alternating current or voltage is the product of 0.707 multiplied by the maximum value.

d. An alternating current has no unit of its own, but is measured in terms of direct current, the ampere. The ampere is defined as a steady rate of flow, but an alternating current is not a steady current. An alternating current is said to be equivalent to a direct current when it produces the same average heating effect under exactly similar conditions. This is used since the heating effect of a negative current is the same as that of a positive current. Therefore, an ac ammeter will measure a value, called the effective value, of an alternating current which is shown in amperes. All ac meters, unless otherwise marked, read effective values of current and voltage.

e. Electrical power for arc welding is obtained in two different ways. It is either generated at the point of use or converted from available power from the utility line. There are two variations of electrical power conversion.

(1) In the first variation, a transformer converts the relatively high voltages from the utility line to a liner voltage for ac welding.(2) The second variation is similar in that it includes the transformer to lower the voltage, but it is followed by a rectifier which changes alternating current to direct current for dc welding.
f. With an alternating flow of current, the arc is extinguished during each half-cycle as the current reduces to zero, requiring reignition as the voltage rises again. After reignition, it passes, with increasing current, through the usual falling volts-amperes characteristic. As the current decreases again, the arc potential is lower because the temperature and degree of ionization of the arc path correspond to the heated condition of the plasma, anode, and cathode during the time of increasing current.g. The greater the arc length, the less the arc gas will be heated by the hot electrode terminals, and a higher reignition potential will be required. Depending upon the thermal inertia of the hot electrode terminals and plasma, the cathode emitter may cool enough during the fall of the current to zero to stop the arc completely. When the electrode and welding work have different thermal inertia ability to emit electrons, the current will flow by different amounts during each half-cycle. This causes rectification to a lesser or greater degree. Complete rectification has been experienced in arcs with a hot tungsten electrode and a cold copper opposing terminal. Partial rectification of one half-cycle is common when using the TIG welding process with ac power.