Fig. 1,321.—Diagram for impressed pressure on circuit containing 5 ohms resistance and inductance of 15 milli-henrys, the current being 10 amperes with frequency of 60.
In fig. 1,321, lay off AB = 50 volts, and BC = 56.6 volts. Using a scale of 20 volts to the inch gives AB = 2.5 ins., and BC = 2.83 ins. Joining AC gives the impressed voltage, which by measurement is 75.4 volts.
In some problems it is required to find the impedance of a circuit in which the ohmic and spurious resistances are given. This is done in a manner similar to finding the impressed pressure.
Fig. 1,322.—Graphical method of obtaining the impedance in circuits containing resistance and inductance, having given the resistance and reactance, that is, the ohmic resistance and spurious resistance. With any convenient scale lay off AB = resistance, and erect the perpendicular BC = reactance (using the same scale); join AC, whose length (measured with the same scale) will give the impedance.
Ohmic resistance and spurious resistance or inductance reactance both tend to reduce an alternating current. Their combined action or impedance is equal to the square root of the sum of their squares, that is,
impedance = √(resistance2 + reactance2)
This relation is represented graphically by the side of a right angle triangle as in fig. 1322, in which the hypothenuse corresponds to the impedance, and the sides to the resistance and reactance.