A D.C. circuit consists of sources of direct current (EMFs), connected to a network of elements. If the elements are ohmic (obey Ohm's Law) then the currents through the elements are directly proportional to the voltages applied across the elements. For ohmic elements, the relation between the current through an element (I), in amps, and the voltage across the element (V), in volts, is V = IR [1] where R is the resistance of the element in ohms. Resistors, which are commonly used in electronic devices, are ohmic. If two or more resistors are connected in series with a voltage source, as illustrated in Figure 18-1, they collectively behave as an equivalent single resistor with resistance, Rser, where Rser = R1 R2 R3 Rn