3-46 Figure 3-49A.—Series LR circuit. View (B) of figure 3-49 shows the waveforms when E_{a} is a square wave. Recall that the inductor acts as an open circuit at this first instant. Current now starts to flow and develops a voltage across the resistor. With a square wave applied, the voltage across the inductor starts to drop as soon as any voltage appears across the resistor. This is due to the fact that the voltage across the inductor and resistor must add up to the applied voltage. Figure 3-49B.—Series LR circuit. With E_{a} being a trapezoidal voltage, as shown in figure 3-49, view (C), the instant current flows, a voltage appears across the resistor, and the applied voltage increases. With an increasing applied voltage, the inductor voltage remains constant (E_{L}) at the jump level and circuit current (I_{R}) rises at a linear rate from the jump voltage point. Notice that if you add the inductor voltage (E_{L}) and resistor voltage (E_{R}) at any point between times T0 and T1, the sum is the applied voltage (E_{a}). The key fact here is that a trapezoidal voltage must be applied to a sweep coil to cause a linear rise of current. The linear rise of current will cause a uniform, changing magnetic field which, in turn, will cause an electron beam to move at a constant rate across a crt.