
We call his grid a system of rectangular coordinates. It defines four rectangular sections or quadrants. Fortunately, we only need only the top right quadrant to map out the usual grid voltage lines that make up the tube manual IV (current/voltage) curves. This is so because tubes are unidirectional current conducting devices: they only conduct from cathode to plate. But before we tackle the vacuum tube's plate curves, we need to cover something much simpler: resistance and straight lines.
One Resistance The movement from the graph's left to right defines increasing voltage; from bottom to top, increasing current. This arrangement allows us to graphically to display Ohm's Law: Resistance = Voltage / Current. For example, 1 ohm equals 1 volt divided by 1 amp and 100 ohms equals 100 volt divided by 1 amp. Graphing these resistances is easy. We know we must start at the bottom left corner where voltage and current equal zero, as no voltage across a resistance means no current. The next point for the 1 ohm resistance would be one unit to the right and one unit up and for the 100 ohm resistance it would be one hundred units to the right and one units up. Remember, the lines continue infinitely, as the relationship between voltage and current defined by the line is constant. So if we follow the 1 ohm line out to 100, 000 volts, we know that the current will be 100, 000 amps. Of course, no 1 ohm resistor on earth could withstand the test; but the issue here was resistance and not necessarily actual resistors. From the plotting of these two resistances, a key feature of graphing resistances is revealed: the lower the resistance, the steeper the line defined; and the higher the resistance, the less steep the line defined. (This will prove valuable when working with tubes, as the principle allows quick inspection of plate resistances, rp's. Given that both graphs have the same XY scales, the steeper set of plate curves belongs to

