When the

curve of a resistor exhibits hysteresis jumps along

or

axis, this resistor can be connected to an energy storage element

or

to form a relaxation oscillator. The criteria of oscillation are derived from a point of view somewhat different from those reported in the literature. It is found that only four different combinations of

or

will support sustained oscillations. Theoretical waveforms and frequencies of oscillations are predicted. Experiments employing an op amp realized

are conducted and agreement between theoretical predictions and experiments is good. A possible application of the theory to the explanation of biological oscillation across an active membrane is proposed.