Usually, the input quantity (electric quantity or other physical quantity) that the relay starts to act and smoothly pulls in is called "action value" and recorded as XL; The input quantity that makes the relay start to release and separate smoothly is called "return value", which is denoted as X2. The current flowing in the controlled circuit after the contact is closed is called the "output" of the relay, which is denoted as Y; When the relay obtains the input signal X, as long as it does not reach the action value Xl, the relay will not act and the output signal Y=0. When the input signal reaches the action value of X 1, the relay will act immediately and give the output signal with the size of yl. After that, even if the input signal continues to increase, the output signal is still Y, and there is no change. After the relay operates, if the input signal weakens, the operating point does not change along the dotted lines B, A and D, but changes along B and C, that is, when X is slightly less than the operating value X, the relay does not release and continues to output signal yl. It is released only when X decreases to the release value X z of the relay, that is, Y suddenly decreases from yl to zero. At this time, the working point of the relay changes along the dotted lines B, C, D and O, and returns to the original state. X is the normal working value and must be greater than Xl, so as to avoid misoperation of relay when input fluctuates. X /x 1 is called "reserve factor" or "safety factor".