In John Donne's poem "Valediction: Forbidding Mourning," the conceit, found in stanzas 7-9, is a compass (a tool used in geometry). Donne, who wrote this poem for his wife when he was about to go on a long trip, compares his wife's soul to "the fixed foot." Just as the fixed part of the compass is planted on the paper, so does his wife stay put. However, just as the fixed part of the compass leans a bit as the other foot leans, so does his wife lean in the same direction that he does. And when he is coming back home, his wife stands upright, just as the fixed part of a compass stands upright when the foot that moves comes closer to it. In the last stanza, Donne says that his wife will be like the fixed foot of a compass as he runs. Just as the fixed part of compass allows the other foot to draw a circle, so will his wife, who stays in place and is steadfast, allowing him to roam in a circle and then come back home.
John Donne cleverly uses on of the most famous of metaphysical conceits in stanza seven of "A Valediction Forbidding Mourning." A metaphysical conceit is like an extended metaphor, in which the poet compares to extremely different objects; usually the comparison involves an abstract concept or emotion, like love, and some other completely random object.
John Donne's conceit in stanza seven definitely features a completely random object to be making an appearance in a love poem-- a compass! Here, Donne compares the lovers' souls to the points of a compass:
"Thy soul, the fix'd foot, makes no show
To move, but doth, if th' other do."
Donne's elegant conceit is both ingenious and moving. He uses the physical object to show the heart-felt closeness of the two lovers; "when the other far doth roam, It leans, and hearkens after it."