In Edgar Allan Poe's essay The Importance of the Single Effect in a Prose Tale, Poe describes the importance of the single effect in a text.
A skillful literary artist has constructed a tale. If wise, he has not fashioned his thoughts to accommodate his incidents; but having conceived, with deliberate care, a certain unique or single effect to be wrought out, he then invents such incidents--he then combines such events as may best aid him in establishing this preconceived effect.
What Poe is saying here is that a writer must have a certain purpose in mind when constructing a text. A true artist (writer) constructs a piece which is meant to be read in one sitting based upon the fact that a
simple cessation in reading would, of itself, be sufficient to destroy the true unity. In the brief tale, however, the author is enabled to carry out the fullness of his intention, be it what it may.
Therefore, Poe's single effect which he creates in The Pit and the Pendulum is one which he speaks of in his essay. Poe wants his reader to be able to take in the complete text in one sitting so that the emotions they are feeling are not stopped given the reader does not require a pause in the reading, which would "destroy the unity" of the text.
As one can easily recognize, Poe's texts were meant to be read in one sitting given his immense repertoire of poetry and short stories). Poe wanted his reader's to be engrossed and engulfed by his words, by the images he created, and by the character's he designed.