What is the single effect in Edgar Allan Poe's "The Purloined Letter?"

A single effect is a paradigm that Edgar Allan Poe created
signifying that a story must produce one specific and consistent effect in the reader.
This is what would make a plot successful, or
unsuccessful.

In the case of "The Purloined Letter", the
single effect is suspense. From the beginning of the story, we know that a letter that
could ruin a man has been stolen. We never find out what the letter says except that it
is highly derogatory. Then, we don't ever find out either the complete name of the
person whom the letter intended to hurt, all we know of him is that his name is
"Minister D----." Finally, when we see the maneuvering of the letters (switching them
from fakes to originals, to copies) the suspense is even more intense. Then in the end,
when we realize what is going on and that it was a revenge we become
relieved.