This magickal helmet comes in 1 size only, which fits any
dwarf’s head tightly yet comfortably. It appears much like a
normal great helm, and has some of the same basic characteristics
(gives AC2 protection to head, but allows vision in only a
60° arc to the wearer’s front). When worn by a dwarf, it bestows
upon the wearer the following benefits:

Quickness and efficiency in detecting facts about an underground
setting are improved, so that a dwarf need only spend
1 round (at most) concentrating on detection of some fact,
instead of as much as 1 turn. Any attempt at detecting a
specific fact (as per Players Handbook, p. 16) is made with a
bonus of +1 to the die roll. The chance of detecting “grade or
[slope] in passage” increases to 100% (4 in 4 instead of 3 in 4), as
does the chance of detecting “new construction or passage/
tunnel.” The chance of detecting “sliding or shifting walls or
rooms” is raised to 5 in 6, and the chances of detecting “traps
involving pits, falling blocks and other stonework” and of determining
approximate depth underground are both 4 in 6 (use
d6, not d4, for these rolls).

It is possible for a gnome, hobbit, or other small humanoid <anthromorph?>
to put on the helmet. But because it doesn’t fit as tightly on
non-dwarves, the magickally enhanced impressions are not
transmitted accurately. Gnomes, halflings, and other similar
creatures who attempt to use the helmet must subtract 2 from
die rolls to determine success. Thus, a gnome wearing the
helmet has his chances (PH, p. 16) of successful detection
reduced to 60%, 50%, 40% and 30% respectively (use d10 for all
determinations). A halfling wearing the helmet has only a 25%
chance of success at determining an up grade or down grade (1
in 4 instead of 3 in 4), and a 30% chance (use d10) of determining
direction.

The Dwarven Detector acts as a normal helmet at all times
when its subterranean detection powers are not being used.