were turned
over at the top toward one another. Hughes tried whether there was any
advantage in making those poles approach one another, and whether
there was any advantage in having as long an armature as 5
centimeters. He tried all the different kinds, and plotted out the
results of observations in curves, which could be compared and
studied. His object was to ascertain the conditions which would give
the strongest pull, not with a steady current, but with such currents
as were required for operating his printing telegraph instruments;
currents which lasted but one to twenty hundredths of a second. He
found it was decidedly an advantage to shorten the length of the
armature, so that it did not protrude far over the poles. In fact, he
got a sufficient magnetic circuit to secure all the attractive power
that he needed, without allowing as much chance of leakage as there
would have been had the armature extended a longer distance over the
poles. He also tried various forms of armature having very various
cross sections.

POSITION AND FORM OF ARMATURE.

In one of Du Moncel's papers on electromagnets[1] you will also find a
discussion on armatures, and the best forms for working in different
positions. Among other things in Du Moncel you will find this paradox:
that whereas using a horseshoe magnet with fat poles, and a flat piece
of soft iron for armature, it sticks on far tighter when put on
edgeways; on the other hand, if you are going to work at a distance,
across air, the attraction is far greater when it is set flatways. I
explained the advantage of narrowing the surfaces of contact by the
law of traction, B squared, coming in. Why should we have for action at a
distance the greater advantage from placing the armature flatway to
the poles? It is simply that you thereby reduce the reluctance offered
by the air gap to the flow of the magnetic lines. Du Moncel also tried
the difference between round armatures and flat ones, and found that a
cylindrical armature was only attracted about half as strongly as a
prismatic armature having the same surface when at the same distance.
Let us examine this fact in the light of the magnetic circuit. The
poles are flat. You have at a certain distance away a round armature;
there is a certain distance between its nearest side and the polar
surfaces. If you have at the same distance away a flat armature having
the same surface, and, therefore, about the same tendency to leak, why