[Illustration: FIG. 51.--HUGHES' ELECTROMAGNET.]
His object was to find out the best form of electromagnet, the best
distance between the poles, and the best form of armature for the
rapid work required in Hughes' printing telegraphs. One word about
Hughes' magnets. This diagram (Fig. 51) shows the form of the well
known Hughes' electromagnet. I feel almost ashamed to say those words
"well known," because on the Continent everybody knows what you mean
by a Hughes' electromagnet. In England scarcely anyone knows what you
mean. Englishmen do not even know that Professor Hughes has invented a
special form of electromagnet. Hughes' special form is this: A
permanent steel magnet, generally a compound one, having soft iron
pole pieces, and a couple of coils on the pole pieces only. As I have
to speak of Hughes' special contrivance among the mechanisms that will
occupy our attention later on, I only now refer to this magnet in one
particular. If you wish a magnet to work rapidly, you will secure the
most rapid action, not when the coils are distributed all along, but
when they are heaped up near, not necessarily entirely on, the poles.
Hughes made a number of researches to find out what the right length
and thickness of these pole pieces should be. It was found an
advantage not to use too thin pole pieces, otherwise the magnetism
from the permanent magnet did not pass through the iron without
considerable reluctance, being choked by insufficiency of section:
also not to use too thick pieces, otherwise they presented too much
surface for leakage across from one to the other. Eventually a
particular length was settled upon, in proportion about six times the
diameter, or rather longer. In the further researches that Hughes made
he used a magnet of shorter form, not shown here, more like those
employed in relays, and with an armature from 2 to 3 millimeters
thick, 1 centimeter wide and 5 centimeters long. The poles 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.