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.
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