Both the magnets and the armature are coated on the sides facing
the distributor with mica or some other non-conductor of heat and
electricity. The distributor is attached to and supported by the cross
arms, so that it turns with the armature.
The distributor is composed of a ribbon of iron or nickel, bent into a
continuous zigzag. This form has the advantage of presenting, in the
cool part of the distributor, an almost direct road for the lines of
force between the poles and the armature, thus diminishing the magnetic
resistance as far as possible. At the same time the Foucault currents
are minimized. To the same end it is useful to slit the ribbon, as in
Fig. 3. This also facilitates the folding into zigzags.
[Illustration: Fig. 3.]
The distributor is heated at two opposite points on a diameter by the
burners, _b b_, above which are the chimneys, _e e_. The cooling of the
alternate section is aided by the circulation of cold air, which is
effected by means of the draught in the chimneys, _e e_. At the points
of lowest temperature a jet of air or water is maintained. The cross
arms are insulated with mica or asbestos at the points where they extend
from the armature to the distributor.
It will now be evident that while the distributor is entirely cool, many
of the lines of force pass from N to S without entering the armature
core; but if heat is applied at the points 1 and 2 in the figure, so as
to increase the magnetic resistance at these points, then a great
portion of the lines will leave the distributor, and pass through the
armature core.
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