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This
invention relates to insulators, and more particularly to insulators
having a low leakage loss. Heretofore
it has been proposed to produce insulators having low leakage losses by
forming the insulators of glass or other dielectric material having very
low loss angles. As is
fully explained in application Serial No. 40,933, filed July 1, 1925, by
H. A. Affel and E. I. Green, the attenuation loss of telephone lines at
high frequencies is, to a considerable extent, due to leakage losses
dependent upon the dielectric material of the insulator. The
nature of the leakage loss, while not fully understood, is apparently
dependent upon the dielectric hysteresis of the material of the
insulator and the dielectric hysteresis decreases as the loss angle of
the dielectric increases. Therefore,
if a dielectric material having a low loss angle is used in making up
the insulator, the attenuation due to the action of the insulator will
be correspondingly decreased. In general, dielectric materials available for insulator construction
tend to be somewhat fragile if their composition is such as to produce a
very low effective loss angle. Those
materials having the desired low loss angle which have not been readily
breakable are more expensive than the flint glasses which have
heretofore been employed for ordinary telephone insulators. It is,
herefore, desirable that some type of insulator be available which may
be constructed of flint or other cheap glass and will at the same time
have a relatively low leakage loss. This
result is accomplished in accordance with the present invention by so
shaping the insulator that the groove in which the conductor is carried
is formed on the outer periphery of a flange which is of a diameter
relatively large as compared with the diameter of the pin upon which the
insulator is supported, the flange being so formed that an air-space
will be interposed between the groove and the surface of the pin. As is
well known, air considered as a dielectric has a loss angle which very
closely approaches zero. Also, its dielectric constant is virtually
unity as compared with a value several times as great for air.
Therefore, the air which constitutes a part of the dielectric material
between the two effective plates of the condenser formed by the
conductor attached to the groove and the supporting pin, reduces the
leakage loss due
to the condenser action of the insulator, first, because the loss in the
air dielectric is negligible, and second, because the capacity of the
condenser is reduced. This
reduction in capacity is important in another respect.
An essential factor producing leakage loss is the so-called
crossarm effect. This loss is proportional to I squared R, where R is
the resistance of the crossarm and I is the current flowing through the
crossarm. Since decrease in
capacity of the insulator decreases the current flowing through the
crossarm, the crossarm effect is relatively greatly reduced for a small
reduction in the current, being proportional to the square of the
current. The
invention may now be more fully understood from the following detailed
description when read in connection with the accompanying drawing, the
figure of which illustrates the preferred embodiment thereof. As
is well understood, the attenuation resulting from the action of an
insulator is largely due to dielectric hysteresis loss inherent in the
condenser action of the insulator.
The line wire and tie-wire connected in the outer groove of the
insulator constitute one plate of a condenser, the area of which becomes
enlarged when the outer surface of the insulator is wet.
The pin upon which the insulator is mounted is sometimes of wood
and sometimes of metal. Even when a wooden pin is used, the wood is to
some extent a conductor, and is always a conductor to a considerable
extent when it is wet. Therefore,
the pin forms the second plate of a condenser and between these two
plates intervenes the dielectric material of the insulator itself. Now,
dielectric hysteresis loss is a function of the capacity of the
insulator, and if the capacity is reduced, the hysteresis loss, and
hence the attenuation, will be correspondingly reduced.
It has been fully pointed out in an application, Serial
No.40,934, filed July 1, 1925, by II. A. Affel and E. I. Green, that
where the insulator is cylindrical in form such that its outer surface
when wet forms an outer cylindrical plate concentric with the inner
cylindrical surface of the pin, the capacity of the insulator may be
materially decreased by making the outer-diameter of the insulator large
as compared with the diameter of the pin. This result is accomplished in accordance with
the present invention by constructing the insulator, whose main body is
indicated at A in the figure of the drawing, with an outwardly extending
flange B at its top, so that the diameter of the flange will be large as
compared with the diameter of the opening C for accommodating the
supporting pin. The flange
may be provided with the usual groove D for accommodating the
transmission conductor and the tie-wire. By arranging this groove in a
flange instead of a cylindrical insulator having extremely thick walls,
a considerable saving of material is effected. Inasmuch as the decrease in hysteresis loss resulting from increasing the diameter of the groove for accommodating the tie-wire is necessarily limited for mechanical reasons, it is proposed in accordance with the present invention to combine this structure with the novel feature of an air-gap interposed between the outer groove and the pin. Accordingly, the flange is provided with a deep groove E extending upwardly from its lower surface, this groove extending well above the upper side of the |
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conductor
supporting groove D. The
groove E not only interposes an air-gap between the outer conductor and
the pin, but it forms a long dry path between the outer conductor and
the skirt F of the insulator for practically preventing direct current
leakage due to moisture particularly in wet weather. It will be clear from the construction above described that a part of the
dielectric intervening between the outer surface of the pin and the
plate formed by the transmission conductor and tie-wire is composed of
air. Air has a loss angle
very closely approaching zero, and as is fully explained in the
application of Affel and Green, Serial No. 40,933, the dielectric
hysteresis is reduced as the loss angle decreases and becomes zero when
the loss angle is zero. If
we consider the insulator as a condenser, its characteristics may be
thought of as a capacity due to the effective plates on the inner and
outer surfaces of the insulator and an effective conductance due to the
dielectric hysteresis. The
insulator then becomes equivalent to a capacity shunted by a
conductance. As is well known, if such a combination is bridged across a
pair of conductors, the transmission through the condenser merely
produces a change in phase and the actual attenuation loss is due to the
leakage through the conductance. If
the conductance is eliminated, there will be merely a change in phase as
the alternating current is propagated along the conductors without any
actual transfer of energy from conductor to conductor. In
other words, the current flowing through the insulator will be a
wattless current if there is no conductance component or dielectric
hysteresis present. It thus
becomes apparent that the interposition of the air dielectric in the
groove E between the
wire mounted on the insulator and the supporting pin very materially
reduces the dielectric hysteresis and this, coupled with the decrease in
capacity due to the mounting of the wire upon the flange B of large
diameter, results in an insulator which, though constructed of ordinary
flint glass, will nevertheless be practically as efficient as an
insulator constructed of some special glass having a very low loss
angle. It
will be noted that the groove E is near the top of the insulator and
hence a considerable distance above the crossarm. This
reduces the possibility of the dry path (the surface within the groove)
being rendered less effective due to splashing from the crossarm. Also,
since the groove E is located close to the wire groove D, the wet
surface conductively connected to the wire is reduced. Consequently,
the variation of capacity, due to change in area of the outer condenser
plate with variation of weather conditions is reduced. In
the above description the invention is set forth as an improvement of
such nature as to enable the insulator to be made of cheap glass, thus
constituting a substitute for an insulator of ordinary form constructed
of glass having a low loss angle. It will nevertheless be obvious that the insulator of the
present invention may be constructed of glass having a low loss angle,
where the additional expense of the glass is not an important
consideration, thus obtaining the advantage of reduction of loss due
both to the improved form of the insulator and to the inherent
characteristics of the glass itself. It
will be obvious that the general principles herein disclosed may be
embodied in many other organizations widely different from those
illustrated, without departing from the spirit of the invention as
defined in the appended claims. What
is claimed is: 1.
An insulator having a flange extending outwardly therefrom, a groove in
said flange for carrying a line conductor and tie-wire, an interior bore
within the insulator whereby it may be mounted on a supporting pin, said
groove being located well below the top of said support pin and the
diameter of said groove being more than twice the diameter of said
internal bore, and a groove extending upwardly into said flange from the
under side thereof to a point a sufficient distance above the external
groove so that an air space will intervene between the said external
groove and the entire surface of the supporting pin. 2. An insulator comprising a body of dielectric material having an
internal bore whereby it may be mounted upon a supporting pin, a flange
extending outwardly and downwardly from near the top of said insulator
to provide an air-space between said flange and said body, an external
groove around.
the periphery of said flange for accommodating a line conductor and
tie-wire, the diameter of said groove being more than twice the diameter
of the internal bore of the insulator, and said groove being so located
upon the outer surface of said flange that the air in the space between
the inner side of the flange and the main body of the insulator will
comprise a part of the dielectric path intervening between the
conducting material mounted in the external groove and each element of
the surface of the supporting pin. 3.
An insulator having a flange extending outwardly therefrom, a groove in
said flange for carrying a line conductor and a tie-wire, an interior
bore within the insulator whereby it may be mounted upon a supporting
pin, said groove being located well below the top of said supporting pin
and the diameter of said groove being more than twice the diameter of
said internal bore, and a groove extending upwardly into said flange
from the underside thereof to a point well above the external groove and
approximately as high as said interior bore so that an air-space will
intervene between the said external groove and the supporting pin. In
testimony whereof, I have signed my name to this specification this 23rd
day of December, 1925. LEON T. WILSON. |