Improvements in Electro-static Measuring Instruments.
Year: 1899

page_text: Date of Application, 20th June, 1899 -Accepted, 10th Feb., 1900
COMPLETE SPECIFICATION.
Improvements in Electro-static Measuring Instruments.
We, THE BRITISH THOMSON-HOUSTON COMPANY LIMITED, of 83, Cannon Street,
in the City of London, Electrical Engineers, do hereby declare the nature
of this invention (which has been communicated to us from abroad by Elihu
Thomson, residing at Lynn, in the County of Essex, State of Massachusetts,
United States of America Electrical Engineer); and in what manner the same
is to be performed to be particularly described and ascertained in and by the
following statement:-
The present invention relates to electrical measuring instruments, and has
particular reference to instruments designed to make electric measurements at
very high potentials. In modern electric installations for the transmission of
power to long distances, potentials in excess of ten thousand volts are commer-
cially used. At such pressure, the consequences of a short circuit are so serious
that every precaution must be taken to prevent it. At the same time, it is highly
desirable that some means be provided for the direct measurement of the electrical
conditions on such lines, and the present invention, though not in any wise
restricted thereto, is particularly useful in such installations.
The best instruments for the purposes of this invention, are those of the electro-
static type, wherein there is no complete electric circuit. It is well known that
these instruments are adapted to measurements of moderate, or even quite high
potential, because they act by induction, and the vanes, and the quadrants which
affect the vanes electrically, are separated by an air space; or they may even be
separated by solid dielectric, although this is not the preferred construction.
The improvement in the construction of such devices, and in the method of
measuring potential with them, which we have devised, consists in connecting the
two sides of the line, or the two points whose difference of potential is to be measured
by an instrument of the class described, and enclosing the instrument in a non-
striking vacuum. To do this, any preferred electro-static voltmeter is taken, the par-
ticular type of which may be varied indefinitely, and placed within a chamber in
which such a vacuum is produced that electrical discharges are prevented from
passing between the opposing surfaces of the electrostatic voltmeter. It is
evident that the chamber may be well constructed of glass. It is well known
that in a Crookes' vacuum a considerable difference of potential is required to
pass the discharge, much higher than is required to pass the discharge in a
Geissler tube, for instance; in instruments designed to measure moderate poten-
tials, those degrees of exhaustion would be sufficient; but if it is designed to measure
the highest potentials which would ever be commercially employed, it is prefer-
able to use the best obtainable vacuum, one which cannot be transversed by an
electric discharge except at a pressure so far beyond the working potential of the
instrument as to render the latter safe.
It will be understood that the form and disposition of parts may be greatly
varied without departing from the invention; and, further, that various arrange-
ments of the moving parts with reference to the stationary parts, and for taking
the current into the vacuum tube, may also be employed.
Not only is it possible by the practice of the invention to render the instru-

page_text: ment safe from discharge in the way pointed out, but, inasmuch as we are enabled
to bring the charged surfaces very much nearer together than if the instrument
were to be used in air, the attractive or repulsive forces are greatly increased and
the instrument rendered very sensitive and reliable.
I
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The drawings show embodiments of the invention and modifications thereof.
Fig. 1 is side elevation of an instrument embodying the invention; Fig. 2 is a
plan in both of these figures the glass is shown broken away. Figs. 3 and 4
are, respectively, a plan and side elevation of parts of the instrument. Fig. 5
is a side elevation of a modified form; and Fig. 6 is a plan of the parts shown
in Fig. 5.
'
In Fig. 1 B is the bulb within which the apparatus is enclosed. Within the
bulb are twofstems or glass supports S, Supon-each-of tliese cismounted an
element of a suitable electrostatic instrument, such as a voltmeter. Terminal
wires are sealed into the glass, and by these terminals connection is made
to the two poles of a source of electricity, the difference of potential of which
is to. be measured. These terminals are arranged at such-a distance apart that
there is no practical-possibility of a spark occurring outside the bulb. It is to
be understood,-in accordance with the statement of invention, that the vacuum
within the chamber-is-so-high that no-spark can pass from one surface to the
other of the voltmeter. The glass stem S, through which the terminal is con-
nected, carries a stationary "quadrant" or curved piece A of aluminum which
is, in shape, the segment of a cylinder, and to this the terminal is connected.
Carried by the stem S¹, and connected in any suitable way to the terminal T, is
a corresponding vane or segment A, also of aluminum or other conducting metal.
In the case illustrated, the connection is made to the terminal T by a wire s which
connects the terminal wire to the shaft X. This shaft, as seen in Figs. 3 and 4,
is mounted in suitable bearings, carried upon the stem S', and upon the shaft is
a disc D of aluminum, or other good conducting metal, which carries the seg-
ment ; and which also swings between the poles of a magnet, acting as a
damper, in a way now well understood in the art. To the shaft is also fixed the
hand or pointer C of the instrument, which plays over a scale N (see Fig. 2),
which is here shown as etched upon the glass of the bulb. The magnet is sup-
ported in any desired manner from the stem $. A spring Z is provided, for
returning the instrument to zero, and its tension is adjusted to any desired delicacy
of movement of the instrument. Pivots or bearings p, p, are also provided, in
which the shaft X is mounted.
The action of the parts, as so far described, is as follows: the difference of
potential between the segments A, A', tends to cause the relative movement of
these two parts, and the segment A swings past the segment A, turning the
shaft, and the pointer C crosses the scale, this movement increasing with increase
of potential. In order to make the spacing of the subdivisions of the scale more
even, the segment A may, if desired, be mounted, as shown in Fig. 3, a little
eccentrically to the axis to the shaft X, so that as the pointer moves over the
scale, the segment A' approaches the segment A, and the torque increases with
the potential, and thus compensates for the increased tension of the spring Z,
caused by the rotation: In the views so far described, the parts of the instru-
ment are shown as being mounted upon the separate supports S, S¹. This is the
best arrangement, because it prevents all creeping of discharges, and the discharge,
if it pass at all, must pass through the vacuous space.
In certain forms of the instrument however, designed to measure potentials
which would subject it to less stress, it is possible to vary from this arrange-
ment, and a modified form is therefore shown in Figs. 5 and 6. In these figures,
the entire instrument is mounted upon a separate stem L, which may then be
sealed into the bulb B, the bulb being afterwards exhausted in the usual way.
For convenience, the bulb may be mounted upon a foot F, which is preferably
also of glass, so that the instrument may be as nearly as possible absolutely insu-
lated. For further protection it may be enclosed in any suitable case, preferably

page_text: of insulating material, but as this forms no essential feature of the invention,
this has not been illustrated.
If desired, the terminals may, as shown in dotted lines at T³, T³ be bent down
and enclosed in vessels of oil so as to prevent accidental discharge from one to
the other. In the particular form of adjustment shown in Figs. 5 and 6, we
have used the so-called "quadrant" form, in which sectors S2, S3 in different
planes are used as the active surfaces; but of course it is immaterial what par-
ticular type of instrument is used, the essential feature of the invention con-
sisting in the protection of the instrument, of whatever form, by a vacuum suffi-
ciently high to prevent "striking" with the particular potential difference to
which the instrument is to be subjected.
Having now particularly described and ascertained the nature of our said
invention (as communicated to us from abroad) and in what manner the same
is to be performed, we declare that what we claim is:-
-
1 The improvement in the art of making electric measurements, which con-
sists in enclosing the instruments by which the measurements are effected in a
practically non-striking vacuum.
2-The combination of an electric measuring instrument and an exhausted
enclosure surrounding the portions between which a high potential difference
exists.
3-The combination of an electrostatic measuring instrument a glass case
therefor, within which a high vacuum is maintained, as set forth.
4 The combination of an electrostatic measuring instrument, a glass case
surrounding the instrument within which a high vacuum is maintained, and a
scale on the case over which the pointer of the instrument moves.
5 In a measuring instrument of the electrostatic type, fixed and movable
elements, to one of which the pointer is fixed, and means for approximating the
elements as the pointer moves from zero.
6-In a measuring instrument of the electrostatic type, a fixed surface, a sur-
face movable relative thereto against the power of a spring, a pointer and a
scale, and means for bringing the surfaces into closer approximation as the pointer
moves over the scale, to compensate for the increased tension of the spring.
7In a measuring instrument of the electrostatic type, a fixed segment, a
segment moving about an axis against the force of a spring and carrying a
pointer; the segments being in eccentric relation, so that, as the pointer moves
over the scale, they approach sufficiently to compensate for the change in
tension of the spring thus permitting the desired spacing of the scale graduation.
8 The method of securing greater attractive effort in an electrostatic instru-
ment, which consists in increasing the induction due to the approach of the
pieces, and, at the same time, of preventing sparking or a discharge from the
parts so approximated, by a high vacuum between them.
9 In an electric measuring instrument, the combination with damping means
of two relatively movable elements mounted within a vacuum chamber, with
means for furnishing a restoring torque or force, and means for indicating the
movement.
نمه
1000