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Патент USA US2406492

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Aug. 2.7, 1946.
c. DoRsMAN
2,406,492
DEVICE FOR MEASURING LOW DIRECT VOLTAGES
Filed DeC. 30, 1942
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Patented Aug. 27, 1946
2,406,492
UNITED STATES PATENT QFFICE
2,406,492
DEVICE FOR MEASURING LOW DIRECT
VOLTAGES
Cornelis Dorsman, Eindhoven, Netherlands;
vested in the Alien Property Custodian
Application December 30, 1942, Serial No 470,685
In the Netherlands January 6, 1941
5 claims; (ci. r11-_95)
2
For measuring small direct voltages, and more
cording to the invention, by taking the steps re
ferred to above.
particularly for measuring the terminal voltage
of a source of direct current having a very high
In order to achieve that the difference in con
internal resistance, devices have come to be known
in which the direct voltage to be measured is
converted into alternating voltage by means of
a condenser which is charged by the direct volt~
age to be measured, means being provided by
tact potential does not alter eventually it has
proved advantageous to house the condenser in
hermetically closed envelope and a chemically
inert atmosphere, preferably in a vacuum, thus
avoiding alterations of the contact potential by
which the condenser capacity is periodically -al
atmospheric and/or chemical agency.
tered. From the condenser an alternating volt 10
By the said eXpedients it is achieved that the
age may then be taken whose amplitude is pro~
difference in contact potential of the electrodes is
portional to the value of the direct valtage sup
constant in contradistinction to that of the known
plied thereto, which alternating voltage, after
devices. Thus the value and polarity of the
having been ampliñed and, if desired, rectified
contact potential can be determined and when
can be measured in a simple manner.
Such de
vices may also be used with advantage for de
16 these factors are taken into consideration in mak
termining the value of direct voltages by means
ing measuremets a much greater measuring ac
curacy is obtained than is feasible by means of
of a compensation method, in which the obtained
known devices.
accuracy depends on the accuracy with which the
In order that the requirements imposed on the
diiierence in value of the unknown direct volt 20 accuracy of the mechanical structure of the con
age and a known direct voltage can be measured.
denser, in conjunction with the desired measur
We have found that the accuracy with which
ing accuracy, shall be as small as possible it is
a direct voltage can be measured by means of the
advisable that the electrodes of the condenser
devices referred to above is comparatively small,
should be made from a metal having such prop
since it generally amounts only to about 0.020 25 erties that the difference in Contact potential of
Volt.
two electrodes made therefrom is naturally very
The invention provides an improvement of de
low. In this respect it has proved advantageous
vices of. the kind referred to above, thus reach
to make the electrodes from electrolytic red cop
ing a much greater accuracy in measuring'.
per.
' _According to the invention the electrodes of the
Since by the above steps the difference in con
condenser, which is loaded by the direct volt
tact potential of the condenser electrodes is con
age to be measured, are hat and parallel and
stant, at variance with that of known devices
so arranged as to be movable with respect to one
comprising variable condensers, for instance by
another solely in a direction normal to the plane
rotation of one of the electrodes, the diiference
of the electrodes by the said means for periodi
in contact potential of the electrodes can be made
cally varyíng the condenser capacity.
The invention is based on the realisation that
the said small measuring accuracy is due to the
difference in contact potential of the two con
denser electrodes, owing to which the condenser
will always exhibit a definite charge even if no
charge is supplied to it. Since the contact po
tential of a metal surface is different from point
t0 point the difference in contact potential be
tween the electrodes, which is approximately in
dependent on the distance between the electrodes,
will be constant only if upon varying the con
denser capacity by relative displacement of the
electrodes the active electrode surface remains f
up for, according to the invention, by supplying
to the condenser a constant direct voltage which
is equal but opposite to the difference in con
tact potential of the condenser electrodes. This
may be made with advantage more particularly
in making measurements according to the com
pensation method. In this method the unknown
direct voltage and a known direct voltage for
comparison are assumed exactly equal, if no al
ternating voltage at all is set up at the condenser
having a variable capacity to which is supplied
the difference of the two direct voltages, which
can be readily ascertained by means of sensitive
amplifying devices controlling a voltmeter or am
meter of the usual type or an indicating device
always the same and, moreover, the ratio between
such as a cathode-ray indicator, so that a very
the contribution towards the total condenser ca
great measuring accuracy can be obtained.
pacity delivered by any part of the active elec
The invention will be further described with
trode surface and the total condenser capacity
reference to the appended drawing the single
always remains the same which is ensured, ac- 55 figure of which illustrates one form of a vo1t~
a/ioasoa
A
means to measure the pulsating voltage produced
at the terminals of the condenser by the variation
in the capacity thereof.
3. Apparatus for measuring a direct voltage
age measuring device embodying the invention.
As shown in the drawing the voltage to be
measured 9 is applied to a variable capacitance
condenser I0 through a charging resistor Il.
Condenser I0 consists of two flat and parallel Ul comprising, a variable condenser member com
prising two flat parallel electrodes spaced apart
electrode plates l2 one of which is íixed and the
and movable with respect to each other solely in
a direction normal to the planes of the electrodes
and an evacuated hermetically sealed envelope
enclosing said electrodes, means to apply the di
rect voltage to be measured to the said condenser
other of which is capable of movement solely in
a direction normal to the plane of the electrode
plates. For moving the said movable electrode
plate and thereby varying the capacitance of the
condenser there is provided a motor unit I3 hav
ing preferably a construction similar to the well
known structure used for a loudspeaker, said
motor unit being energized by a suitable source
of alternating current.
The condenser l0 and the driving motor I3
electrodes, means to periodically move one elec
trode relative to the other electrode solely in a
direction normal to the planes of the electrodes
and through a substantially constant displace
ment value to thereby vary the spacing between
the electrodes and the capacitance of the con
are enclosed within a hermetically-sealcd con
denser between substantially fixed limits, and
tainer I6 which may be evacuated or ñlled with
means to measure the pulsating voltage produced
a chemically-inert atmosphere.
The alternating voltage produced across con 20 at the terminals of the condenser by the vari
ation in the capacity thereof.
denser lû during the periodic varying of the ca
4. Apparatus for measuring a direct voltage
pacitance thereof is amplified and subsequently
comprising, a variable condenser member com
operates a suitable indicating instrument, such
prising two flat parallel electrodes consisting of
For compensating the contact potential of the 25 electrolytic red copper and being spaced apart
and movable with respect to each other solely
condenser electrodes there is provided a source
in a direction normal to the plane of the elecof compensating voltage l5 the polarity and in
trodes, means to apply the direct voltage to be
tensity of which is adjustable to produce the de
measured to the said condenser electrodes, means
sired cancellation.
as a voltmeter, ammeter or a cathode-ray tube.
30 to periodically move one electrode relative to the
What I claim is:
otherrelectrode solely in a direction normal to
1. Apparatus for measuring a direct voltage
the planes of the electrodes and through a sub~
comprising, a variable condenser member com~
stantially constant displacement value to thereby
prising two flat parallel electrodes spaced apart
and movable with respect to each other solely in
a direction normal to the planes of the electrodes,
means to apply the direct voltage to be measured
to the said condenser-electrodes, means to peri
vary the spacing between the electrodes and the
capacitance oi> the condenser between substan
tially ñxed limits, and means to measure the pulu
sating voltage produced at the terminals of the
condenser by the variation in the capacity
odically move one electrode relative to the other
electrode solely in a direction normal to the
planes of the electrodes and through a substan
thereof.
40
tially constant displacement value to thereby vary
the spacing between the electrodes and the ca
pacitance of the condenser between substantially
ñxed limits, and means to measure the pulsating
voltage produced at the terminals of the con- ‘
denser by the variation in the capacity thereof.
p
5. Apparatus for Ymeasuring a direct voltage
comprising, a variable condenser member com
prising two ilat parallel electrodes spaced apart
and movable with respect to each other solely in
a direction normal to the planes of the elec
trodes, means to apply the direct voltage to be
measured t0 the said condenser electrode, means
2. Apparatus for measuring a direct voltage
comp-rising, a variable condenser member com
to periodically move one electrode relative to the
denser electrodes, means to periodically mol/'e
one electrode relative to the other electrode> solely
in a direction normal to the planes of the electrodes and through a substantially constant dis
connected to said electrodes and having a Value
otherelectrode solely in a direction normal to
the planes of the electrodes and through a sub
prising two flat parallel electrodes spaced apart
and movable with respect to each other solely 50 stantially constant displacement Value to thereby
vary the spacing between the electrodes andl the
in a direction normal to the planes of the elecm
capacitance of the condenser between substan
trodes, an hermetically sealed envelope enclos
tially fixed limits, means to cancel the inherent
ing said electrodes and a chemically inert atmos
differential contact potential of the electrodes
phere nlling said envelope, means to apply the
comprising a source of constant directJ voltage
direct voltage to be measured to the said con- '
equal and opposite to the difference in contact
potential of the condenser electrodes, and means
to measure the pulsating voltage produced at the
placement value to thereby vary the spacing be- 60 terminals of the condenser by the variation in
the capacity thereof.
tween the electrodes and the capacitance of the
CORNELIS DORSMAN.
condenser between substantially fixed limits, and
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