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Jan. ‘14, 1947.
Filed June 19, 1944
10% 4
Patented Jan. 14, 1947
2,414,224 .
Alan Douglas, Burnham Green, England
Application June 19, 1944, Serial No. 541,022
In Great Britain June 3, 1943
4 Claims.
(Cl. 177-351)
This invention relates to the measuring and
gap is ‘normal, and when said air gap is varied,
indicating of small displacements, particularly,
voltage is applied to said indicating instrument.
but not exclusively, for the purpose of measuring
and indicating mechanical forces causing such
, '
Many methods have been proposed for measur
ing and indicating displacements by electrical
means. In some such methods the arrangement
has been such that the displacement causes the
' variation of a gap in a magnetic circuit constitut
ing the flux path through an inductance, and
thereby causes variation in said inductance,
which variation in ‘turn causesa corresponding
' variation in an. electrical quantity and thereby
said circuits are no longer so tuned so that a.
It will be seen that the magnetic circuit is re
lieved of all stress and the speed of response is
limited only by the inertia of the component
It the percentage change in the variation of
the air gap is kept small, even for the maximum
10 displacement to be measured, to say 1% of the
initial air gap, then a substantially linear rela
tionship obtains between the ch= age in gap and
the change in inductance of the coil.
In order to ensure operational stability, onev
15 procedure would be, not to earth the frequency
generating source, and to employ a balanced
actuates a measuring device. In most arrange‘
ments of this character two of such magnetic
and screened transformer ‘to connect the ire
circuitsv and inductances- have been employed " quency
generator to the circuit.v In practical
which normally balance one another, thereby
however, a high degree of stability
giving a zero readlngon the measuring device,
and when the gap in one of said magnetic cir
cuits is varied the balance is distributed and the
measuring device gives a reading whichis a func
tion of the gap variation.
The present invention has for its object the
provision of an improved measuring and indicat~
ing device of the above character, in which,
among other advantages, only one inductance
will be required. Further objects of the inven-,
tion are the provision of a device which will be
20 may be achieved it a common earth connection
is used for the frequency source. the, coil and
the device which detects the electrical variation
in the coil circuit, and in order that the invention
may be more clearly understood an embodiment
thereof involving this last named arrangement
will now be described with reference to the accom
panying diagrammatic drawing.
Referring to this drawing the displacement
which is to be measured is physically transmitted
a U-shaped piece of iron I which is correlated
highly sensitive and will give great rapidity of 30 to
with a T-shaped piece of iron 2 having a coil 3
response without distortion, also substantial
wound on its centre member, in such a way that,
linearity between the displacement to be meas
.ured and the electrical variation applied to the
as shown, a magnetic circuit for the ?ux of said
coil is constituted having three equal air gaps 4,
measuring device.
two between the respective extremities of the
The invention consists broadly of apparatus for
piece i' and the cross member of the piece 2, and
measuring small displacements wherein an air
one between the crutch member of the piece I and
gap, in a magnetic circuit forming the core of a
the extremity of the centre member of the piece 2.
coil, is physically varied in accordance with the
The displacement of the piece I is indicated
displacement to be measured, and said coil is con-i
graphically in the drawing by the double-ended
nected, in commomboth across a source of con 40 arrow a, and this displacement is in such a di
stant frequency alternating current supply, in se
rection as to simultaneously reduce or simulta-i
ries with a capacity, and across an indicating
neously increase all these air gaps 4,.
instrument in series with another capacity, the
Two equal variable condensers 5 and 6 (which
arrangement being such that said instrument re
may be mechanically coupled together) are pro
sponds in accordance with variations in said air
vided having a plate of the one connected to a
gap and therefore in accordance with the dis
plate of the other. One end of the coil 3 is con
placements to be varied.
nected to the junction point of these two plates
The circuit across said source of alternating
and the other end of said coil is connected to
current supply, and including said coil and the
?rst-named capacity, and also the circuit across 50 earth. A source of constant ircquency oscilla
tions has one of its terminals ‘I connected to the
the indicating instrument, and including said coil
other plate of the condenseri and its other ter
and the second named capacity, are both tuned,
minal 8 connected to earth. A detector device
when the air gap is normal, to resonate at the ‘fre
or measuring instrument I has one of its ter
quency of said supply, so that no voltage is ap
plied to the indicating instrument when said air 55 minals 9 connected to the other plate of the con
9,414,924; .
denser ii, and its other teinai iii connected to '
earth. :A variable resistance ii is connected
across the two condensers as shown for balancing
the resistance or the coil.
which a source or vconstant frequency alternat¢
_ ing' current may be connected, a condenser. said
condenser and coil being connected in series
across saidv terminals, and a measuring circuit
- When the air gap 6 is ‘normal the circuit 1, B,’
including an indicating instrument and a second
8, l is tuned to resonate at the frequency or the
condenser connected in series across said coil.
source vand the circuit of 9,.8, 3, i0 is also tuned
2. Apparatus as recited in claim 1, wherein said
to resonate at that frequency, andthus no voltage
are adjustable and tuned at a pre
appears across the terminals 9. Hi. It the air
gap 4' is varied the inductance of‘ the coil 3 is 10 selected normal length 0! air gap to'resonate the
input circuit and the measuring circuit at the fre
varied ‘and the said two circuits no longer reso
quency of said alternating current source: where~
nate at-the frequency of the source and a voltage
no voltage is applied to the indicating instru»
is applied to the terminals 8. it or the indicating >
ment when the air gap is of normal length, and
instrument. This voltage is a function oi’ the
a voltage is applied upon detuning oi said circuits
a _ gap variation and therefore of the displacement 15 by change in the ‘air gap length.
or the piece i and or the applied iiorce.
3. Apparatus as‘ recited in claim 1, in combi-v
What I claim and desire to secure by Letters
nation with a resistance connected across said
' Patent is:
1. Apparatus- for measuring small
_ ments comprising a pair of magnetic material 20
elements forming a magnetic circuit having an air
' gap, one or said elements being movable to vary
the air gap in accordance withthe displacement '
to be measured, a coil onione of said magnetic ele»
ments, a pair of input circuit te 111:: ale across
condensers for balancing the resistance or said
4. ‘Apparatus as recited in claim 1, wherein one I -
_ input circuit terminal is earthed, and said coil and
said indicating instrument each have a terminal
directly connected to sai earthed inputcircuit ‘
\; n‘ouoms.
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