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Патент USA US2123045

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July 5, 1938.
's'. c.. HOARE
Filed July 8. 1936
3 Sheets-Sheet l
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Inventxm~ :
Stephen C. Hoer'e,
July 5, 1938.
s. c. HOARE
Filed July 8. 1936
3 Sheets—Sheet 2
Sm PTm
0 n26
July 5, 1938.
s. c._ HOARE
Filed July s. 1936
‘5 Sheets-Sheet s
95 @l
mi Law“
j 2,123,045
Patented July 5,' ‘1938
Stephen 0. Hoare, Manchester, Mass, assignor ‘to
General Electric Company, a corporation or
New York
Application July 8,.1936, Serial No. 89,620
3 Claims. (01.1-75-183)
‘ My invention relates to a detector of magnetic
material and weak magnetic ?elds, such as may
be caused by hidden magnetic material or hidden
neutralize the in?uence of‘the earth's ?eld on
nonmagnetic material in or about which a mag
The detector apparatus is useful in‘ locating pipes
the compass needle ll. Then, by adjusting the
bar magnet towards the. magnetic north, it can 5
neutralize the in?uence of the earth’s ?eld on
or other metal objects hidden from view in the
the compass needle ll.
ground‘ or in walls‘and the like.
In carrying my invention into eifect, I make
Then, by adjusting the bar magnet towards the
compass needle, overcompensation of the earth's
5 netic ?eld is created by a ?ow of» electric current.
such size and strength that, with its north
seeking pole towards the magnetic north, it can .
10 use of a special sensitive compass device, which
ismade nearly immune to the earth's magnetic
?eld and which, therefore,-isespecially sensitive
as a magnetic detector.
The features of my invention which are be
“ lieved to be novel and patentable will be pointed
out in the claims appended hereto. For a better
‘understanding of my invention, reference is made
?eld can be obtained, causing the north' end of 10
the compass needle to point south, or, by ad
justing the bar magnet away from the compass
needle, undercompensation may be obtained.‘ \ H
In the use of the apparatus as a‘ detector, it is
ordinarily undesirable to exactly compensate for 15
in the‘ following description to the accompanying‘
the earth's ?eld because then the compass needle becomes so sensitive that it requires too long a time for the needle to come to rest in a detect
drawings, Fig. -1 of which represents one form of
ing position.‘ I have found that, if the earth's
go my invention which employs in its construction
an ordinary surveyor’s compass; Fig. 2 is a curve
explanatory‘of the use of the detector of Fig. 1
in ‘locating a buried iron pipe when no current
?ows through the pipe; Fig. 3 is a curve ex
25 planatory of the use oi?the detector of Fig. 1 in
locating a buried pipe whencurrent flows there
in; Fig. 4 shows the use of this device as applied
for measuring the depth of pipe after its‘lateral
location has been determined; Fig. 5 represents
30 another form of the invention which is suitable
for locating hidden magnetic parts; Fig. 6 is a
perspective view of a magnetic detector which has
a freely pivoted compensating magnet and which
is self-compensating; Fig. 7 shows a sectional
95 view of a detector of the type shown in Fig. 6
with modi?ed structural details; and Fig. 8 is an
explanatory diagram of the action of the self
Referring to Fig. 1, It! represents the box of an
ordinary surveyor's compass having a magnetic
needle l'l, leveling gauges l2, orienting sights l3,
and tripod ll. The extra parts which may be
used to convert this compass into a sensitive
_\ magnetic detector comprise parts indicated‘ at
IS, IS, l1, and l8. I5 is a nonmagnetic tube
?eld is over or undercompensated'by an amount 20
which causes the compass needle to have a period
of about 6 times the period of the uncompen
sated compass, satisfactory‘results will be ob
tained for ordinary uses. By a period, I mean
that, when de?ected from a balanced magnetic 25
position, the compass needle will oscillate back
and forth on either side of a zero-de?ecting posi
tion a few times before coming to rest. The
time of ‘a complete oscillation is called the period.
The ordinary surveyor's compass with which I 30
have experimented when uncompensated has a
period of about ?ve seconds and when compen
sated for practical use as a detector has a period
of about thirty seconds.
It may be stated that, for a given de?ection; 35
'the sensitivity of a compass needle varies as the
square of the period. Increasing the period from
' ?ve to thirty seconds represents an increase in
sensitivity of thirty-six times normal for such a
compass. The degree of sensitivity which will be 40
most satisfactory in any particular case will
depend. somewhat on the nature of the investiga
tion undertaken, the strength and remoteness
of the magnetic ?eld being located, the time
available for the investigation, the degree of 45
such as brass secured across the central portion accuracy desired, and the presence or absence
of the compass box. Fastened thereto and ex of other disturbing in?uences. Thus, while I
1 tending vertically above the pivot of the com
pass needle is a support ii for a small bar mag
50 net H. The support I6 is made of telescoping
. parts so that the bar magnet may be adjusted
towards and away from the‘ compass needle II,
also the upper telescoping part of support It may
be rotated so as to orient the bar magnet l1 in
5‘ any desired direction. The bar magnet I ‘I is of.
_have .indicated above a range of sensitivity
which is generally satisfactory for the location
of underground pipes, it may be desirable to 50
choose a different range of sensitivity for other
The operation of a detector of the typeshown
in Fig. 1 can also be improved for certain pur
poses by the addition of long magnetic ?ns ex- 55
tending horizontally in opposite directions from
?ection c. After a sufficient number ‘of readings
were taken, the curve of Fig. 2 was plotted and
the pipe :0 located below the point where the
maximum de?ection was noted, namely, at four
of the equipment, are tubular housings for such teen feet from the left curb. A ?fteen foot length
magnetic ?ns. The tubes I! may be of brass of two and one-half inch pipe was located at at,
and each contains one or more rods orstrips of four feet below the detector as set above this
magnetic material. ' The disposition .of these point. After locating the pipe, its in?uence on
the detector. The parts II, which have a tele
scoping ?t into the ends of tube I! so that they
may be readily pulled out to facilitate portability
magnetic strips or ?ns are indicated by dotted
10 lines. They extend in opposite directions from
approximately the outer edges of the compass
box. These ?ns are preferably made of a mag
netic material having high permeability and low
hysteresis loss. They serve as a sort of magnetic
15 antenna.
These ?ns tend to collect any stray
?ux lines in the vicinity of the instrument which
have a direction generally parallel to the fins and
direct these ?ux lines symmetrically across the
Their presence makes a ?ne ad
Justment of the bar magnet i1 unnecessary and
would considerably improve the operation of the
compass as a detector if the bar magnet H were
omitted. In the use of the instrument, these
fins are oriented, for example, in a direction gen
25 erally parallel with the direction of the magnetic
an ordinary compass was tried out and it was
estimated that, when brought to the fourteen
foot station, the ordinary compass changed its
de?ection (indicated by the dotted line curve
between the 12 and 16 foot stations) by not more
than two-tenths of a degree.
/ _
When a small direct current can be sent 15
through a buried pipe line or the like, its location
is facilitated, but a somewhat different pro
cedure is recommended as will be described in
connection with Fig. 3. In Fig. 3, :1: represents
a buried water main known to run parallel with 20
the street, the left curb of which is indicated at
the zero foot station. A small direct current can
be sent through this main by connecting a storage
battery 2! to exposed connections such as the
faucet l9 and the ?re hydrant 20 spaced apart
lines of force emanating from the object to be ' along the street at points between which the de
located as will be explained in examples of the tector is to be used. With a current ?owing
use of the detector.
through the main 3:, ?ux lines will be set up
Fig. 2 is ‘an explanatory graph taken from ac
about the main as indicated by the dotted line
tual ‘data of the use of the detector of Fig. l encircling :r. In order to best in?uence the de
in locating an iron pipe buried in a street. The tector by -such ?ux lines, the antennae or ?ns
horizontal scale represents distance in feet from l8 thereof are oriented in a line‘ generally cross
one curb used as a reference point. The vertical wise of the pipe line and the detector is moved
scale represents de?ection in degrees of the de
in a straight line across the street from station
35 tecting instrument. It was known that the pipe to station with the ?ns so positioned. Also, the
ran generally parallel with the ‘street but its needle of the instrument is initially magnetically
distance from the curb and depth were unknown. biased to approximately right angles to the line
It was impracticable in this case to send an elec
of the antennae. This is indicated at the three
tric current through the pipe and the detector foot station. Readings of the magnetic needle
was used for more de?nitely locating the pipe. de?ection of the detector are taken at each sta
The detector was set up on one side of the street, tion both with current ?owing through the pipe
such as at the point corresponding to the reading line and with no current ?owing therethrough,
a, known or believed to be on one side of the pipe. and the angular difference in the readings of the
The directions of the ?ns it were generally par
needle at all stations under these two conditions
allel with the suspected direction of the pipe, is recorded together with the distance measured a
the bar magnet H was oriented so as to cause from the curb or other reference point. As the
the compass needle ii to point approximately at detector is moved near the pipe line, the an
right angles to the direction of the ?ns l8 and tennae catches more and more of the ?ux lines
the indication of the compass needle on the angu-_ existing about the pipe line when the current is
lar scale of the device was noted. The detector ?owing therethrough and the angular difference
was then moved to other stations, crossing the in the detector reading with the current on and
street, and readings were noted at each station, off increases. Thus, at the three foot station, this
the sights i3 being used to sight on some distant angular difference is thirteen degrees and, at the
objects so as to move from station to station twelve foot station, it is about ?fty—two degrees.
along a straight line and to keep the orientation A su?icient number of readings are taken to plot
of the compass box constant. The distances of the curve, such as is shown in Fig. 3, whereupon
each station from the curb or starting point were the pipe line is located beneath the point of
also noted so that the curve abcd of Fig. 2 could maximum de?ection. A pipe line four feet below
be plotted.
the road surface and ?ve feet below the detector
In Fig. 2, the detector is indicated as being was so located.
set up at the nineteen foot station for obtaining
It will be understood that the exposed electric
the reading d and, at the nine foot station, a top circuit between connections If! and 20 should be
view of the detector is shown to illustrate the run well back, away from the street, otherwise
orientation of the ?ns and the nature of the magnetic in?uences therefrom when current is .
detector de?ection at this point. As the detec— ?owing will distort the readings and result in
tor was moved across the street and readings erroneous results.
were taken at the different stations, the needle
After having located the lateral position of the
of the de?ector tended to turn more nearly par
water main as above described, its approximate
allel with the ?ns it! as these ?ns collected more, depth can be determined as follows:
70 and more magnetic lines emanating from the
The detector is set up under the return elec
‘ buried pipe indicated at 3:. Thus, at the nine tric circuit and the wire thereof lifted above the 70
foot station, the de?ection was increased 1! de
detector as indicated, in Fig. 4. The wire is given
grees. This in?uence was, of course, strongest the same bearing as the pipe line, the latter hav
at the station which was nearest the pipe line, ing been determined by two or more separated
is namely, at the station corresponding to the de courses of detector travel. De?ection readings
' ;' are'taken with the same current ?owing
" needles? In the illustration, aldetectm‘ with only
"the-return wire as was used in obtaining the ‘two such ' needles'_~3_l and 32 is‘ shown. These
. curvepf Fig. 3. By raising-‘the wire above the‘ _ needles being magnetized are connected together
detector such a distance as‘ to obtain. the same‘ magnetically but not mechanically except in so'
maximum de?ection as wasobtained when-direct ‘ far as, their pivots 33and‘34 are in axial alignment
ly'over the water main, ‘it can be assumed that and ‘have a common supporting structure. The
V -_."-the. circuits were the same distance away from‘, _'shorter needle 3|‘ is the detector and'the longer
Thus, if is
needle 32 is the compensator'for the'effect of the
'the :detector
ins both instances.
earth’s magnetic ?eld. This compensator 32 is
necessary to lift the wire, ?ve feet above the de
tector tojobtain this maximum de?ection, it is‘ ‘freely-pivoted instead of 'b'eing‘?xed as in the‘
reasonable to' assume that the detector was ?ve device of Fig. 1. _'The compensator needle 32 is
I _ feet above the water main when the same de?ec
made long so that it can substantially compensate
tion "was previously obtained. This assumption
the‘ short needlewith respect‘to the in?uence of
, _ 'works'out. to be approximately correct, and the
the earth's ?eld ‘and yet be able to take up a
water mainv :cwas found' within
a few inches of
'thedep'th so deduced.
1 ‘ In Fig. 5, I have shown the essential elements
position nearly in the earth’s magnetic meridian. 15
In this. arrangement, advantage .is taken-of the
sensitivity of the detector needle to the adjust
f oftanother form of magnetic detector that may be ‘. ment of the compensator, small changes in ad
used.for ?nding quite small masses of hidden justment of the latter producinga greater change
in the detector. The pivoted compensator acts as 20
magnetic material-aswell as larger masses.
does not require the somewhat unwieldly ?ns of further ampli?cation ‘on the detector., A' weak.
~ the device of Fig. 1' to make it highly responsive to
‘ .weak magnetic ?elds. It consists of a long piv
oted spindle 25 of nonmagnetic material with
I25 ‘magnetic ‘needles‘26 and Y21 on either end. The
'- needles 26 and 21 are iixed- to spindle 25 so that
they lie in the same‘plane but with their mag
netic polarities reversed as indicated by the desig~
nations N and S. If these needles are of equal
strength and length and ?xed at their centers at
right angles to the spindle‘25, and the device is
‘ i used with the spindle 25 in a vertical position the‘
device will be substantially immune from in?u
ence by_ the earth’s magnetic ?eld. If, however,
magnetic ?eld to be detected'produces an appre
ciable de?ection ‘of the detector needle as before
and also a slight de?ection of the compensator‘
needle, but a slight de?ection of‘ the compensator 25
makes for still greater de?ection of the detector
and ampli?cation is the result.
This type‘ of ‘detectorhas certain‘ advantages as
The sensitivity of the‘ detector needlci'is ad 30
justed once for all when the device is‘manufac- .
tured. .The compensator needle is pivoted ,and'
needs no adjustment other than that which auto
matically takes place. All that is necessary in
35 one of the needles 26 is‘ adjustable so thatit can ‘ conducting‘ a search is to turn the compass box 35
be more or less inclined to the spindle 25 in the or support so that the longer needle reads zero on
same plane with needle 21, the radius to its pole the scale 35 and note the de?ection of the smaller .
tip and its in?uence by the earth’s magnetic ?eld
are reduced as'compared to needle 21. In this
40 vway the device may have its sensitivity adjusted
to any ‘desired degree of astaticism.
The needles are positioned a su?icient distance
needle on-its scale 36. The de?ection _of the
shorter needle increases with stronger ?elds. I
have obtained goods'results with this type, of de 40
tector where the ‘shorter needle was one and
three-eighths inches in- length and the longer
needle four. and one-half inches in length with
as indicated at28', located below the device will ' the needles located in planes two and three-quar
_ .
45 have a relatively greater in?uence on the nearer ters inches apart.
Suchan instrument may be built with a single
or' lower needle 21. This greater in?uence on
needle .2‘l'is augmented by the fact that its torque scale 35 as shown in Fig. 7 where the short needle
arm is slightly longer than that of the inclined 3| is provided with aluminum extensions‘ 31 which
needle 25. The spindle 25 will, .therefore, be extend‘ over and indicate on scale 35 with the
turned until the lower needle points toward the long needle 32.
‘ "
Fig. 8 is typical of the positions of the pointers '
hidden mass of iron. The de?ection may be noted
on a scale-indicated at 23.‘
- ~
of the form of ‘detector shown in Fig. 6 when ad
This form of detector is usedby exploring the justed for good results and when unin?uenced by region where the hidden magnetic 'material is external ?elds except the earth’s ?eld. The po
55 suspected. The magnetic'mass is located at the sition of 32 is mostly determined by the earth’s 55
position of maximum de?ection.
?eld but is oil the magnetic meridian by about
I have found that a spacing of about ?ftee
?ve degrees because of the in?uence of‘ the‘short
inches between needles 26 and 21 and the relative needle 3|. The short needle is mostly in?uenced
proportions indicated in Fig. 5 gives a satisfactory by the large needle 32, to a lesser extent by the
detector for general use.
earth’s ?eld, and takes up the position shown at
The device can be used as above described; an angle of about forty degrees from the magnetic
however, I have found it convenient to provide a meridian. The forces determining the position of
small stationary but adjustable bar magnet 30 needle 3i are a counter-clockwise force due to the
pivoted on the same axis near and preferably earth’s ?eld and a clockwise force due to the atabove the upper needle 26. This is convenient as traction between-the unlike poles of the pointers 65
a zero adjuster for needle 21 and as an adjust
and a repulsion between the like poles of the
ment for the desired degree of sensitivity‘ When pointers. These needles will have other zero bal
the bar magnet 30 is included as a part of the de
anced positions where needle 32 is rotated ten
vice, the inclination adjustment of needle 25 may degrees counterclockwise and ne'edle.3l eighty de
70 be permanently made at the time of manufacture. grees clockwise. The needles will come to rest in 70
In Fig. 6, I have shown still another form of either one of such balanced positions. By mov
detector which is particularly suited for con . ing needle 3l._ away from 32, it will be less in
_. apart on spindle 25 sothat a mass of iron, such
venience and rapid work in locating water pipes
and the like. This detector comprises a cascade
75 arrangement of a plurality of pivoted magnetic
?uenced by 32 and the ?ve degreeangle will be
decreased and theyfor'ty degree angle increased.
The relative lengths of the pointers will also have 75
an in?uence on‘ the zero balanced positions of the
Now assume that I bring a mass of magnetic
material Ill within" the in?uence or the detector.
5 Pointer 32 tends to turn toward it. The pointer
Ii also tends to turn toward it. This upsets the
magnetic forces between the two pointers and
changes their ratios with the result that the new
de?ection of pointer 3| due‘to the presence of
10 the magnetic mass 40 is further ampli?ed by rea
son of the shift in pointer 32. ‘
The cascade arrangement last described may,
if desired, be provided with the antenna of Fig. 1.
'l‘heantenna is ‘indicated by the parts l8a, Fig. 8.
15‘ ,In accordance with the provisions of the patent
' statutes, I have described the principle of opera
tion of my invention together with the apparatus
which I now consider to represent the best embod~
' iment thereof but I desire to have it understood
20 that the apparatus shown is only illustrative and
that the invention may be carried out by other
What I claim as new and desire to secure by
letters Patent of the United States is:
1. A magnetic detector comprising, a magnetic
compass having a polarized pivoted needle, an
lecting weak stray ?ux lines and directing them
across the axis of rotation of the ?rst mentioned
needle so as to amplify their in?uence on such
2. A magnetic detector comprising, a magnetic
compass having a pivoted magnetic needle, and
another ‘magnetic needle of shorter length than
the ?rst, mounted adjacent the ?rst needle, and
adjustable about the same axis of rotation and
towards and away from the ?rst mentioned nee
die, the shorter needle serving to substantially '
compensate ‘the ?rst for the in?uence of the
earth’s magnetic ?eld thereon, whereby the ?rst
mentioned needle becomes sensitive to weak mag
netic ?elds in the vicinity which would have a
negligible in?uence thereon ‘without such com
3. A magnetic detector comprising in combina
tion, a surveyor’s compass having a pivoted mag
netic needle, a permanent magnet or. shorter 20
length than the magnetic needle mounted so as
to be adjustable about the axis of rotation of the
pivoted needle and toward and away therefrom
for the purpose of neutralizing the‘in?uence of
the earth’s magnetic ?eld on said pivoted needle 25
to the extent that the period of oscillation of said
other polarized needle mounted adjacent the ?rst . compass needle in the earth’s magnetic ?eld is
needle and adjustable about the axis ‘of rotation approximately thirty seconds, and'magnetic an
of the ?rst needle, said second needle having such tenna means comprising strips of high-permea
30 location, polarized strength, and dimensions as bility magnetic material having a low-hysteresis 30
to substantially compensate the ?rst needle for loss extending in opposite directions from said
the in?uence of the earth’s magnetic ?eld there
compass at right angles to the axis of rotation of
on, the degree of suchcompensation being adjust;
able, whereby the ?rst mentioned needle becomes
35 su?iciently sensitive to weak‘ magnetic ?elds in
the vicinity thereof to be used in detecting the
location thereof, and a magnetic antenna for col
its needle for increasing the sensitivity of said
needle with respect to weak magnetic in?uences
in its vicinity.
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