Superconducting magnetic shield

This invention provides a superconducting magnetic shield and a magnetic shielding apparatus including the same, thereby accurately measuring an extremely weak magnetic field such as magnetoencephalographic waves by reducing the influence of magnetic field of the earth or magnetic noises.

BACKGROUND OF THE INVENTION 
(1) Field of the Invention 
This invention relates to a magnetic shield utilizing a superconducting 
phenomenon and, more particularly, to a magnetic shield in which a 
superconductor is composed of bismuth high temperature oxide 
superconductor for accurately measuring a magnetoencephalographic wave or 
other external alternating magnetic field to be measured. 
(2) Description of the Prior Art 
Magnetic shields generally have an active shield and a passive shield. The 
passive shield has a ferromagnetic shield and a superconducting shield 
known per se. The characteristics of the respective magnetic shields have 
features. (See "Cryogenic Engineering" written by Ogasahara, pages 
135-147, Vol. 18, Nov. 4, 1978.) 
Among the magnetic shields, the ferromagnetic shield eliminates 
introduction of a magnetic field into an inner space by shielding an 
external magnetic field by a ferromagnetic material such as, for example, 
a nickel iron alloy commercially available as Permalloy, trade name. On 
the other hand, the superconducting shield excludes an external magnetic 
field toward the exterior without shielding it in a superconductor to 
obviate the introduction of the magnetic field into the inner space, and 
its magnetic shielding effect is much larger than that of the 
ferromagnetic material. More specifically, the ferromagnetic shield has a 
limit due to the presence of a residual magnetization. A magnetic shield 
of a range exceeding the limit is expected by a superconducting magnetic 
shield. However, since the critical temperature Tc at which an electric 
resistance becomes zero, of an Nb metal superconductor of, for example, 
Nb.sub.3 Sn, Nb.sub.3 Ge, etc., is approximately 20K of cryogenic 
temperature, it is necessary to employ liquid helium as a refrigerant. 
Thus, construction of a magnetic shield has a barrier of its cost, and the 
superconducting shield is not yet realized except an extremely small field 
at present. 
Recently, researches for trying an analysis of a mechanism of human brains, 
a headache, a diagnosis of brains, etc., have been activated by 
measurements in magnetic waves generated from the brains. Heretofore, 
searches in the interiors of human brains by MRI, positron CT, etc. have 
been clinically executed, but limited in its resolution, radioactive rays 
to be used, etc. Therefore, needs for detection of magnetoencephalographic 
waves themselves are abruptly raised. 
However, the intensity of the magnetoencephalographic waves is 10.sup.-3 
gauss, extremely weak, while the magnetic field of the earth or the 
terrestrial magnetism is as strong as 0.3 gauss. There are large magnetic 
noises generated from various electric devices and equipments in addition 
to the magnetic fields of the earth in the environment. 
It is required for a magnetic sensor of high sensitivity and a magnetic 
shield for shielding the magnetic field of the earth and the magnetic 
noises under such circumstances to detect extremely weak 
magnetoencephalographic waves. To this end, a magnetic sensor or a 
superconducting magnetic shield called SQUID (superconducting quantum 
interference device) has been recently developed. However, since the 
superconducting magnetic shield necessitates expensive liquid helium as 
its refrigerant as described above, it is not yet realized. 
The development of a practical superconducting magnetic shield is desired 
not only to detect an extremely weak signal of a biomagnetism but to 
protect Josephson elements, IC circuits against external magnetic noises 
as described above. 
It is an object of the present invention to provide a superconducting 
magnetic shield or a magnetic shielding apparatus which can shield various 
external magnetic noises by using inexpensive liquid nitrogen without 
expensive refrigerant such as liquid helium, etc. 
SUMMARY OF THE INVENTION 
According to one aspect of the present invention, there is provided a 
superconducting magnetic shield comprising a bismuth high temperature 
oxide superconductor having a cylindrical or rectangular parallelepiped 
shape at least opened at one end thereof and 1.times.10.sup.-5 or less of 
a magnetic shielding effect defined by the ratio of intensity of a 
magnetic field at 77K detected by a magnetic field detecting coil 
installed in said cylindrical or rectangular parallelepiped shield to 
intensity of a magnetic field at an ambient temperature when the ratio of 
bore/length is 1 or less and ah external alternating magnetic field is 
objective. 
According to another aspect of the present invention, there is also 
provided a magnetic shielding apparatus comprising a bismuth high 
temperature oxide superconducting shield having a cylindrical or 
rectangular parallelepiped shape at least opened at one end thereof and 1 
or more of ratio of bore/length, and a ferromagnetic material magnetic 
shield or Helmholz coil provided outside said superconducting shield, 
whereby a magnetic shielding effect defined by the ratio of intensity of a 
magnetic field at 77K detected by a magnetic field detecting coil 
installed in said cylindrical or rectangular parallelepiped shield to 
intensity of a magnetic field at an ambient temperature is 
1.times.10.sup.-3 or less when an external alternating magnetic field is 
objective.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Typical high temperature oxide superconductors generally include various 
superconductors such as La-Sr-Cu-O, Bi-Sr-Ca-Cu-O, Y-Ba-Cu-O, Tl-Ba-Cu-O, 
etc. Among them, the La superconductors have Tc=30-40K, and hence liquid 
nitrogen cannot be employed as a refrigerant similarly to the Nb 
superconductors. The Tl superconductors have high Tc up to 125K, but have 
a problem in toxicity in its application. The high temperature oxide 
superconductors practically include Y and Bi superconductors. As a result 
that the present inventors have variously researched the Y and Bi high 
temperature superconductors, it has been found that the Bi oxide 
superconductor has superior properties to the Y oxide superconductor by 
forming the Bi oxide superconductor in a specific shape and exhibiting 
specific magnetic shielding effect. One reason is that the Bi high 
temperature oxide superconductor easily obtains a homogeneous 
superconducting phase due to the difference of oxygen sensitivity at the 
time of manufacture of the shield. If even a small amount of 
nonsuperconducting phase exists, lines of magnetic force are invaded 
therefrom to inhibit the effect of the magnetic shield. Another reason is 
considered that, since a Bi-O layer is easily slip-deformed in the Bi high 
temperature oxide superconductor, a stress at the time of manufacture is 
easily alleviated. It is also considered that the Bi oxide superconductor 
is stronger against water content than the Y oxide superconductor and has 
excellent aging stability. 
The reason why the ratio of the bore/length of the shield is 1 or less in 
one aspect of the present invention is that it has small influence of 
invasion of magnetism from the open end to sufficiently shield the 
magnetic field of the earth and the magnetic noises without the necessity 
of other external magnetic shield. The reason why the one end is at least 
opened is that a SQUID as a sensor is inserted. Normally, the one end is 
closed in a vessel shape, but in order to measure magnetoencephalographic 
waves, etc., both ends are open, and a human head is inserted thereinto. 
Since the SQUID employs liquid helium, the size of its detector is 
increased, and a sufficiently large opening diameter is required. The 
diameter of the opening is reduced as much as possible as long as the 
above relation is satisfied. 
The reason why the external alternating magnetic field is objective in the 
present invention is because magnetic fields such as those generated from 
a human brain is an alternating magnetic field having a frequency up to 
approximately 10 Hz. The SQUID serves to detect only the alternating 
magnetic field due to its mechanism. The SQUID is effective to detect 
magnetoencephalographic waves due to high sensitivity and the 
characteristic of the alternating magnetic field. If the SQUID also 
detects a DC magnetic field, it is necessary to shield the DC magnetic 
field of the magnetic shields, resulting in impossibility of detecting 
magnetoencephalographic waves from a noise. Therefore, even if the 
magnetic field of the earth exists, it is substantially a magnetostatic 
field, and therefore not a problem. A subject is fluctuations of the 
magnetic field of the earth. In order to detect magnetoencephalographic 
waves by suppressing the external alternating magnetic field represented 
by the fluctuations of the magnetic field of the earth, a SQUID having a 
gradiometer and performance of 10.sup.-5 or less of magnetic shielding 
effect are required. This performance cannot be satisfied by the 
ferromagnetic shield which merely has a shielding effect of 10.sup.-2 to 
10.sup.-3. 
Here, the magnetic shielding effect is defined as the ratio of the 
intensity of magnetic field at 77K (-196.degree. C.) detected by a 
magnetic field detecting coil mounted in a cylindrical or rectangular 
parallelepiped shield to the intensity of magnetic field at an ambient 
temperature. In the present invention, the superconducting magnetic shield 
of the dimension and shape described above formed through steps of 
molding, cutting, sintering and coating from bismuth oxide powder having a 
composition such as, for example, Bi.sub.1.8 Pb.sub.0.4 Sr.sub.2 Ca.sub.2 
Cu.sub.3 O.sub.y obtained, for example, by an oxalic acid-ethanol 
coprecipitation process to be described in detail later has 
1.times.10.sup.-5 or less of magnetic shielding effect defined as 
described above, thereby making it possible to detect low frequency, low 
magnetic field such as magnetoencephalographic waves having a frequency up 
to 10 Hz and 10.sup.-9 gauss. 
According to another aspect of the present invention, there is provided a 
magnetic shielding apparatus comprising a bismuth high temperature oxide 
superconducting shield having a cylindrical or rectangular parallelepiped 
shape opened at least at one end thereof and 1 or more of ratio of 
bore/length, and a ferromagnetic material magnetic shield or Helmholz coil 
disposed outside the bismuth high temperature oxide superconducting 
shield. Even though the ratio of the bore/length is 1 or more and the 
influence of invasion of a magnetic field from the opened end is large, 
the ferromagnetic material magnetic shield or Helmholz coil is provided 
outside the superconducting magnetic field to reduce magnetic noises in 
so-called two stages, thereby making it possible to measure accurately an 
extremely weak magnetic field such as magnetoencephalographic waves. 
When a magnetic field (up to 10.sup.-9 gauss) generated from a human brain 
is detected by a SQUID fluxmeter, the diameter (D) of the opening end of a 
high temperature oxide superconducting magnetic field is set to such a 
size that the head of a person to be a patient to be measured is easily 
inserted, and the length (L) of the shield is reduced to be smaller than 
the D. In this case, it is theoretically impossible to reduce the ratio 
Hi/Ho of the intensity Ho of an external alternating magnetic field to a 
magnetic field Hi in the shield to a value smaller than 5.times.10.sup.-4. 
Therefore, this magnetic shield cannot detect magnetoencephalographic 
waves. In other words, with variations in the magnetic field of the earth 
being 0.3.times.10.sup.-1 gauss sit cannot be reduced to be smaller than 
0.3.times.10.sup.-1 .times.5.times.10.sup.-4 .times.10.sup.-4 
=1.5.times.10.sup.-9 even by using a second-order gradiometer (assuming 
that a noise can be reduced by four figures). Since its signal has up to 
10.sup.-9 gauss, its S/N ratio is not smaller than up to 1, and the 
signal cannot be detected in this state. 
In the magnetic shield of the present invention, the diameter of the 
opening end thereof is not always the same over the entire length of the 
cylindrical or rectangular parallelepiped shape. The length necessary to 
preferably measure magnetoencephalographic waves by inserting a human head 
has a diameter to accommodate the head, but the other portion is reduced 
in diameter of the degree necessary to insert the end of a SQUID fluxmeter 
to desirably reduce the influence of magnetic noises as small as possible. 
In any case, the magnetic shield can be formed through the steps of 
pulverizing, molding and sintering. 
Therefore, according to the present invention, a ferromagnetic material 
magnetic shield made of a ferromagnetic material such as, for example, 
Mumetal or a Helmholz coil is installed outside the high temperature oxide 
superconducting magnetic shield of cylindrical or rectangular 
parallelepiped shape having, a D/L of 1 or less as described above so as 
to cancel magnetic field noises. Here, the Mumetal is nickel-iron-copper 
alloy having high permeability and low hysteresis loss characteristics. 
The Helmholz coil is formed by disposing a pair of circular coils having 
equal radius and number of turns at a predetermined interval with respect 
to a common axis and connecting the circular coils in series with each 
other, thereby making it possible to obtain much more uniform magnetic 
field than the magnetic field made by a sole coil. 
The direction of the cancelling coils is set to a direction for cancelling 
magnetic field noises to be invaded. External magnetic field can be 
reduced up to 10.sup.-2 to 10.sup.-3 or less by the ferromagnetic material 
magnetic shield or Helmholz coil. In this manner, the magnetic shielding 
apparatus has a value of 1.times.10.sup.5 or less of magnetic shielding 
effect defined as described above. If a strong noise exists, it is 
preferable to employ the Helmholz coil. Assume that the magnetic noise of 
10.sup.-3 can be reduced by the Mumetal, with the variations in the 
magnetic field of the earth being 0.3.times.10.sup.-1, sufficient S/N 
ratio can be obtained for a signal up to 10.sup.-9 gauss by the level of 
0.3.times.10.sup.-1 .times.10.sup.-3 .times.5.times.10.sup.-4 
.times.10.sup.-4 =1.5.times.10 -12 gauss. 
FIGS. 1 and 2 show concept view of the case of reducing magnetic noises at 
two stages and detecting an extremely weak magnetic field generated from a 
brain. FIG. 1 illustrates installation of a Mumetal cylinder as a 
ferromagnetic material magnetic shield, and FIG. 2 illustrates 
installation of a Helmholz coil. 
In FIGS. 1 and 2, a superconducting magnetic shield 1 is formed in a 
cylindrical shape opened at both ends thereof. In the drawings, the 
diameter of the lower end of the superconducting magnetic shield 1 is 
formed to be larger, and the diameter of the upper end of the magnetic 
shield 1 is formed to be smaller. The magnetic shield 1 is covered with a 
heat insulating vessel 2 supported by suitable means, the vessel 2 having 
a shape and dimensions corresponding to the magnetic shield, and 
refrigerant, liquid nitrogen 3 is filled in the vessel 2. 
On the exterior of the superconducting magnetic shield 2 formed as 
described above is disposed a cylindrical ferromagnetic material magnetic 
shield 4 made of a ferromagnetic material such as, for example, Mumetal to 
surround the periphery of the superconducting magnetic shield 1 (FIG. 1), 
or Helmholz coils 5 of a pair of coils having the same radius and number 
of turns are disposed at the same distance on the same axis through the 
magnetic shield 1 (FIG. 2). 
A SQUID fluxmeter 6 is installed above the superconducting magnetic shield 
1, and the end 7 thereof is extended into the opening above the 
superconducting magnetic shield 1. 
A patient 8 to be measured for his magnetoencephalographic waves is located 
under the superconducting magnetic shield 1, and his head 9 is inserted 
into the lower opening of the magnetic shield 1. The end 7 of the SQUID 
fluxmeter 6 is approached to the head 9 of the patient 8 to measure the 
magnetoencephalographic wave on a head skin. 
The SQUID is an ultrahigh sensitivity fluxmeter containing a Josephson 
element. Since the intensity of a magnetic field generated from a brain is 
extremely weak such as a level of 10.sup.-9 gauss, it cannot be measured 
except the SQUID fluxmeter. In order to detect the magnetoencephalographic 
waves, the following procedure is executed. 
1) Liquid helium is transferred to the SQUID in an operating state. 
2) The head of the SQUID is set to the head skin of the patient to be 
measured as near as possible. 
This is because the distance attenuation of the magnetoencephalographic 
wave is avoided as much as possible. 
3) A magnetic signal inputted to the Josephson element in the SQUID is 
detected as a voltage signal through electronics. 
4) The above signal is integrated to form a magnetoencephalography (MEG). 
5) A "reverse problem" is solved from the magnetoencephalography, and the 
generating position of the magnetic signal is fixed. 
As described above, the generated magnetic signal can be detected in an 
extremely localized state. 
It is necessary to avoid a mechanical vibration as much as possible in the 
procedure. Because the mechanical vibration disorders the magnetic signal. 
When the ferromagnetic Mumetal cylinder is employed as shown in FIG. 1, 
the magnetic shield may be, as required, box type to contain all of the 
patient to be measured, a fluxmeter, and a magnetic shield in the box. In 
this case, the magnetic shield is further completely performed, but 
designed in view of its cost performance. 
A method of manufacturing the superconducting magnetic shield in a vessel 
shape according to the present invention comprises five steps of (1) 
manufacturing Bi high temperature oxide powder, (2) molding it by a cold 
hydrostatic press, (3) cutting it, (4) sintering it, and optionally, (5) 
coating the surface with resin. The steps of the manufacturing method will 
be described in detail. 
(1) Manufacturing of powder 
An oxalic acid-ethanol coprecipitating process improved from a known 
coprecipitating method as a wet powder manufacturing method is employed to 
coprecipitate the oxalate of Bi-Pb-Sr-Ca-Cu. The coprecipitated oxalate is 
filtered, dried and baked. In the baking step, bismuth high temperature 
oxide powder having a desired composition can be obtained by thermal 
decomposition. The size of the powder is in the order of submicron. The 
baking step is conducted at a temperature in a range of 800 to 850.degree. 
C. to be set under optimum conditions of the desired composition. Pb is 
added to the powder. The Pb is known as an indispensable element to obtain 
so-called 110K phase in high volumetric rate in the Bi superconductor. The 
repetition of the baking steps is effective to exhibit high temperature to 
110K phase, and a pressing step on the way is further effective thereto. 
As described above, the powder to be provided for the present invention 
intends to satisfy submicron size, content of 80 to 90% of up to 110K 
phase, homogeneity and high Tc phase. 
(2) Pressing 
In order to raise baking density, a cold hydrostatic pressing is employed. 
The Bi superconductor feasibly cracks at the baking time due to anisotropy 
of the Bi superconductor by uniaxial pressing. The pressing pressure is 
desirably 1 ton/cm.sup.2 or more. Since the vessel is cylindrical, a core 
of rigidity is required at the pressing time. Therefore, a difference of 
molding densities between the outside and inside of the cylindrical vessel 
tends to occur. Since the difference of the densities causes the vessel to 
crack, the thickness in the vessel is limited in this respect. 
(3) Cutting 
Since a rubber mold is employed in the hydrostatic press, the surface of 
the vessel is uneven, and it is necessary to grind it in a desired size. 
In this case, it is considered that the influence is given on 
superconducting characteristics by applying a strain to the molded piece 
at the time of grinding. Therefore, it is necessary to reduce a cutout 
angle, a feeding speed as much as possible. Typically, the conditions are 
provided according to the following Table. 
______________________________________ 
A cutter with diamond tip is used to cut 
Spindle speed: 105 rpm 
Feeding Cutout amount 
______________________________________ 
Bottom face 0.175 mm/r 0.5 mm 
Rough finishing of 
0.80 mm/r 0.25-0.5 mm 
outer periphery 
Finishing 0.20 m/r 0.1 mm 
______________________________________ 
(4) Sintering 
The sintering temperature is 800 to 850.degree. C., and the optimum 
condition is determined depending upon the composition. The sintering time 
is typically 20 hours. The most important point for homogeneously 
sintering is to make temperature distribution in a furnace uniform. To 
this end, an electric furnace having a four face heater is preferable. 
(5) Resin coating 
The surface of the vessel is coated, as required, with silicone or varnish, 
etc. 
As described above, the thickness of the vessel has a limit in view of 
pressing step and cutting step. In this meaning, it is preferable to set 
the thickness of the vessel to 1 mm or thicker. If the thickness of the 
vessel is less than 1 mm, the molded piece is impossible to be cut, and 
the sintered material has pores and it is accordingly necessary to enhance 
its surface density, and desired surface density cannot be obtained. 
Examples will be described. 
EXAMPLE 1 
Powder was manufactured to obtain composition of Bi.sub.1.8 Pb.sub.0.4 
Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.y by an oxalic acid-ethanol 
coprecipitating process. The ratio of the ethanol to the entire liquid at 
the time of coprecipitation was set to 4. Thus, component metal ions can 
be all coprecipitated in a range of pH 3 to 6. It is sucked and filtered, 
then dried at two stages of 100.degree. C. and 500.degree. C., heat 
treated at 845.degree. C. for 94 hours and pressed three times. 
It is confirmed that almost all is obtained in so-called up to 110K phase 
by an X-ray diffraction pattern of the powder obtained as described above. 
Then, the powder was cold hydrostatically pressed to mold a cylindrical 
vessel. The pressing pressure was 1.5 tons/cm.sup.2, the diameter of a 
core was 45 mm, the diameter of the opening of the molded piece was 46 mm, 
the length was 60 mm (bore/length = 0.77), and the thickness was 2 mm. The 
thickness of 2 mm was adjusted by cutting. 
This molded piece was baked at 845.degree. C. for 20 hours, and sintered. 
In the case of the Bi oxide superconductor, almost no shrinkage upon 
baking was confirmed. 
In order to check whether the manufactured vessel was cracked or not at the 
time of manufacturing, it was observed via an X-ray transmission 
photograph. As a result, it is confirmed that no crack occurred. The 
sintered vessel was coated with silicone, further wound with a Teflon 
tape, and the magnetic shielding effect was measured. The measuring device 
is shown in FIG. 3. 
The cylindrical superconducting magnetic shielding vessel 11 is laid down 
laterally, a detecting coil 13 is provided in a ceramic powder 12, and an 
exciting coil 14 and a reference coil 15 are disposed adjacently outside 
the detecting coil 13. A bath of liquid nitrogen 16 is filled outside the 
ceramic powder bath 12, and an FET amplifier 17 is disposed therein. In 
order to prevent the high temperature superconductor vessel from cracking 
upon cooling to cool the superconductor vessel to a liquid nitrogen 
temperature, the vessel is not directly dipped in the liquid nitrogen, but 
indirectly cooled through ceramic (alumina) powder. 
In the measurements, the ferromagnetic material magnetic shielding effect 
can be known in a state that the liquid nitrogen is not poured (in which 
state the superconductor vessel is not in a superconducting state), 
An alternating magnetic field was generated by an exciting coil 14, a 
signal detected by the detecting coil 13 was supplied to a lock-in 
amplifier (not shown) through the FET amplifier 17, and the exciting 
voltage was measured by using a signal from the reference coil 15. 
The result of the magnetic shielding effect measured by a measurement 
system in FIG. 3 is shown in FIG. 5. The magnetic shielding effect of 
1.times.10.sup.-5 or less was confirmed in a measured frequency range of 
10 to 1000 Hz. In V.sub.77K /V.sub.RT, V indicates an induction voltage 
excited in the detecting coil to be proportional to the intensity of the 
magnetic field. Since the vessel is not in a superconducting state at the 
ambient temperature (RT), this ratio can be reread as being the ratio of 
the presence of the superconducting vessel to the absence of the 
superconducting vessel. 
EXAMPLE 2 
Powder of a composition of Bi.sub.1.8 Pb.sub.0.4 Sr.sub.2 Ca.sub.2 Cu.sub.3 
O.sub.y was produced by an oxalic acid-ethanol coprecipitating process as 
described in Example 1. 
Then, the powder was cold hydrostatically pressed to mold a cylindrical 
vessel. The pressing pressure was 1.5 tons/cm.sup.2, the diameter of a 
core was 400 mm, the diameter of the opening of the molded piece was 400 
mm, the length was 350 mm (bore/length = 1.14), and the thickness was 10 
mm. 
This molded piece was baked at 845.degree. C. for 20 hours, and sintered. 
In the case of the Bi oxide superconductor, almost no shrinkage upon 
baking was confirmed. 
In order to check whether the manufactured vessel was cracked or not at the 
time of manufacturing, it was observed via an X-ray transmission 
photograph. As a result, it is confirmed that no crack occurred. 
Then, a Mumetal cylinder having 400 mm of diameter and 740 mm of length was 
manufactured. The thickness was set to 0.5 mm. These vessels are disposed 
as shown in FIG. 4. 
The cylinder was constructed the same as above FIG. 3 except that a 
superconducting magnetic shield 11a is disposed and a Mumetal cylinder 18 
are provided. Similarly to the Example 1, the exciting voltage was 
measured. As a result, the ratio of the voltage induced in the reference 
coil in a state that the liquid nitrogen is not poured to the voltage 
induced in the detecting coil was up to 10.sup.-9 in frequency of 20 H. 
This represents the shielding effect by the cylinder. A further large 
shielding effect can be expected by forming the Mumetal cylinder double. 
Then, liquid nitrogen was poured to cool it at the ratio of average 
20.degree. C./hr. When the superconductor vessel is set to a 
superconducting state, the above measurement was conducted. As a result, 
the shielding effect of 20 Hz of frequency was V.sub.77K /V.sub.RI of up 
to 1.times.10.sup.-5. 
The measurement was executed in an aluminum electromagnetic shielding room. 
EXAMPLE 3 
In the Example 2, the case that Helmholz coils are disposed instead of the 
Mumetal will be described. 
The coils are designed as below. 
With a coil constant C (=0.716/R)N, R: radius, N: number of turns), when R 
= 0.4 m and N = 10, a current value to be supplied by using equations of 
C=18Im.sup.-1 J, H =CiIA/mJ is: 
I=(0.3.times.10.sup.3 /4.pi.)/18=1.3 A 
and a current density to be supplied is up to 1 A/mm.sup.2. Therefore, the 
radius r of a copper wire to be employed is: 
##EQU1## 
Thus, a current of 1 A/mm.sup.2 was supplied to cancel the magnetic field 
of the earth of 0.3 gauss. 
In this case, the obtained magnetic shielding effect was up to 
1.times.10.sup.-5. The measurement was performed in an aluminum 
electromagnetic wave shielding room. 
According to the present invention, the influence of the magnetic field of 
the earth and magnetic noise is small with the magnetic shield of the 
bismuth high temperature oxide superconductor having the shape and 
dimensions described above, and extremely weak magnetoencephalographic 
waves can be preferably measured by the SQUID fluxmeter. 
Even if the ratio of the bore/length of the superconducting magnetic shield 
is set to 1 or more, when the ferromagnetic material magnetic shield or 
the Helmholz coil is disposed outside the superconducting magnetic shield, 
the influence of the magnetic field of the earth or the magnetic noise is 
reduced and a biomagnetism such as a magnetoencephalographic wave can be 
preferably measured. 
Even if the effective superconducting magnetic shield has 1 or less or more 
of the ratio of bore/length as described above, the superconducting 
magnetic shield can be manufactured advantageously in ordinary 
manufacturing process having the steps of pulverizing to powder, molding 
and sintering. 
The present invention is not only effective for the detection, measurements 
of the biomagnetism as described above, but also can be used to 
effectively protect a Josephson element, IC circuits against external 
magnetic noises. 
Further, the present invention is applicable not only in the magnetic field 
of an alternating current but also in that of a direct current.