Method and apparatus for optically measuring a current

A current is measured by making use of a rotation of polarization plane in a magnetic field, that is, a Faraday effect. A conductor under measurement is transversely inserted into a center opening of a Faraday effect glass which has at least two total reflection areas on a periphery thereof so that a light directed into the Faraday effect glass is circulated around the conductor and emitted externally. The light emitted from the Faraday effect glass is split into at least two light beams, which are converted to electric signals by photoelectric elements, and A.C. components contained in the electric signals are extracted and compared. The electric signals are corrected based on the comparison result. The present measuring apparatus and method attain high precision current measurement in a stable manner over an extended period.

The present invention relates to method and apparatus for optically 
measuring a current, and more particularly to an optical current measuring 
apparatus suitable to measure a current flowing in a high voltage 
conductor. 
A winding type current transformer has been used in current measurement for 
the measurement or protection of a commercial power system. The prior art 
winding type current transformer, however, has problems in insulation and 
structure when a transmitted voltage is very high such as 500 kV-700 kV, 
and the apparatus becomes large in size and expensive. 
Recently, a current measuring method based on a principle different from 
that of the prior art apparatus has been put into practice. In one 
approach, a current measuring method which uses an optical current 
measuring apparatus or a so-called optical current transformer which 
utilizes a physical phenomenon called a Faraday effect, that is, a 
rotation of a polarization plane in a magnetic field has been proposed. 
In principle, when a polarized light passes through a glass which exhibits 
the Faraday effect such as lead glass (hereinafter referred to as a 
Faraday effect glass) disposed in a magnetic field created by a current, a 
polarization plane is rotated by an angle .theta.=V.multidot.H.multidot.L 
(where V is a Verdet's constant, H is a magnetic field strength in a 
direction of light propagation and L is a length of the glass in the 
direction of the light propagation). This rotation is detected by a known 
method to measure the magnetic field strength H to measure a current 
flowing in the vicinity of the glass. (See U.S. Pat. Nos. 3,708,747; 
3,743,929 and 3,753,101.) 
However, when a current is to be measured in a conventional current 
measuring site where a plurality of conductors are arranged in the 
vicinity of the Faraday effect glass by the optical current transformer 
manufactured in accordance with such a principle, a distinction between 
the conductor under measurement and the other conductors is not attained 
in principle and the magnetic fields created by the conductors other than 
the conductor under measurement are sensed. As a result, a large error is 
obtained. 
In order to reduce the measurement error, it has been proposed by the 
Japanese Utility Model Publication No. 44-7589 to arrange polarization 
members such as the Faraday effect glass in a rectangular pattern around 
the conductors, arrange reflection mirrors between the polarization 
members, apply only a linearly polarized light ray extracted from light 
transmitted through a light conduction rod by a polarizer to the 
polarization members, apply lights transmitted through the polarization 
members to an analyzer to detect a Faraday rotation angle and extract a 
signal light from the light conduction rod. The magnetic fields created by 
the other conductors are cancelled out by summing the polarization 
rotation angles of the plurality of polarization members and only the 
polarization rotation angle by the magnetic field of the conductor under 
measurement is added. In this manner, the current of the conductor under 
measurement is distinguished from the currents of the other conductors. In 
the above construction, light conduction rods may be arranged in place of 
the reflecting mirrors, analyzers may be provided in input paths of the 
light conduction rods and polarizers may be provided in output paths. 
However, because of complexity of light paths, the above construction has 
the following disadvantages. It is difficult to maintain a relative 
position of the polarization member and the reflecting mirror or the light 
conduction rod at a high precision. The linearly polarized light is 
elliptically polarized by a phase change of the light by the reflection in 
the reflection mirror or the light conduction rod and a detectable 
polarization rotation angle is significantly reduced making the current 
detection difficult. Further, in order to distinguish the current of the 
conductor under measurement from the currents of the other conductors to 
precisely measure the current, it is necessary that the polarization 
members have the same sensitivity, but the sensitivities of the 
polarization members to the magnetic fields substantially differ from each 
other because the lights transmitted through the reflection mirrors or the 
light conduction rods are no longer linearly polarized. This makes the 
high precision measurement of the current of the conductor difficult. 
As another approach to measure a large current, the U.S. Pat. No. 3,746,983 
proposes to surround a current flowing conductor by a rectangular Faraday 
effect member having mirror surfaces at four corners and an inner surface 
having reflecting surfaces on both sides thereof at a portion along a 
linear side. In the proposed apparatus, a light beam from a light source 
is polarized by a polarizing filter and applied to one side of the inner 
surface of the Faraday effect member and reflected thereby at an angle of 
45 degrees and reflected by each of the four corners to circulate the 
member, and emitted outwardly from the other side of the inner surface. 
The emitted light beam further passes through another Faraday effect 
member having a coil wound thereon so that a polarization rotation angle 
can be detected. A current is supplied to the coil from a variable current 
source so that a magnetic field produced thereby rotates the polarization 
plane of the light beam passing through another Faraday effect member to 
return to a polarization plane of an incident light on the rectangular 
Faraday effect member surrounding the conductor. A current of the coil is 
metered under this condition to measure the current of the conductor which 
is proportional to the current of the coil. 
However, in the above apparatus, the total reflection of the light beam in 
the rectangular Faraday effect member surrounding the conductor has a 
phase difference between a component perpendicular to the reflection plane 
and a component parallel to the reflection plane. As a result, the linear 
polarization at the incident point is changed to the elliptic polarization 
as described above, making the detection of the current of the conductor 
quite difficult. 
A method for measuring a current of a conductor by winding an optical fiber 
directly on the conductor under measurement instead of using the Faraday 
effect glass has been proposed. (See U.S. Pat. Nos. 3,605,013 and 
3,810,013.) In this method, however, when a linearly polarized light 
passes through the optical fiber, the polarization angle is readily varied 
by a mechanical influence such as bend, distortion and vibration of the 
optical fiber and an elliptic polarization occurs irregularly. As a 
result, a high precision and highly stable holding mechanism for the 
optical fiber is required. This is not practical. 
As discussed hereinabove, an optical current measuring apparatus having a 
high precision and high stability has not been attained in the prior art. 
It is an object of the present invention to provide an optical current 
measuring apparatus which attains a light path circulation with a single 
polarization member and allows precise and stable current measurement. 
It is another object of the present invention to provide a magneto-optical 
current measuring apparatus and a measuring method which allow precise and 
stable current measurement with a small aging effect. 
It is a further object of the present invention to provide an optical 
current measuring apparatus which enables precise and wide range of 
current measurement. 
In accordance with the present invention, a Faraday effect glass has an 
opening at a center thereof so that a conductor under measurement passes 
through the opening to cross perpendicular to the Faraday effect glass, 
and the Faraday effect glass has at least two total reflection surfaces on 
a periphery thereof to cause a light to circulate a light path around the 
conductor under measurement so that the light circulated around the light 
path is made a polarization rotation by a Faraday effect so as to 
constitute the optical current measuring apparatus. In order to enhance a 
sensitivity, the reflected light at a changing point of the light path 
direction in the Faraday effect glass is totally internally reflected 
twice in order to prevent the distortion of the linear polarization (i.e. 
to prevent the linear polarization from converting to the elliptic 
polarization). 
In accordance with another aspect of the present invention, there are 
provided means for constantly monitoring and detecting a change of a 
proportional constant caused when the light transmitted through the 
Faraday effect glass is applied to an analyzer and an optical current 
output from two lines of light energy emitted from the analyzer is 
converted to an electric signal, and means for adjusting amplification 
factors of amplifiers in accordance with the detected change of the 
proportional constant so that a high precision current measurement is 
attained stably over an extended period. 
In accordance with a further aspect of the present invention, a plurality 
of Faraday effect glasses of different materials are arranged in parallel 
and one of the Faraday effect glasses is selected in accordance with a 
range of measurement to allow a wide range of measurement.

A first embodiment of the optical current measuring apparatus of the 
present invention is shown in FIG. 1. An input light path comprises a 
light source 10 for emitting light, an optical fiber 10A for transmitting 
the light from the light source 10, a condenser lens 11 for condensing the 
light emitted from the optical fiber 10A and preventing scattering of the 
light and a polarizer 12 made of a high molecule film, a vapor deposition 
film or a polarizing prism for linearly polarizing the light from the 
condenser lens 11. The light source 10 may be a light emitting diode, a 
laser diode or a laser of another type. The output light from the 
polarizer 12 is directed to a Faraday effect glass 13. FIG. 2 shows a 
front view of the Faraday effect glass 13. An incident light goes straight 
along a light path of the Faraday effect glass 13 as shown by a solid 
line, it is reflected by a first total reflection plane (having an angle 
of 45 degrees to the incident light), goes straight toward a second total 
reflection plane, it is reflected by the second total reflection plane and 
then reflected by a third total reflection plane. The reflected light then 
goes out of the Faraday effect glass 13 transversely to the incident 
light. A center area of the Faraday effect glass 13 is cut away to permit 
the insertion of a conductor 20 thereinto. 
The light circulates around the conductor 20 under measurement by the three 
total reflection planes provided in the Faraday effect glass 13. The 
linearly polarized light propagates through the Faraday effect glass while 
it inter-links with the conductor 20 under measurement (primary 
conductor). A polarization rotation angle .theta. by the Faraday effect 
after one circulation around the conductor 20 is given by 
EQU .theta.=V.intg.Hdl=VI 
where V is a Verdet's constant, H is a magnetic field strength around the 
conductor and I is a current under measurement. 
Thus, the polarization rotation is exactly proportional to the current 
under measurement. Accordingly, the current is measured more precisely 
than the prior art apparatus. 
The light emitted from the Faraday effect glass 13 is sent to an output 
light path. The output light is received by an analyzer 14 such as a 
polarization prism which splits the light into two light beams. The two 
light beams are condensed by condenser lenses 15 and 16. The linearly 
polarized light condensed by the condenser lens 15 is transmitted to a 
light detector 19 through an optical fiber 17, and the linearly polarized 
light condensed by the condenser lens 16 is transmitted to the light 
detector 19 through an optical fiber 18. The light detector 19 calculates 
a Faraday rotation angle from (Pa-Pb)/(Pa+Pb) based on the two linearly 
polarized lights Pa and Pb. 
A material of the Faraday effect glass may be flint glass such as lead 
glass or heavy flint glass, fused glass such as crown glass or silica 
glass, or porcelain glass. The material of the Faraday effect glass should 
be properly selected because the Verdet's constant and a detectable 
maximum current differ depending on a wavelength of the light source 10 as 
shown in Tables 1 and 2. 
TABLE 1 
______________________________________ 
Wavelength 830 nm (infrared light 
emitting diode or semiconductor laser) 
Verdet's constant 
Maximum detectable 
(min/A) current (kA) 
______________________________________ 
Fused silica 
-0.006 -400 
Crown glass 
-0.012 -200 
Flint glass 
-0.03 -90 
______________________________________ 
TABLE 2 
______________________________________ 
Wavelength 633 nm (He--Ne laser) 
Verdet's constant 
Maximum detectable 
(min/A) current (kA) 
______________________________________ 
Fused silica 
-0.012 -230 
Crown glass 
-0.02 -120 
Flint glass 
-0.05 -50 
______________________________________ 
Since the light path is formed by the Faraday effect glass, it is hardly 
subjected to a surrounding temperature and a mechanical vibration and a 
variation of the light path can be substantially neglected. As a result, a 
high precision measurement is stably attained over an extended period. 
In the illustrated embodiment, the number of times of the total reflection 
in the Faraday effect glass is three. Since the present invention can be 
realized by circulating the light path around the conductor, a Faraday 
effect glass having a shape as shown in FIG. 3 may be used to reduce the 
number of times of reflection. 
The total reflection planes in the Faraday effect glasses shown in FIGS. 2 
and 3 may be formed by optically polishing or plating silver or aluminum. 
The Faraday effect glasses shown in FIGS. 2 and 3 are simple in structure 
and easy to manufacture but the total internal reflection occurs only once 
for each deflection of the light path. As a result, the linearly polarized 
light is readily changed to the elliptically polarized light for each 
total internal reflection and hence the Faraday rotation angle may be 
reduced and the sensitivity may be lowered. An embodiment which resolves 
the above problem is described below. 
A configuration of a second embodiment of the optical current measuring 
apparatus of the present invention is shown in FIG. 4, in which the like 
elements to those shown in FIG. 1 are designated by the like numerals. In 
the present embodiment, the total internal reflection planes of the 
Faraday effect glass are improved such that the total internal reflection 
occurs twice at each light path in the Faraday effect glass to prevent the 
reduction of the sensitivity. 
An example of a shape of the Faraday effect glass used in the present 
embodiment is shown in FIGS. 5 to 9. The total internal reflection occurs 
six times, that is, at points b, c, d, e, f and g. It is desirable that 
axial distances between the points b and c, d and e and f and g are as 
small as possible in order to prevent the affect by the magnetic fields 
created by other conductors. The light from the polarizer 12 is applied to 
the point a, goes straight as shown by a solid line, totally reflected at 
the point b and deflected by 90 degrees, totally reflected at the point c 
and goes leftward perpendicular to the incident light. The light is 
totally reflected at the point d and axially deflected (and goes straight 
in the opposite direction to the light going to the point c), and it is 
totally reflected at the point e downward in the opposite direction to the 
incident light. The linearly polarized light is reflected at the point f 
frontward and then reflected rightward so that the light is emitted from 
the point h with an angle of 90 degrees to the incident light. The light 
circulates around the conductor 20 under measurement six times by total 
internal reflection. By total internal reflecting the light twice at a 
short interval at each light path deflection point, an ideal Faraday 
rotation can be attained. 
The rotation of the linearly polarized plane by the Faraday effect is 
proportional to a magnetic field strength parallel to the light path and a 
light path length. Since the currents flowing in the other conductors than 
the conductor 20 which are in the vicinity of the conductor 20 are nulled 
by the integration along the light path in the Faraday effect glass 30, an 
effect thereby can be neglected. Accordingly, the Faraday rotation angle 
.theta. by the linearly polarized light circulated around the conductor 20 
is essentially proportional to the current flowing in the conductor 20. 
Since the total internal reflection planes of the Faraday effect glass 30 
can be readily prepared even on a peripheral area by optically polishing, 
they can be formed without bonding. 
The present embodiment has an improved sensitivity over the embodiment of 
FIG. 1. 
The Faraday effect glass 30 shown in FIGS. 5 to 9 may be further improved 
to facilitate the manufacture. Examples thereof are shown in FIGS. 10 to 
13 and FIGS. 14 to 16. 
In a first modification shown in FIGS. 10 to 13, a corner of a Faraday 
effect glass 40 which forms a light input section and a light output 
section is cut away and a rectangular prism 41 is bonded to that area. As 
a result, the polishing of the right side is facilitated (because each of 
the optically polished surfaces is planar and no stepping is required) and 
the manufacture is facilitated. In the present modification, the light 
transmitted through the polarizer goes into the Faraday effect glass 40 
from one side of the rectangular prism 41 as shown in FIG. 10 and is 
totally internally reflected six times, that is, twice at each of three 
corners at a short interval as shown by a chain line, and goes out of the 
other side of the rectangular prism 41. 
In a second modification shown in FIGS. 14 to 16, rectangular prisms 51 and 
52 are bonded to a corner of a Faraday effect glass 50 which functions as 
a light input section a and a light output section b. With this structure, 
all of the optically polished surfaces are planar and the cutting of the 
corner is not necessary. Therefore, the manufacture is further 
facilitated. 
As described above, according to the optical current measuring apparatus of 
the present invention, the Faraday effect glass inter-links (traverses) 
the conductor under measurement and the portions which cause the Faraday 
rotation are in union and not affected by the currents flowing in the 
other conductors. Accordingly, a high precision current measurement is 
stably attained over an extended period. 
In the optical current transformer of the optical current measuring 
apparatus described above, when a directly polarized light passes through 
the Faraday effect glass, the polarization plane is rotated by an angle 
proportional to a product of a length of the glass and a magnetic field 
component along the direction of the light propagation. In order to detect 
the rotation angle of the polarization plane, a polarization prism such as 
a Wollaston prism or a polarization beam splitter is used as an analyzer. 
By properly selecting an angle relative to the polarizer which linearly 
polarizes the light, the two light beams emitted from the analyzer are 
split as lights of energies k(1+sin2.theta.) and k(1-sin2.theta.) where 
.theta. is the Faraday rotation angle and k is a constant representing a 
light transmission efficiency. 
In a normal condition, 2.theta.&lt;&lt;1 and the above values are approximated by 
k(1+2.theta.) and k(1-2.theta.), respectively. Light energies P.sub.1 and 
P.sub.2 which are proportional to those light energies are represented by 
EQU P.sub.1 =a.sub.1 k(1+2.theta.) and P.sub.2 =a.sub.2 k(1-2.theta.), where 
a.sub.1 and a.sub.2 are proportional constants. By transmitting the light 
energies P.sub.1 and P.sub.2 through optical fibers and converting them to 
electric signals by photo-diodes, photo-current outputs represented by 
EQU I.sub.1 =a.sub.1 b.sub.1 k(1+2.theta.) and I.sub.2 =a.sub.2 b.sub.2 
k(1-2.theta.) 
where b.sub.1 and b.sub.2 are light constants, are obtained. 
Theoretically, if the characteristics of the optical fibers and the 
photo-diodes are identical, 
EQU a.sub.1 =a.sub.2 and b.sub.1 =b.sub.2 and the Faraday rotation angle is 
proportional to (I.sub.1 -I.sub.2)/(I.sub.1 +I.sub.2), but normally 
a.sub.1 .noteq.a.sub.2 and b.sub.1 .noteq.b.sub.2 and hence they must be 
compensated. Output voltages V.sub.1 and V.sub.2 of the amplifiers are 
given by 
EQU V.sub.1 =a.sub.1 b.sub.1 C.sub.1 k(1+2.theta.) and V.sub.2 =a.sub.2 b.sub.2 
C.sub.2 k(1-2.theta.) 
Thus, by adjusting the amplification factors C.sub.1 and C.sub.2 of the 
amplifiers, a.sub.1 b.sub.1 C.sub.1 is made to be equal to a.sub.2 b.sub.2 
C.sub.2 so that the Faraday rotation angle is proportional to (V.sub.1 
-V.sub.2)/(V.sub.1 +V.sub.2). Since this method uses a ratio of the 
outputs of the two light beams, it is not affected by a variation of the 
transmitted light. 
However, if a.sub.1 b.sub.1 and a.sub.2 b.sub.2 deviate from the values 
adjusted to meet the relation of a.sub.1 b.sub.1 C.sub.1 =a.sub.2 b.sub.2 
C.sub.2 by a temperature characteristic of the optical parts such as the 
optical fibers or the condenser lens or the aging effect, a.sub.1 b.sub.1 
C.sub.1 is not equal to a.sub.2 b.sub.2 C.sub.2 and a detection error for 
the Faraday rotation angle increases. 
In the optical current measuring apparatus of the present invention, an 
A.C. (Alternating Current) component of the current is utilized to 
constantly monitor and detected the variations of a.sub.1 b.sub.1 and 
a.sub.2 b.sub.2. When the current under measurement is I.sub.DC +I.sub.AC, 
the Faraday rotation angle .theta. is represented by Ve(I.sub.DC 
+I.sub.AC) and hence 
EQU V.sub.1 =a.sub.1 b.sub.1 C.sub.1 k(1+2VeI.sub.DC +2VeI.sub.AC) 
EQU V.sub.2 =a.sub.2 b.sub.2 C.sub.2 k(1-2VeI.sub.DC -2VeI.sub.AC) 
where Ve is a Verdet's constant. The above equations can be rewritten as 
EQU V.sub.1 =a.sub.1 b.sub.1 C.sub.1 k(1+2VeI.sub.DC)+a.sub.1 b.sub.1 C.sub.1 
k2VeI.sub.AC 
EQU V.sub.2 =a.sub.2 b.sub.2 C.sub.2 k(1-2VeI.sub.DC)-a.sub.2 b.sub.2 C.sub.2 
k2VeI.sub.AC 
Noting the second terms of the right sides of the equations for V.sub.1 and 
V.sub.2, the A.C. component of V.sub.1 is equal to the A.C. component of 
V.sub.2 after the adjustment has been made to meet the relation of a.sub.1 
b.sub.1 C.sub.1 =a.sub.2 b.sub.2 C.sub.2. Thus, if a.sub.1 b.sub.1 or 
a.sub.2 b.sub.2 changes by the deterioration of the optical fibers or the 
deterioration of the photo-diodes, the A.C. component of the electric 
signal V.sub.1 and the A.C. component of the electric signal V.sub.2 are 
detected and C.sub.1 and C.sub.2 are adjusted such that the A.C. component 
of V.sub.1 is equal to the A.C. component of V.sub.2. In this manner, the 
error due to the deterioration of the optical fibers and the photo-diodes, 
the temperature characteristic and the aging effect can be corrected. 
Since a D.C. (Direct Current) current used in a commercial power system 
necessarily includes a ripple current component I.sub.AC, the present 
invention can be applied to such a system. 
A further embodiment of the optical current measuring apparatus and method 
of the present invention is explained with reference to FIG. 17. The 
conductor 20 through which the current under measurement flows extends 
through a Faraday effect glass 13 like in the previous embodiments. The 
light from the light source 10 is transmitted through the optical fiber 
10A, condensed by the condenser lens 11 and the condensed light is 
directed into the Faraday effect glass 30 through the polarizer 12. The 
light emitted from the Faraday effect glass 30 goes into the analyzer 14 
where it is split into two light beams which are directed to the condenser 
lenses 15 and 16, respectively. The lights condensed by the condenser 
lenses 15 and 16 pass through the optical fibers 17 and 18, respectively 
and are directed to photo-diodes 60 and 61, respectively, which function 
as light detectors. The outputs of the photo-diodes 60 and 61 are supplied 
to current-to-voltage conversion amplifiers 62 and 63, respectively. An 
output of the first current-to-voltage conversion amplifier 63 is supplied 
to a band-pass filter 64, a subtractor 65 and an adder 66, and an output 
of the second current-to-voltage conversion amplifier 62 is supplied to a 
band-pass filter 67 and a multiplier 68. An output of the band-pass filter 
64 is supplied to a divider 71 through a detector 69 and an integrator 70, 
and an output of the band-pass filter 67 is supplied to the other terminal 
of the divider 71 through a detector 72 and an integrator 73. An output of 
the divider 71 is supplied to the other terminal of the multiplier 68 and 
an output of the multiplier 68 is supplied to the subtractor 65 and the 
adder 66, and an output of the adder 66 is supplied to a divider 74. An 
output of the subtractor 65 is also applied to the divider 74. 
The operation of the present embodiment is now described. In order to 
detect the Faraday rotation angle .theta. when the light from the light 
source 10 goes into the Faraday effect glass 30 through the optical fiber 
10A, the condenser lens 11 and the polarizer 12 and goes out after one 
circulation, the emitted light is split into the two light beams by the 
analyzer 14 and these two light beams are condensed by the condenser 
lenses 15 and 16 and efficiently directed into the optical fibers 17 and 
18, and the lights transmitted therethrough are converted to the 
photocurrent by the photo-diodes 60 and 61 to produce the electric 
signals. The current outputs from the photo-diodes 62 and 63 are converted 
to the voltage signals by the current-to-voltage conversion amplifiers 62 
and 63. By adjusting the amplification factors C.sub.1 and C.sub.2 of the 
current-to-voltage conversion amplifiers 62 and 63, the condition of 
a.sub.1 b.sub.1 C.sub.1 =a.sub.2 b.sub.2 C.sub.2 described above can be 
met. The A.C. components of the output voltage signals V.sub.1 and V.sub.2 
of the current-to-voltage amplifiers 62 and 63 are extracted by the 
band-pass filters 64 and 67, and they are converted to D.C. signals by the 
detectors 69 and 72 and the D.C. signals are integrated by the gated 
integrators 70 and 73. Through the integration, the affect by the noises 
included in the D.C. signal can be reduced. 
If the output of the integrator 70 becomes lower than the output of the 
integrator 73 by the temperature characteristics or the aging effects of 
the optical fibers 17 and 18 and the photo-diodes 60 and 61, a ratio of 
those outputs is calculated by the divider 68 and it is multiplied to the 
output signals of the current-to-voltage amplifiers 62 and 63 by the 
multiplier 68 to correct the error. The adder 66, the subtractor 65 and 
the divider 74 are well-known arithmetic circuits to detect the Faraday 
rotation angle based on the fact that the Faraday rotation angle .theta. 
is proportional to (V.sub.1 -V.sub.2)/(V.sub.1 +V.sub.2). The ratio of the 
outputs of the integrator 70 and the integrator 73 which is initially set 
to approximately unity may exceed unity due to dew condensation on one of 
the optical fibers or breakage of one of the optical fibers. This abnormal 
condition can also be monitored by the illustrated circuit to diagnose the 
failure when the operation condition deviates from a normal range. 
In accordance with the present embodiment, the light emitted from the 
Faraday effect glass is photo-electrically converted by the photo-diodes 
60 and 61 and the photo-currents are supplied to the arithmetic circuit 
through the current-to-voltage conversion amplifiers 62 and 63 having the 
amplification factors C.sub.1 and C.sub.2, respectively, and the 
arithmetic circuit extracts the A.C. components of the outputs and 
corrects the measurement error due to environment condition variation such 
as the temperature characteristics and the aging effects of the components 
by the divider 71 and the multiplier 68 based on the change of the ratio 
of the A.C. components so that a high precision current measurement is 
stably attained over an extended period. By monitoring the ratio of the 
A.C. components for the two light paths, the failure in which the 
operation condition deviates from the normal range can be diagnosed. 
Further, since the present embodiment uses the analog arithmetic circuit, 
the calibration speed for the error is fast. 
Referring to FIG. 18, another embodiment of the optical current measuring 
apparatus of the present invention is explained. The light emitted from 
the light source 10 goes into the Faraday effect glass 30 through the same 
path as that of FIG. 17, and the light emitted therefrom goes into the 
photo-diodes 60 and 61 through the same path. The output current signals 
from the photo-diodes 60 and 61 are supplied to the current-to-voltage 
conversion amplifiers 62 and 63, and the outputs therefrom are supplied to 
analog-to-digital (A/D) converters 75 and 76, and the outputs thereof are 
supplied to a digital signal processor 77 such as a microprocessor. 
The analog voltage signals from the current-to-voltage conversion 
amplifiers 62 and 63 are converted to digital signals V.sub.1 and V.sub.2 
by the A/D converters 75 and 76, and A.C. components V.sub.1AC and 
V.sub.2AC in the digital signals V.sub.1 and V.sub.2 are extracted by the 
digital signal processor 77, which carries out the following arithmetic 
operations: 
##EQU1## 
The signal V is used as a digital output of the optical current measuring 
apparatus. 
In accordance with the present embodiment, since the microprocessor is used 
as the digital signal processor 77 the construction of the arithmetic unit 
is simpler than that of FIG. 17 and an error in the arithmetic operation 
is small because of the digital processing. Accordingly, the measuring 
apparatus of higher precision can be provided. 
It should be understood that the present invention, like the embodiment of 
FIG. 17, can measure the current stably over an extended period without 
increase of precision error due to the temperature characteristics and the 
aging effects of the components. 
As discussed hereinabove, the optical current measuring apparatus which 
uses the Faraday effect glass has an advantage that it allows the 
measurement of the current flowing through a particular conductor without 
being affected by the currents flowing in the other conductors. However, 
there are many kinds of material to be used, and when a different material 
is used, the Faraday rotation angle differs. On the other hand, if a 
Faraday effect glass of a single kind of material is used in the optical 
current measuring apparatus or the optical current transformer, a range of 
measurement is limited. For example, the Faraday rotation angle of a lead 
glass is 0.12 min./AT for wavelength of 633 nm, and the Faraday rotation 
angle of a silica glass of the same shape and same dimension is 0.012 
min./AT, which is one tenth (1/10) of that of the lead glass. Thus, the 
lead glass having a larger rotation angle is suitable for the measurement 
under a low magnetic field but the rotation angle saturates in a high 
magnetic field and correct measurement is not attained. On the other hand, 
the silica glass having a smaller rotation angle is suitable for the 
measurement under a high magnetic field but a sensitivity under a low 
magnetic field is too low to assure precise measurement. 
In order to utilize the different characteristics of different materials of 
the Faraday effect glass, an embodiment of the optical current measuring 
apparatus of the present invention shown in FIG. 19 uses parallelly 
arranged first Faraday effect glass 80 made of lead glass having an 
internal light path which links to a conductor 20 and second Faraday 
effect glass 90 made of silica glass having an internal light path which 
links to the conductor 20. 
A linearly polarized light is applied to the first Faraday effect glass 80 
from a light source 81 through an optical fiber 82, a condenser lens 83 
and a polarizer 84. The incident linearly polarized light is totally 
internally reflected twice at each corner of the first planar Faraday 
effect glass 80, circulates around the conductor 20, is emitted from a 
light output plane, is split by an analyzer 85 such as a Rochon prism, a 
Wollaston prism or a polarization beam splitter, and the split light beams 
are directed to light detectors 88A and 88B such as photo-diodes through 
condenser lenses 86A and 86B and optical fibers 87A and 87B and they are 
converted to electric signals. The electric signals are then converted to 
a rotation angle signal Va by an arithmetic circuit 89, an output of which 
is supplied to an OR gate 100. 
Similarly, a linearly polarized light is applied to the second Faraday 
effect glass 90 from a light source 91 through an optical fiber 92, a 
condenser lens 93 and an analyzer 94. The incident linearly polarized 
light is totally internally reflected twice at each corner of the second 
Faraday effect glass 90, circulates around the conductor 20, is emitted 
from a light output plane and is split by an analyzer 95, and the split 
light beams are directed to light detectors 98A and 98B through condenser 
lenses 96A and 96B and optical fibers 97A and 97B and they are converted 
to electric signals. The electric signals are similarly converted to a 
rotation angle signal V.sub.b by an arithmetic circuit 99, an output of 
which is supplied to the OR gate 100. 
A maximum value V of the rotation angle signals V.sub.a and V.sub.b is 
produced by the OR gate 100. The maximum value signal V from the OR gate 
100 or the measuring apparatus changes as shown by a solid line curve in 
FIG. 20. When the first Faraday effect glass 80 of the lead glass having a 
larger Faraday rotation angle per ampere-turn and a higher sensitivity is 
used, a characteristic curve V.sub.a of FIG. 20 which shows a maximum 
output when a magnetic field created by the current in the conductor 20 is 
B is obtained. On the other hand, when the second Faraday effect glass 90 
of the silica glass having a smaller Faraday rotation angle per 
ampere-turn and a low sensitivity is used, a characteristic curve V.sub.b 
of FIG. 20 which shows a maximum output when a magnetic field is 10B. 
Thus, the output V of the measuring apparatus derived from the OR gate 100 
shows a characteristic as shown by the solid line, which allows the 
current measurement in a range from a low magnetic field to a high 
magnetic field. 
As described hereinabove, the optical current measuring apparatus of the 
present invention has an advantage that the current can be measured at a 
high precision over an expanded current range without being affected by 
the currents in the other conductors. 
In order to enhance the detection sensitivity of the measuring apparatus, 
it is necessary to properly select a material of the glass used. 
Alternatively, a shorter wavelength light may be used to attain the same 
effect. However, when the shorter wavelength light is used, the light 
source is expensive and the transmission loss in the optical fiber 
increases. Accordingly, those approaches may be used in appropriate 
combination.