Abstract:
A lightning detector featuring improved reliability over the conventional radar-type or coherer-type lightning detector that measures the intensity of static electricity or the intensity of electromagnetic waves, includes a coherer with an automatically restoring decoherer. The coherer is equipped with a separate coherer for a lightning circuit. The lightning detector detects and operates the (static) position and/or approaching/separating condition (dynamic) data of the thundercloud while protecting its own circuit.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The Present invention relates to a lightning detector and, more specifically, to a lightning detector by utilizing the characteristics of a coherer equipped with a decoherer 
     2. Description of the Related Art 
     There have heretofore been put into practice or proposed a variety of lightning detectors which can roughly be classified into (1) those without using a coherer and (2) those using a coherer. 
     Some devices pertaining to (1) uses {circle around (1)} a lightning wire of, for example, a high-tension transmission line. In this case, however, the detector line (lightning wire) inevitably becomes lengthy for maintaining a precision for detecting the potential relative to ground, and the detector line itself is subject to be destroyed by a thunder-bolt. 
     There has further been proposed a method or a device {circle around (2)} which confirms the presence of a thundercloud based upon the data from a so-called meteorological radar (cloud radar) to estimate the degree of danger accompanied thereby. However, the device becomes expensive though the reliability is not so high. Concerning the method or the device {circle around (2)}, it has been attempted to render an overall judgement by taking into consideration the data obtained from artificial satellites and telemeter-measured values of precipitation without, however, satisfactory results. 
     Further, {circle around (3)} a device, SKYSCAN (™), taught in U.S. Pat. No. 5,541,501 has been placed in the market in the U.S.A. and in other countries. This is a lightning detector which displays the distance from the lightning position by comparing and analyzing the frequency and intensity of electromagnetic waves due to lightning. 
     As the manual of the product states “BE AWARE THAT STORMS CAN FORM DIRECTLY OVER YOUR LOCATION, OFFERING LITTLE OR NO ADVANCE WARNING EVEN WHEN USING A SKYSCAN”, however, this device is not capable of detecting lightning just overhead, and it Lust be said that its reliability is very low. 
     The method or the device (2) was developed based on the re-discovery that the coherer sharply reacts to a change in the intensity of the electromagnetic field inclusive of the static electric field, and there have been proposed many variations as taught in, for example, {circle around (4)} Japanese Unexamined Patent Publication (Kokai) No. 50154/1996, {circle around (4)} Japanese Unexamined Patent Publication (Kokai) No. 180911/1997 and {circle around (5)} U.S. Pat. No. 5,399,962. However, the device of the publication {circle around (4)} is often affected by alternating electric field noise from, for example, a high-tension transmission line, by the modulated carrier waves of broadcast or communication and, particularly, by PWM (pulse width modulated) intermittent wave noises, and, hence, malfunctions, accounting for a major cause of inhibiting the widespread use as the lightning detector with coherer. 
     The device of the publication {circle around (5)} is a highly sensitive coherer having a low self-operation voltage (threshold voltage), which is very effective in detecting a very weak spark discharge. This device, however, is rather cumbersome to use for the spontaneous aerial discharge of large electric power such as of thunder discharge (lightning). This device has not been proposed for detecting lightning and, besides, involves a problem in practice. 
     The device of the publication {circle around (6)} is a surge protector for a high-tension transmission line and uses a coherer. This device detects damped wave disturbances but is not designed for detecting lightning. The damped wave disturbances are caused by spark discharge due to a thunder-bolt or a short-circuit (abnormal discharge inclusive of arc discharge or corona discharge caused by salt damage). The device of the publication {circle around (6)} has been designed for detecting abnormal conditions on the high-tension lines. 
     That is, the coherer has wide frequency characteristics and exhibits a considerable degree of sensitivity even for artificial spark discharge noise. The frequency spectrum of artificial spark discharge noise exists chiefly in a high-frequency region of about 1 Mhz or higher. On the other hand, the electromagnetic waves of lightning discharge have low frequencies in the regions of from VLF to MF. According to the prior art, the frequencies could not be distinguished. Since high-frequency noises were easily picked up, the sensitivity to the electromagnetic waves of lightning became relatively low. 
     With the conventional techniques {circle around (1)} to {circle around (3)} and {circle around (4)} to {circle around (6)}, it is difficult to reliably detect the intensity of the electromagnetic waves of not smaller than a predetermined value caused by thunder-bolt. 
     There has not yet been known a lightning detector equipped with a lightning circuit for protecting itself and with a coherer, capable of detecting thunder that is approaching or is separating way. 
     SUMMARY OF THE INVENTION 
     The present inventors, therefore, have accomplished a lightning detector capable of more correctly detecting lightning by incorporating a lightning circuit having a low-pass filter (hereinafter often abbreviated as LPF or is often called “filter transformer”) and/or an aerial discharge gap between the antenna and the coherer, to offer the function of detecting thunder that is approaching or is separating away. 
     It is therefore an object of the present invention to provide a lightning detector which is capable of reliably predicting lightning free from the danger of being struck by lightning by using a relatively simply constructed coherer with a decoherer. 
     Another object of the present invention is to alert the degree of danger of lightning by using an indicator by detecting and storing lightning data and thunder data of a plurality of thunder-bolts at a distance, and by comparing at least two pairs of these data. 
     A further object of the present invention is to produce an indication/alarm by statically detecting the fact that the lightning detector is approaching the lightning range and/or by dynamically detecting whether the thunder-bolt is approaching/moving away. 
     The invention provides a lightning detector comprising an antenna, a coherer with a decoherer, a low-pass filter installed between the antenna and the coherer, an indicator and a power source, the lightning detector detecting the intensity of impulse electromagnetic waves of not smaller than a predetermined value produced by a thunder-bolt outside the lightning range, and producing an indication and/or an alarm on an indicator to tell that the detector is approaching the lightning range. 
     The invention provides a lightning detector as met forth above  1 , wherein the coherer comprises one filled with metal particles or metal particles coated with an oxide film between a pair of electrodes in an insulating tube that is sealed, and the decoherer comprises one that gives mechanical vibration to the coherer from the external side. 
     The invention provides a lightning detector as set forth above, wherein the coherer is equipped with a lightning circuit capable of adjusting the aerial discharge gap. 
     The invention provides a lightning detector as set forth above, wherein the lightning circuit is equipped with an exclusive coherer (coherer No.  2 ) for the lightning circuit, separate from the coherer (coherer No.  1 ) for the alarm. 
     The invention further provides a lightning detector as set forth above, wherein a plurality of thunderbolts stronger than a predetermined level are measured and recorded as electromagnetic wave intensity difference ΔEn and sound pressure difference ΔPn, the moments of inputs of these signals are stored in a storage medium in time series, a red lamp is turned on when a gradient of time difference ΔTn of ΔEn and ΔPn of when a pair of electric signal and sound signal are input, is (−) over a time of at least m pairs (m is a positive integer), a yellow lamp is turned on when the gradient of time difference ΔTn is close to (0) and a green lamp is turned on when the gradient of time difference ΔTn is (+), to indicate the danger of a thunder-bolt. 
     The invention will now be described in detail. 
     Lightning emits electromagnetic waves of countless frequencies which are distributed in a relatively low range (VLF to MF bands). 
     The field intensities of the electromagnetic waves of these frequencies can be detected in a synthesized form. Since the intensities of electromagnetic fields of various frequencies sharply change with the passage of time, the intensity of the electromagnetic field that is detected usually changes sharply. 
     It has been known that in a typical lightning discharge, the intensity of the electromagnetic field changes as much as 120 V/m at the greatest 100 μS as measured at a point 20 Km away from the point where the discharge is taking place (as calculated from a maximum waveform of main discharge, H. Norinder, Handbook of Wireless Engineering, 10, Chapter 6, 10-69, FIG.  10-94 : Lightning Discharge waveform, Ohm Co., May 25, 1964). 
     Such a sharp and large change in the intensity of the electromagnetic field is unthinkable in the artificial electromagnetic waves that are used in the daily broadcasts and communications. In fact, however, the coherer reacts to a sharp and large change in the electric field intensity. In fact, a coherer that was fabricated for testing favorably reacted to a change in the field intensity of 60 V/m in 100 μS (in an electric field due to artificial spark discharge). 
     The coherer, in contrast, is very insensitive and does not react to electromagnetic waves of a predetermined field intensity that is accompanied by a mild change in the electromagnetic field intensity. It was confirmed that the coherer fabricated for testing did not react even under a predetermined electromagnetic field of 10 V/m (such an electromagnetic field intensity is very larger than those that are usually used). 
     The present inventors have re-discovered such characteristic properties of the coherer, and have applied such properties to the lightning detector and have further added some contrivances. 
     In the coherer fabricated for testing, a mixture powder of Ni and Ag (95% by weight of Ni, 5% by weight of Ag, the particles having an average diameter of about 100 to 1500 μm) was sandwiched by Ag electrodes, and was sealed in a glass tube together with the dry air. 
     The coherer comprises particles of a single metal such as Ni, Co, Fe, Mn, Zn, Cu, Ag, Au, Pd, Al or Pt or mixture particles thereof that are filled in a fluidizing manner in an insulating tube such as of ebonite, glass or plastic, and a pair of electrodes are provided at both ends of the insulating tube. Usually, the surfaces of these metal particles are covered with an oxide film having a low electrically conducting property. Therefore, the contact resistance is so large that no current flows across both terminals. 
     In the case of a stable metal such as Au or Pt, the effect of the oxide film is small and a generally good electrically conducting property is exhibited. Therefore, the initial insulating property is maintained due to gaps among the particles and the lowly electrically conducting substance such as silicone oil that is added. When an impulse voltage due to lightning is applied across both terminals, the metal particles in the coherer are destroyed for their insulation and are cohered to conduct the electricity. 
     The decoherer is a device for recovering the insulating property of the coherer that is cohered. In its most primitive form, the glass tube is hit by hand using a wood hammer or a plastic hammer. This, however, may be automatically carried out by mechanically vibrating the glass tube relying upon ultrasonic waves (20 KHz or higher) generated by a piezo-electric element or an electromagnet after every predetermined period of time. 
     The dynamic detector means accomplishes dynamic detection only independently of the static detection, and could become an effective lightning detector means for indicating danger (see embodiment 4). That is, there exists an invention in the dynamic detector means itself. 
     Concerning other respects, the invention will be described in further detail with reference to the embodiments below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram of a first embodiment of the present invention; 
     FIG. 2 is a block diagram of the circuit of FIG. 1; 
     FIG. 3 is a circuit diagram of a second embodiment of the present invention; 
     FIG. 4 is a block diagram of the circuit of FIG. 3; 
     FIG. 5 is a front view illustrating a coherer with a decoherer having a discharge gap; 
     FIG. 6 is a side view illustrating a coherer with a decoherer according to embodiments  3  and  4 ; 
     FIG. 7 is a front view illustrating a coherer with a decoherer; 
     FIG. 8 is a circuit diagram of athird embodiment of the present invention; 
     FIG. 9 is a block diagram of the circuit of Fig. 8; 
     FIG. 10 is a flowchart illustrating the operation of the invention; 
     FIG. 11 is a circuit diagram of a fourth embodiment according to the present invention; 
     FIG. 12 is a block diagram of the circuit of FIG. 11; 
     FIG. 13 is a flowchart illustrating the operation; 
     FIG. 14 is a view schematically illustrating how to judge that the thundercloud is approaching or is separating away; 
     FIG. 15 is a circuit diagram of a fifth embodiment of the present invention; 
     FIG. 16 is a block diagram of the circuit of FIG. 15; and 
     FIG. 17 is a flowchart illustrating the operation. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiment 1 
     Described below is an embodiment 1 of the present invention of claim 2. FIG. 1 is a circuit diagram of the present invention and FIG. 2 is a block diagram thereof, wherein reference numeral  12 A denotes an antenna No.  1  for trapping electromagnetic waves generated by a thunder-bolt,  13 A denotes earth,  1 A denotes a coherer No.  1  for detecting electromagnetic waves due to lightning,  15  denotes a filter transformer (a kind of low-pass filter, the same holds hereinafter) which removes ranges other than the frequency range of the electromagnetic waves due to lightning and insulates an alarm circuit and the antenna  12 A No.  1  from each other,  104 C denotes a transistor which operates according to a change in the voltage applied to the coherer  1 A No.  1 , reference numeral  104 A denotes a variable resistor for adjusting a bias voltage applied to the transistor  104 C, reference numeral  11 A denotes a decoherer No.  1  which vibrates the coherer No.  1  to decohere it,  17  denotes an indicator that indicates an alarm, and  16 A denotes a timer which transmits the detection of lightning to the indicator  17  for a predetermined period of time. 
     The electromagnetic waves of lightning trapped by the antenna No.  1   12 A are transmitted to the coherer No.  1  through the filter transformer  15 . The filter transformer absorbs electromagnetic waves of frequencies higher than about 1 MHz. Namely, the frequency components of the electromagnetic waves are selected. The filter transformer also plays the role of protecting the coherer from induced lightning which is a weak thunder-bolt. Thus, the coherer No.  1   1 A is shifted to a cohered electrically conducting state. Next, the transistor  104 C of which the bias voltage has been adjusted in advance by the variable resistor  104 A, is for protecting the coherer. When the coherer No.  1  is cohered, the transistor  104 C is turned off to interrupt the electric current. 
     The interruption of current by the transistor  104 C is transmitted to the timer circuit  16 A and to the decoherer No.  1   11 A through a photo coupler to actuate them. The photo coupler is for electrically insulating and protecting the circuits of the subsequent stages. The photo coupler also removes electric noise that causes the circuits in the subsequent stages to malfunction. The coherer No.  1   11 A is decohered by the decoherer No.  1   11 A, restores its electrically insulated state and becomes ready to receive electromagnetic waves of subsequent lightning. 
     Effect of the embodiment 1. 
     This embodiment has proved the validity of the lightning detector that utilizes characteristics specific to the coherer that is insensitive to noise, such as electromagnetic waves (carrier waves) of signals, but that very sharply reacts to impulse disturbed electromagnetic waves such as those of lightning. 
     Embodiment 2 
     Described below is an embodiment 2 of the present invention. 
     FIG. 3 is a circuit diagram of the embodiment 2 of the invention and FIG. 4 is a block diagram, wherein reference numeral  12 B denotes an antenna No.  2  for trapping static discharge and corona discharge just before the thunder-bolt,  13 B is earth,  14  is a discharge gap,  18  is a coherer No.  2  for detecting the electric discharge taking place at the discharge gap  14 , reference numeral  104 D is a transistor that operates according to a change in the voltage applied to the coherer No.  2   1 B, reference numeral  104 B is a variable resistor for adjusting the bias voltage applied to the transistor  104 D, reference numeral  1 B denotes a decoherer for giving vibration to the coherer No.  2  to decohere it, and reference numeral  16 B is a timer for transmitting the detection of lightning to the indicator for a predetermined period of time. 
     Near the antenna No.  2   12 B, a static discharge or corona discharge occurs on the antenna No.  2   12 B due to a rapid change in the static electric field between the atmosphere and the ground just before a thunder-bolt. Therefore, a small spark discharge occurs at the discharge gap  14 . The coherer No.  2   1 B is cohered due to the electromagnetic waves produced by the spark discharge. Then, the transistor  104 D of which the bias voltage is adjusted by the variable resistor  104 B is turned off, to actuate the timer circuit  16 B and the decoherer  11 B. The timer circuit  16 B actuates the indicator for a predetermined period of time. The coherer No.  2   1 B is decohered by the decoherer  11 B to be ready for the next occurrence of electric discharge. 
     Effect of the embodiment 2. 
     The device according to the second embodiment traps a sharp change in the static electric field between space and ground just before the thunder-bolt to highly reliably detect (forecast) the lightning. 
     Embodiment 3 
     Described below is an embodiment 3 of the invention. 
     FIG. 8 is a circuit diagram of the embodiment 3 of the invention, FIG. 9 is a block diagram thereof, and FIG. 10 is a flowchart of the operation. In addition to those elements of the above-mentioned embodiments 1 and 2, there are provided an antenna change-over unit No.  1   102 A and an antenna change-over unit No.  2   102 B for shutting off the antenna No.  1   12 A and the antenna No.  2   12 B from the alarm circuit in order to protect the alarm unit from lightning at a close distance. There are further provided a non-directive microphone  109  for detecting thunder, an amplifier  110  for amplifying the signal of the microphone  109 , a low-pass filter  111  for improving the selectivity of thunder of a low frequency, a diode  112  for converting AC thunder signals into DC voltages, a close distance judging circuit  105 R for judging the thunder-bolt at a close distance based on the timer operated by the turn-off of the transistor  104 C and the thunder signal from the diode  112 , an intermediate distance judging circuit  105 Y for judging the thunder-bolt at an intermediate distance based on the timer that starts operating upon the end of operation of the close distance judging circuit  105 R and the thunder signal from the diode  112 , a remote distance judging circuit  105 G for judging the thunder-bolt at a remote distance based on the timer that starts operating upon the end of operation of the intermediate distance judging circuit  105 Y and the thunder signal from the diode  112 , red, yellow and green alarm lamps  106 R,  106 Y and  106 G for indicating degrees (static) of danger of a thunder-bolt, a decoherer timer circuit  107  for operating the decoherer A No.  1  and the decoherer B No.  2  by turning the transistor  104 C off and turning the transistor  104 D off, a power distribution line  115 , a power interrupt relay  114 , a battery-supported failure-proof power source device  113 A for the coherer power source unit and a battery-supported failure-proof power source device  113 B for the alarm unit, and an interrupt timer circuit  108  which operates upon the turn off of the transistor  104 D and actuates the antenna change-over unit No.  1   102 A, antenna change-over unit No.  2   102 B and power source interrupt relay  114 . 
     Alarming Operation. 
     The electromagnetic waves of lightning received by the antenna No.  1   12 A are transmitted through the antenna change-over unit No.  1   102 A to the filter transformer  15  where high-frequency components are attenuated, and are further transmitted to the coherer No.  1   1 A. The coherer No.  1   1 A is cohered by the input of electromagnetic waves of lightening. The transistor  104 C of which the bias voltage is adjusted through the variable resistor  104 A is turned off by the coherer No.  1   1 A that is cohered to become conductive, whereby the close distance judging circuit  105 R and the decoherer timer circuit  107  start operating. 
     The close distance judging circuit  105 R turns the close distance alarm lamp  106 R on when a thunder signal is received from the diode  112  within a timer period. After the timer period has passed, the next intermediate distance judging circuit  105 Y starts operating and turns the intermediate distance alarm lamp  106 Y on when a thunder signal is received from the diode  112  within the timer period. The remote distance judging circuit  105 G, too, operates in the same manner for the remote distance alarm lamp  106 G. 
     The decoherer timer circuit  107  actuates the decoherer No.  1   11 A and the decoherer No.  2   11 B, and adds mechanical vibration to the coherer No.  1   1 A and to the coherer No.  2   1 B, so that the coherer returns from the cohered electrically conducting state to the insulation-recovered state to be ready for receiving the electromagnetic waves of subsequent lightning. 
     Lightning Operation. 
     When the electrostatic discharge or corona discharge takes place on the antenna No.  2   12 B just before the lightning, the discharge current flows through the antenna change-over unit No.  2   102 B causing an electric discharge to take place at the discharge gap  14 . The coherer No.  2   1 B is thus cohered. The transistor  104 D of which the bias voltage is adjusted in advance by the variable resistor  104 B is turned off depending upon a change-in the state of the coherer No.  2   1 B, whereby the interrupt timer circuit  109  starts operating and the above-mentioned decoherer timer circuit  107  starts operating, causing the coherer No.  1   1 A and the coherer No.  2   1 B to be decohered. 
     The antenna change-over unit No.  1   102 A, the antenna change-over unit No.  2   102 B and the power source interrupt relay  114  are operated for a predetermined period of time due to the interrupt timer circuit  109 , to prevent the internal circuitry in the detector from being destroyed by violent impulse electromagnetic waves due to lightning at a close distance passing through the antenna No.  1   12 A, antenna No.  2   12 B and power distribution line  115 . 
     After the passage of the predetermined period of time, the interrupt timer circuit  108  permits the antenna change-over unit No.  1   102 A, antenna change-over unit No.  2   102 B and power source interrupt relay  114  to return to execute the alarming operation. 
     Effect of the embodiment 3. 
     This embodiment enables the detector itself to automatically conduct the lightning to ground in addition to obtaining the effects of the embodiments 1 and 2, and, hence, makes it possible to safely continue the detection of lightning. 
     Embodiment 4 
     Described below is an embodiment 4 of the invention of claim 5. FIG. 11 is a circuit diagram of the embodiment 4, FIG. 12 is a block diagram thereof, FIG. 13 is a flowchart of the operation, and FIG. 14 is a diagram schematically illustrating how to judge that the thundercloud is approaching or is moving away. 
     In addition to those elements of the above-mentioned embodiments 1, 2 and 3, there are provided a lamp  120 R which indicates that the thunder is approaching, a lamp  120 Y which indicates neither the thunder is approaching nor the thunder is separating away, a lamp  120 G which indicates that the thunder is separating away, and a CPU  119  which controls the turn-on of lightning danger (dynamic) alarm lamps  120 R,  120 Y and  120 G as shown in a flowchart of FIG. 14 relying upon the input of lightning electromagnetic waves from the photo coupler  30 A and upon the input of thunder front the diode  112 , and controls the buzzing sound of a buzzer  116 . Reference numeral  118  denotes a clock for measuring the passage of time. 
     Initial Setting. 
     When the power source is turned on, the CPU  119  operates the decoherer No.  1   11 A as an initial setting, resets the passage of time to the clock  118 , and erases the contents stored in the memory. When the coherer  1 A is in the cohered conducting state, the CPU  119  repeats the initial setting. 
     Alarming Operation. 
     The electromagnetic waves of lightning received by the antenna No.  1   12 A are transmitted to the filter transistor  15  where high-frequency components are attenuated and are further transmitted to the coherer  1 A. The coherer  1 A is cohered by the input of electromagnetic waves of lightning. 
     The transistor  104 C of which the bias voltage has been adjusted in advance by the variable resistor  104 A is turned off by the coherer  1 A that is cohered and is rendered conductive, whereby the CPU judges that the electromagnetic waves of lightning have been received. The CPU reads out the time from the clock, stores it in the internal memory A, and operates the decoherer No.  1   11 A to decohere the coherer No.  1   1 A. 
     The CPU waits for the input of a thunder signal from the diode  112  and stores it in the memory B. A distance to the point where the thunder has generated is estimated from the time difference between the time of receiving the lightning electromagnetic waves and the thunder time stored in the memory A and in the memory B, and is stored in the internal memory D. When no thunder signal is input even after having waited for more than 30 seconds, the time difference is set to be 30 seconds, and the distance to the point where the thunder is generated is estimated and is stored in the internal memory D. 
     The CPU stores the value D in the internal memory D 1  and stores the value A in the internal memory T 1  as lightning generation hysteresis data. Then, every time when a new lightning electromagnetic wave is detected, the CPU additionally stores them as hysteresis data in the internal memories D 2  to D 100 , T 2  to T 100 . When the detection of lightning electromagnetic waves has exceeded 100 times, the CPU shifts the contents stored in the memories D 2  to D 100  and T 2  to T 100  to D 1  to D 99  and T 1  to T 99 , stores the memory D in the memory D 100 , and stores the memory A in T 100 , to store them as the latest lightning generation hysteresis data. 
     The CPU estimates the approach or separation of the thundercloud from the lightning generation hysteresis data T 1  to T 100  and D 1  to D 100  in the memory. In practice, however, the points where lightning is occurring are considerably dispersed as shown in FIG.  14 . Described below is how to judge the approach or separation of the thundercloud from these dispersed values. 
     First, at a moment where the oldest hysteresis distance value D 1  is stored in FIG. 14, the CPU waits for the next thunder-bolt. When T 2  and D 2  are stored at the next generation of a thunder-bolt, the CPU calculates the inclination of a line that connects a point T 1 , D 1  to a point T 2 , D 2  in FIG.  14 . When the inclination is (−), the CPU judges that the lightning is approaching and turns the alarm lamp  120 R on. When the inclination is near (0), the CPU judges that the lightning is neither approaching nor moving away and turns the alarm lamp  120 Y on When the inclination is (+), the CPU judges that the lightning is moving away and turns the alarm lamp  120 G on. 
     When D 3  and T 3  are stored at the third generation of lightning, the CPU operates average-value points Tc, Dc of T 1 , T 2  and D 1 , D 2 . When the inclination of a line connecting these points to the points T 3 , D 3  is (−), the CPU judges that the lightning is approaching and turns the alarm lamp  120 R on. When the inclination is nearly 0, the CPU judges that the lightning is neither approaching nor separating away and turns the alarm lamp  120 Y on. When the inclination is (+), the CPU judges that the lightning is separating away and turns the alarm lamp  120 G on. 
     Thereafter, every time when lightning newly occurs, the CPU finds an inclination of a line connecting average points Tc, Dc of the two preceding thunder-bolt generation hysteresis data to new points Tn, Dn, and repeats the alarming operation. 
     Though the indicator was designed to turn on red, yellow and green lamps, so that the degree of danger can be easily comprehended, there can be employed any similar display means. However, that there may be further used an alarming sound, a recording paper, a telemeter automatic emission displayed on CRT, or a combination thereof. 
     Effect of the embodiment 4. 
     This embodiment makes it possible to judge and display the degree of danger of a thunder-bolt based on whether the thundercloud is approaching or is moving away, as well as to automatically detect whether the thundercloud is in a lightning range. 
     When taking a measurement while mounting the device (lightning detector) of this embodiment on a body that moves at a constant speed, it can be presumed that the measurement might be adversely affected due to Doppler effect. This, however, can be corrected by suitable means such as adding or subtracting the moving speed of the device to or from the measurement value. 
     Embodiment 5 
     Described below is an embodiment 5 of the invention of claim 5. The embodiment 5 is a combination of the above-mentioned static lightning detector means and the dynamic lightning detector means. FIG. 15 is a circuit diagram of the embodiment  5 , FIG. 16 is a block diagram thereof, FIG. 17 is a flowchart illustrating the operation, and FIG. 14 is a diagram schematically illustrating how to judge the approach or separation of the thundercloud. 
     In addition to those elements of the above-mentioned embodiments 1, 2, 3 and 4, there are provided a lamp  121 R for indicating the lightning range, and a CPU  122  which controls the turn-on of lightning danger (dynamic) alarm lamps  120 R,  120 Y and  120 G as shown in a flowchart of FIG. 17 relying upon the input of lightning electromagnetic waves from the photo coupler  30 A and photo coupler  30 B and upon the input of thunder from the diode  112 , controls the turn-on of lightning danger (static) alarm lamp  121 R that indicates the lightning range, controls the buzzing sound of the buzzer  116 , and controls the antenna change-over unit No.  1   102 A, antenna change-over unit No.  2   102 B and power source interrupt relay  114 . Reference numeral  118  denotes a clock for measuring the passage of time. 
     Initial Setting. 
     When the power source is turned on, the CPU  122  operates the decoherer No.  1   11 A and the decoherer No.  2   11 B as an initial setting, resets the passage of time to the clock, and erases the contents stored in the memory. When the coherer No.  1   1 A and the coherer No.  2   1 B are cohering, the CPU  122  repeats the initial setting. 
     Alarming Operation. 
     The electromagnetic waves of lightning received by the antenna No.  1   12 A are transmitted, through the antenna change-over unit No.  1   102 A to the filter transistor  15  where high-frequency components are attenuated and are further transmitted to the coherer  1 A. The coherer  1 A is cohered by the input of electromagnetic waves of lightning. 
     The transistor  104 C of which the bias voltage has been adjusted in advance by the variable resistor  104 A is turned off by the coherer  1 A that is cohered and is rendered conductive, whereby the CPU judges that the electromagnetic waves of lightning have been received. The CPU reads out the time from the clock, stores it in the internal memory A, and operates the decoherer No.  1   11 A and the decoherer No.  2   11 B to decohere the coherer No.  1   1 A and the coherer No.  2   1 B. 
     The CPU waits for the input of a thunder signal from the diode  112  and stores it in the memory B. A distance to the point where the thunder has been generated is estimated from the time difference between the time of receiving the lightning electromagnetic waves and the thunder time stored in the memory A and in the memory B, and is stored in the internal memory A. When no thunder signal is input even after having waited for more than 30 seconds, the time difference is set to be 30 seconds, and the distance to the point where the thunder is generated is estimated and is stored in the internal memory D. 
     The CPU stores the value D in the internal memory D 1  and stores the value A in the internal memory T 1  as lightning generation hysteresis data. Then, every time when a new lightning electromagnetic wave is detected, the CPU additionally stores them as hysteresis data in the internal memories D 2  to D 100 , T 2  to T 100 . When the detection of lightning electromagnetic waves has exceeded 100 times, the CPU shifts the contents stored in the memories D 2 -D 100  and T 2 -T 100  into D 1 -D 99  and T 1 -T 99 , stores the memory D in the memory D 100 , and stores the memory A in T 100 , to store them as the latest lightning generation hysteresis data. 
     The CPU estimates the approach or separation of the thundercloud from the lightning generation hysteresis data T 1 -T 100  and D 1 -D 100  in the memory. In practice, however, the points where lightning is occurring are considerably dispersed as shown in FIG.  14 . Described below is how to judge the approach or separation of the thundercloud from these dispersed values. 
     First, when D 1  is within the lightning range at a moment where the oldest hysteresis distance value D 1  is stored in FIG. 14, the CPU  122  informs of the generation of a thunder-bolt by turning the alarm lamp  121 R on and energizing the buzzer  116 . When the oldest hysteresis distance value D 1  is outside the lightning range, however, the CPU waits for the generation of a next thunder-bolt. When T 2  and D 2  are stored at the next generation of thunder-bolt, the CPU  122  turns the alarm lamp  121 R on and energizes the buzzer  16  provided D 2  is within the lightning range. When D 1  is outside the lightning range, the CPU  122  calculates the inclination of a line that connects a point T 1 , D 1  to a point T 2 , D 2  in FIG.  14 . When the inclination is (−), the CPU judges that the lightning is approaching and turns the alarm lamp  120 R on. When the inclination is near (0), the CPU judges that the lightning is neither approaching nor separating away and turns the alarm lamp  120 Y on. When the inclination is (+), the CPU so, judges that the lightning is separating away and turns the alarm lamp  120 G on. 
     When D 3  and T 3  are stored at the third generation of lightning, the CPU turns the alarm lamp  121 R on and energizes the buzzer  116  provided D 3  is within the lightning range. When D 3  is outside the lightning range, the CPU operates average-value points Tc, Dc of T 1 , T 2  and D 1 , D 2 . When the inclination of a line connecting these points to the points T 3 , D 3  is (−), the CPU judges that the lightning is approaching and turns the alarm lamp  120 R on. When the inclination is nearly 0, the CPU judges that the lightning is neither approaching nor separating away and turns the alarm lamp  120 Y on. When the inclination is (+), the CPU judges that the lightning is separating away and turns the alarm lamp  120 G on. 
     Thereafter, every time when lightning is newly generated, the CPU finds an inclination of a line connecting average points Tc, Dc of the two preceding thunder-bolt generation hysteresis data to new points Tn, Dn, and repeats the alarming operation. 
     Though the indicator was designed to turn on red, yellow and green lamps, so that the degree of danger can be easily comprehended, there can be employed any similar display means. However, there may be further used an alarming sound, a recording paper, a telemeter automatic emission displayed on CRT, or a combination thereof. 
     Lightning Operation. 
     When the electrostatic discharge or corona discharge takes place on the antenna No.  2   12 B during the alarming operation, the discharge current flows through the antenna change-over unit No.  2   102 B causing an electric discharge to take place at the discharge gap  14 . The coherer No.  1   1 A is thus cohered. 
     The transistor  104 D of which the bias voltage is adjusted in advance by the variable resistor  104 B is turned off due to the coherer No.  2   1 B that is adhered and is rendered conductive, and the CPU detects the generation of electric discharge. 
     The CPU operates the antenna change-over unit No.  1   102 A, the antenna change-over unit No.  2   102 B and the power source interrupt relay  114 , to prevent the internal circuitry in the detector from being destroyed by violent electromagnetic waves due to lightning at a close distance passing through the antenna No.  1   12 A, antenna No.  2   12 B and power distribution line  115 . The CPU further turns the red alarm lamps  120 R and  121 R on, and intermittently energizes the alarm buzzer  116  to indicate that the lightning is being conducted to ground. 
     After the passage of the predetermined period of time, the CPU  122  permits the antenna change-over unit No.  1   102 A, antenna change-over unit No.  2   102 B and power source interrupt relay  114  to return, whereby the coherer No.  1   1 A and the coherer No.  2   1 B are decohered to resume the alarming operation. 
     Effect of the embodiment 5. 
     This embodiment makes it possible to automatically judge and indicate the degree of danger of lightning depending upon the approach or separation of thundercloud and to automatically detect whether the thundercloud is within the lightning range in addition to the effect of the embodiment 4. 
     When taking a measurement while mounting the device (lightning detector) of this embodiment on a body that moves at a constant speed, it can be presumed that the measurement might be adversely affected due to Doppler effect. This, however, can be corrected by suitable means such as adding or subtracting the moving speed of the device to or from the measured value. 
     The above-mentioned objects are all accomplished by implementing the present invention. That is, by using a coherer with a decoherer of a relatively simple structure, it is allowed to reliably predict the danger of thunder-bolt without the probability of being struck by lightning. 
     Further, a plurality of lightnings (electric signals) and thunders (sound signals) due to thunder-bolts at a distance are detected and stored, and at least two pairs of these data are compared to reliably notify to a user the degree of danger of a thunder-bolt by using an indicator.