JPS58191403A - Voltage non-linear resistor and method of producing same - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is directed to a voltage non-straight type III motor which is made of a sintered body mainly composed of zinc oxide, which can be used in a surge arrester, etc.
Concerning resistors and their manufacturing methods.

Zinc oxide-based voltage nonlinear resistors are generally manufactured using well-known ceramic sintering techniques.

Its main component is zinc oxide powder, bisma oxide, X (B15on), and antimony oxide (8B).
sOs), cobalt oxide (0oz), chromium oxide (Or*Os), manganese oxide (MnO2), silicon oxide (810,), boron oxide (B鵞Os) 1
．． Add nickel chloride (NiO) etc. and mix thoroughly.
Water and a suitable binder such as polyvinyl alcohol are added to this, followed by granulation and molding. Baking is done using an electric furnace.
It is carried out at a temperature of 0 to 1400°C.

For the purpose of preventing creeping discharge, a borosilicate lead-based low melting point glass group [400 to 700℃] is coated on the side of the fired resistor to prevent creeping discharge.
The baking process is performed by polishing both end faces of the resistor, which will form electrodes, to a predetermined thickness, and then forming the electrodes using a bath irradiation or baking method to form a voltage resistor. Furthermore, examples are known in which lead glass having a softening point of 350 to 650° C. is mixed with a refractory filler as the glass.

However, the nine-resistor obtained by this method has the following drawbacks. First, because the moisture resistance and acid resistance of the glass used is poor, the glass may become damaged or eroded during use at high temperatures or during the etching process before forming the electrodes, or the glass may become eroded. If the resistor is used sealed in nitrogen, the glass is corroded by the nitric acid gas generated by the corner discharge, resulting in a decrease in creeping resistance and pressure of the resistor O. Second, when glass is subjected to ψ deposition at a temperature of 50Ω to 800° C., there is a drawback that the non-electrostatic coefficient of the resistor O becomes smaller than before.

The present invention has been made in view of the above, and an object of the present invention is to provide a voltage non-direct radiation resistor with stable characteristics such as non-direct radiation number and creepage withstand voltage, and (1) a method for manufacturing the same.

That is, if the present invention is summarized, the present invention summary 10 inventions (
1! Piezo nonlinear resistor) A glass layer is formed on at least the side surface of a sintered body mainly composed of zinc oxide, with or without a ceramic high-resistance layer, and electrodes are provided on both the upper and lower end surfaces of the sintered body. In the formed nine-voltage non** resistor, the glass layer is below OJi! Minutes: Zinc oxide (ZnO): 30 to 71 weight - boron oxide (Blam): S to 20 weight, silicon oxide (810 weight) = 10 to 50 weight, lead oxide (PI) O):
The present invention relates to a voltage nonlinear resistor characterized in that it is a zinc borosilicate glass containing 1% to 23ti.

Then, according to the 20th invention of the present invention (method for producing a voltage non-direct radiation resistor), a high resistance layer is formed on the sintered body 011 whose main component is zinc oxide, and an oxidized layer is formed on the sintered body 011. @30~
71 coulometric capacity - 1 boron oxide 5 to 20 weight - 1 silicon fluoride 1
A paste consisting of a binder and a borosilicate zinc-based glass component containing 0 to 50 coul-1 lead oxide 5 to 25111 t% and tantalum oxide 2 to 6 t% is spread and heated to 850°C or higher in an oxygen-containing gas atmosphere. is lower than the firing temperature of the ψ feeder, and the softening temperature of the glass is m degrees and the working temperature is O
The present invention relates to a method for manufacturing a voltage non-linear resistor C, characterized in that a glass layer is formed by baking at a temperature of about 100 liters, and electrodes are formed on the upper and lower end faces of the sintered body.

The feature of the present invention is that the voltage nonlinear resistor 1lIPj7J, which is mainly composed of zinc oxide and composed of a burnout element, is coated with zinc oxide 30 to 7
100% boron oxide, 5-20% boron oxide, 10% silicon oxide
~50 if arrogant, lead oxide 5~15 electric current and, if necessary, sand tantalum oxide 2~6i1-11Tt, borosilicate zinc-based glass O @ 850℃ or higher (the firing temperature of the sintered body) is cauterized at a low temperature. In addition, electrodes are formed on both the upper and lower end surfaces of the resistor. Furthermore, the nonlinear coefficient is improved by firing in an oxygen-containing gas atmosphere, particularly in an oxygen atmosphere. An embodiment of the present BAO voltage non-straight chain resistor O structure
- As shown in Figures 1 and 2.

That is, FIG. 1 is a cross-sectional view showing an example of the structure in the case where the Cera-based high resistance layer is not used in the invention. In Figure 91, 1 is a sintered body, 2 is a −JI2 layer, and 5 is a sintered body.
is an electrode.

Further, FIG. 2 is a cross-sectional view showing an example of the movement when a ceramic high-resistance layer is used in the present invention. Second
In the figure, 1 to S have the same meanings as in FIG. 1, and 4 is a mixed high resistance layer.

As shown in Figure 2, Zny is added to the interface between the glass layer and the sintered body.
A high-resistance ceramic layer made of 8blO11, Znl a i 04, or the like may be provided to further improve the adhesion between the glass layer and the sintered body. Furthermore, it goes without saying that the glass layer may partially cover the upper and lower surfaces O where the electrodes are provided.

In addition, No. 51!3 shows the voltage non-direction-resistance O heat treatment temperature (horizontal axis, °C) and the non-direction ai coefficient (IQ
This is a graph showing the relationship between FA@111m) O change rate (vertical axis, 96). In the diagram, 1111111 shows the case where the heat treatment was performed at each temperature for 1 hour in the atmosphere, and m@2 shows the case where the heat treatment was performed at each temperature for 1 hour in oxygen.

As a result of various studies by the present inventors, (1) In order to improve the adhesion between the resistor and the electrode, the O resistor imt may be etched with an acid such as nitric acid in hydrochloric acid after polishing. For this reason, it is necessary to use IIK acidic K as the side coating glass. ■As shown in Figure 3, when the resistor is heat-treated after KX scaling, the non-linearity coefficient of the resistor decreases in the temperature range of 40O to aQθ℃, but at temperatures above 850℃, it is different from that before heat treatment. = So to be the same or to become larger. (2) Generally, the acid resistance of glass increases as 8101 and pbo increase to a certain extent in the glass. The composition of the glass is oxide oxide.

5ates, lead oxide 3-23 weight - 1 boron oxide 5-2
0iit, if the etching is slow, there should be no problem with the corrosion resistance to K, and the firing temperature is 850°C.
Since the above high melting point zinc - JI TSU O heat I# is usually a resistor), there is a risk that there will be cracks in the glass, or even if the glass is wired zinc. 60～
It was found that if the glass contains 711, the coefficient of thermal expansion will increase and cracks will occur.When heat treated at a temperature of 400 to 80 degrees Celsius, a B110 phase change occurs, the nonlinear coefficient decreases, and the Big False Toru, 4 (82
0° C.) or higher, the same 〇 phase as after ** is formed, and the nonlinear coefficient is considered not to decrease. In addition, when heat treated in oxygen, a large amount of oxygen ions are released on the surface of the zinc oxide O particles.
As a result, the non-direction gk number increases. In addition, the temperature for glass baking is the same as the softening temperature of the glass and the working temperature.

In addition, I mentioned that the glass has excellent acid resistance in the resistor of the AJ4, but rainbows and corona may occur when the resistor is inserted into a nitrogen atmosphere like an arrester. There is no risk of being etched by the nitric acid produced, and the percentage is stable.

The present invention @O voltage non-WLIII resistor is made by adding a[11 to 7
Mol% OIM Antimony oxide, ilization: f)
Then, it is baked at 1000 to 1400℃, coated with WIg-resistant zinc borosilicate glass powder, and heat treated at a temperature of 50 to 1300℃ to form the glass T-*A, and one end surface of the plate is made to have a shrinkage strain thickness. It is obtained by etching the ground surface, preferably the first surface, with nitric acid in hydrochloric acid, and then forming an electrode. Here, if glass is applied to the electrodes rlX, vk, the electrodes will be oxidized and will not be drawn when the glass is attached.

Ma°hiki, glass firing! 'jgA degree is 1350℃ or higher
When exposed to heat, the glass and sintered body react violently, causing the glass to diffuse into the sintered body multiple times, impairing the characteristics of the resistor.
It's not right.

The crow used in the present invention is a telegraph as follows.

That is, firstly, in order not to deteriorate the inherent non-linear coefficient of the resistor, it is necessary to bake the glass at a high temperature of 1300° C. for the resistor K115Q, and the glass must be a single-point glass tube.

Secondly, in order to improve the t-pole resistor's own property, it is desirable to etch the glass on the resistor using hydrochloric acid, nitric acid, etc. after baking the glass. In addition, it must be made of acid-resistant glass to prevent surface creepage when it enters a nitrogen atmosphere. Thirdly, K11a, K, L in glass 0 components
It is required that the amount of alkali metals such as i and t is small. In addition, in order to improve the adhesion between the resistor and the glass, and to prevent creepage, the thickness of the glass film should be approximately 20 mm.
There is a fee to make it more than μm, and it is desirable that the resistor and the glass O thermal 1# expansion coefficient are similar. Since the thermal expansion coefficient of the densified lead resistor O is 50~7QX10-7/℃, the thermal coefficient of glass is 30~? A range of OX 10-'/l is preferable. If there is a large difference in the coefficient of thermal expansion, should it be heat treated?
6II or when a thick 11EfIL is applied, stress acts on the resistor or the glass, causing cracks or peeling of the glass, making it impossible to achieve sufficient stability against current flow and protection against creepage. To prevent this from cracking or peeling, it is especially recommended that the glass be made into kAi.

The main composition of the zinc borosilicate glass of the present invention is
, zinc oxide weighs 30-71%, boron oxide weighs 5-201%
ul silicon oxide 10-5.0 arrogant, oxidized mental 2
~6tt fight, lead oxide 5~23! ! It is desirable that the amount is in the sO range. The silicon oxide accumulation is higher than this range,
If the amount of lead oxide or boron oxide is too small, the softening point and working temperature of the glass will be high, and the glass O baking temperature will be 135%.
Inconveniences arise when the temperature exceeds 0°C. On the other hand, if there is too much lead oxide or boron oxide, the baking temperature of the glass will be 800° C. or lower, and the glass will have a disadvantage of poor acidity. Note that crow rich in lead oxide and containing a small amount of boron oxide generally has better moisture properties.

Moisture resistance, acid resistance, and wettability between the resistor and the glass film improve when the lead oxide content ranges from 3 to 25@1i%.

9. Tantalum oxide in the glass component is glass O crystallized.
ii It has the effect of increasing the IfrC influence qII and improving the thermal strength of the glass. In particular, the thermal strength of the glass is 2 to 6 F [amount].

The present invention O glass #'i30~71wt sO Containing zinc oxide improves the mechanical strength of the glass by %Kii
Delicious. Glass containing zinc oxide sometimes becomes crystallized glass when baked, increasing the strength of the glass layer, preventing glass O cracks, and improving element O resistance. If zinc oxide O1 is less than the above range, the O effect will not be sufficient, and if it is too much (below the above range), the acid resistance of the glass will decrease.

Therefore, the desirable composition for the glass O% of the present invention is 40
≦jsno≦60 electric power, 7≦B, 03≦12 weight-1
2Ω≦8101≦40 electric power, 2≦i&! Os≦6 weight whisper, 5≦pbo≦15 weight arrogance.

The present invention will be illustrated below using Example t1, but the present invention is not limited thereto.

Implementation f1i1 Duplex oxide (MnO) 7452 f % 592 Bismuth oxide (Bi, O,) 324? , cobalt oxide (como
s) 1bbt, 11-in-gay (Mn01) 5
7 f-, ell-formed Annamon (sb, ox) 2
92t, Wl conversion 10m (0r203) ” '
% Nickel alumide (NiO) 75FXel silicon (810,) ? Of is mixed in a ball oil at 12 o'clock. After drying, the mixed powder is granulated to give a size of 12mmφ×
Shape into 4■. The molded body was fired at 1250°C in the air for 2 hours.

Specially made of high melting point crystallized glass and acid resistant OJL GP・
14 (O product name manufactured by Nippon Electric Insulators) Glass powder was suspended in ethylcellulose-trichloroedelene #suk, and this was baked on the 011 surface of the resistor with a thickness of -g50-300μm.
It was applied using a basket coating or dipping method to achieve the desired results. This was heat treated at 1000℃ for 30 minutes in the air. The rate of temperature rise and fall at zero is 100°C/h. Both end faces of a resistor made of glass were polished with a Lap Oster to a depth of approximately α5cm and then cleaned. After washing, the antibody will form an injection electrode. Table 1 shows a comparison of the O nonlinear coefficients of the product of the present invention and a conventional product (made of borosilicate lead-based low melting point crystallized glass and baked at a low temperature of 650° C.).

Table 1 It can be seen that the products of the present invention have extremely large nonlinear coefficients and are superior to the conventional products.

Example 2 Same as Example 1, zinc oxide (ZnO), vinyl oxide, Xw
(Bt, am) 163 f, cobalt oxide (C
! 0,0. ) 34 f, oxidation'? 7 gun (MnOl
)29t1 antimony oxide (S'btO1) 195
fX chromium oxide (0rlO1) 38 f, nickel oxide (Mlo)S a F, silicon oxide (1101
) 45fX boron oxide (BmOs) 5t are mixed in a ball mill at 13:00 in a wet manner. After drying, the mixed powder was formed into granules of 12 mm x 6 mm and heated at 1270℃2.
It was held for a time and fired. The fired resistor was a of Example 1.
PE14 glass paste thickness 15G-200p1Km
It was heat-treated at 1020° C. for 1 hour in an oxygen atmosphere. The rate of temperature rise and fall at this time was 40°C/h. Both end faces of the glass-shaped resistor were polished by approximately 1 inch using a lap master and cleaned. The cleaned resistor is great! A friend that forms a radiation electrode. Table 2 shows a comparison of the non-correlation coefficients between this invention and a conventional product (made of lead borosilicate based low single point crystallized glass and baked at a temperature of 750 cOIl). It can be seen that the nonlinear coefficient of the product of the present invention is significantly lower than that of the conventional product.

General 2 Table Example 3 Same as Fruiting Example 1 Zinc oxide (ZnO) 3816f
uoz) 29f1 antimony oxide (8b10g) 1
95t.

Chromium oxide (0rlO1) 58 f, nickel oxide (Mlo) 511rX @ oxidation collection (810) 45
F.

Mix in an IhOn 5ft ball mill for 15 hours. After drying, the mixed powder is granulated and 12-φ×
The molded product was molded into a 4-haze color and fired at 1250° C. for 2 hours in the air. Baked resistor Fi Example 10GP/14 glass paste with thickness 100~200111! Apply IK,
Heat treatment was performed at 1000° C. for 1 hour in the air. glass be
Both end faces of the resistor were wrapped around 1111 mm using a lap master.
Polish and clean one by one. Here, in the conventional product, the housing tH injection electrode was directly formed on the resistor after polishing and cleaning.

In the product of the present invention, a polished and cleaned O resistor is immersed in salt V:Water 70 and Etching 11 for 3 minutes to etch the polished surface, and then a mulch bath is formed.

Table 3 shows a comparison of the characteristics of the product of the present invention and the conventional product.

Table 3: The products of the present invention have larger nonlinear coefficients and varistor voltages than conventional products, and have a smaller rate of voltage change due to energization, and 5e
It can be seen that the tIt tolerance is also greatly superior.

Example 4 Similar to Examples 1 and 3, nitrous oxide (ZnO) 581
6? , bismuth oxide (Bi, Os) 1M port f1
(cobal oxide) (00103) 34 f, manganese oxide (mno, ) 291X scissored antimore (Elbt
on ) 225 f, chromium oxide (0rzO1) 3
8t, graded nickel (lli, o) 58t, silicon oxide (to 81) 55t1 boron oxide (B0.) 5
F. was mixed in a ball mill for 15 hours. After drying, the mixed powder is granulated and shaped into 12 φ×5- pieces. The molded body is 81
A paste containing 01-ah, Os-Bi, and Ol was applied and then fired at 1300° C. for 2 hours.

For the fired resistor, paste 1 was prepared in the same manner as in Example 1 using various glasses 1 shown in Table 4, and 1o.

~20Qp grave thickness Kll cloth, heat treated in air at a given temperature for 1 hour.

The glass-blown resistor has both end faces approx. aS using a lap master.
, - polish. The polished resistor was prepared using nitric acid difluoric acid:water-5:
After etching the polished surface by immersing it in a 1:410 etching chamber for 3 minutes, the mold was sprayed to form an electrode.

From this, as seen in the wJz diagram, one resistor O@
In the image Zny8blO11 and Znl II i 04
A resistor was obtained in which a high-resistance ceramic layer was formed, and a glass film was further formed on the high-resistance ceramic layer. The results are shown in Tables 5 and 6.

As shown in Table 5, the acid resistance of glass O is! 7
Depending on the composition of the lath), the present invention has a composition range of O.
The etching rate with respect to P.14 glass 0llK is 01/2 to 1/10 or less of other glasses.

#! As is clear from 6, the impulse tolerance (waveform 4×
10μ days), ap・14 glass (number 4) and liI were also crushed, followed by G1・351 (number 1), OT・1 (
No. 2), Gmu・4 (No. 1 5), BH・5 (No. II 6)
It is 0@.

On the other hand, glass with a large amount of boron saponide (No. 1.2.4) and glass with a large amount of lead oxide O (No. 5.5) have poor resistance to 0ITIt and moisture resistance, and the impulse resistance is lower than that when using the O glass of the present invention. It is as follows.

t*, impulse 1 amount is larger when using crystalline glass (No. 1.2.4) than when using amorphous glass (No. 1! S, 5, RI); That's all.

Example 5 A 12mmφx4mKJiK shape was prepared in the same manner as in Example 1.
It was held at 12110°C for 2 hours to form v8.

The glass shown in Table 4 was used for the resistor in Example 1.
5 and 4 and 1ift were applied to a thickness of 10G to 2QQpmO, and heated in the air at 750 to 1000°C for 1 hour. The rate of temperature rise and fall during this time was 70°C/h.

Both end faces of the glass 1-coated resistor were polished by α5m. The resistor for polishing is HMO@2Hν: Hlo -5
: 1 : 4Q Etsudenda i [K5 minute soaking and polishing I
After etching and rounding, a mutwm injection electrode was formed. In this way, the resistor was exposed to corona discharge in a nitrogen atmosphere and the changes in its characteristics were investigated. Table 7 shows the vkO% after 1 hour of corona discharge.

When GP-14 is used, the impulse withstand capacity remains unchanged before and after the corona discharge test. On the other hand, with GIN-351 and 0〒・1 glass, which have slightly lower acid resistance, the impulse withstand capacity decreased by about 101i before and after the test1. This is because the resistor can be encapsulated in a nitrogen atmosphere and cause corona discharge. A large amount of water in the atmosphere reacts with nitrogen to produce nitric acid, and this nitric acid etches and deteriorates the glass film. Furthermore, when a device coated with a conventional glass, P.810 glass, was tested, the impulse withstand capacity decreased by more than 20 points before and after the test.

Example 6 It was formed into a size of 561φ x 24- by the same method as in Example 1,
It was held and fired at 1250°C for 3 hours.

For the fired resistor, a paste consisting of glass having the composition shown in Table 8 was applied to Examples 1.5.4 and 5 and M.
It was applied to a 200 μm rhinoceros and baked in the air at a OU to 1050° C. for 1 hour.

This and 1! The O rising/falling rate is 40°C/h.

Glass is coated and the antibody is usually polished by polishing both end faces (15 m each. The polished resistor is HMO. Fill:
The polished surface was etched by immersing it in an etching chamber containing H, O-4:1:4 for 3 minutes to form a thermal sprayed electrode. In this way, we obtained the non-linear coefficient for the current of 9 elements ranging from 10 pm to 1 mm, the initial impulse withstand capacity, the impulse withstand capacity after the corona discharge test, and the impulse withstand capacity after being left in boiling water for 10 hours. , -40℃5150℃O heat cycle 1000
The O impulse withstand capacity after rotation is as shown in table @a.

For Aresfu, the cumulative O impulse withstand capacity is 11' [100 ICA below 288 to M connection, and 150 to 100 ICA or more is required for 420XV system or above.

As seen in Table 8, number 5 within the scope of the invention
．． 4.7.8.9.12.15.1.7 and 184D samples have a large nonlinear coefficient, and the impulse withstand capacity is 150 μm or more after the corona test, boiling test, and thermal digital test, and the 420I matrix It can be used as an arrester element larger than a wire. Furthermore, as is clear from Table 8, ZnO
In the sample (number 5) with an amount of 601 it% or more, the glass Oi
1 has poor acidity, and the impulse resistance to deterioration after the corona discharge test is greater than other samples. Mae, day 10 goose and Btu
Sample with too many s t (number 2.6.10.14.15
At y, the water resistance of the glass O deteriorates, and the deterioration of the impulse resistance after the boiling test is greater than that of the loosened O sample. Furthermore, Pt
When the amount of 1O is too large and the sample is not sampled (No. 11, 19.20), the non-linearity coefficient tends to become obvious when the temperature of the glass 0 firing reaches around 800°C.

As detailed above, the present invention has the effect of eliminating the round creepage that coats the resistor surface with acid-resistant and moisture-resistant glass, and also allows the glass to be baked at a higher temperature than conventional methods. There is no deterioration of the non-linear coefficient of the resistor characteristic O.

Furthermore, the glass has excellent mechanical strength and acid resistance, and no electrical equipment or chips occur during the resistor manufacturing process, and nitric acid is not generated when used in a nitrogen atmosphere like an arrester. It also has a remarkable effect on the preparation of children, as there is no need to worry about being etched by the glass attendant.

[Brief explanation of drawings]

FIGS. 1 and 2 are cross-sectional views showing a voltage nonlinear resistor structure 0-implementation O11 of the present invention. FIG. 5 is a graph showing the heat treatment temperature of the #IK voltage nonlinear resistor O, the rate of change of the nonlinear coefficient, and 01[I]. 1: Sintered body, 2 Glass layer, 3: Electrode, 4: Cell high resistance layer/Fig. Procedural amendment (method) August 24, 1980 Kazuo Wakasugi, Commissioner of the Patent Office - Indication of case 1982 Patent Application No. 75571, title of invention Voltage nonlinear resistor and 7 am method 5 amendment person Incident and O relationship Patent applicant address: 5-1 Marunouchi-chome, Chiyoda-ku, Tokyo
Name (510) Hitachi, Ltd. Representative 3 1) Shigeru Kou Address 302 Nishi-Shinbashi Chuo Building, 15-8 Nishi-Shinbashi S-chome, Minato-ku, Tokyo Telephone (457)-5467 July 9, 1982 (Shipping date) July 27, 1981) Full text of the specification subject to melon amendment O

Claims (1)

[Claims] t At least m1t of a love body whose main segment is zinc oxide.
In a voltage non-linear resistor in which a glass layer is formed with or without the iK Ryu Lamix high-resistance layer and electrodes are formed on both the upper and lower end surfaces of the sintered body, the glass layer contains the following components: oxidation Substance (ZnO): 50-71 Electron (B-Os): S-20 Electron Silicon Oxide (81
0m): 10~50 @ Lead oxide (PbO)
: A voltage nonlinear resistor characterized in that it is a zinc borosilicate glass containing 5 to 25 weight bags. 2 #Zinc borosilicate glass has tantalum oxide (tea)
gOi) The voltage non-linear resistor according to claim 1, which is 10 containing t2 to 6 weight. 5. Special 1FFii1 where the zinc borosilicate glass is made of crystallized glass! Voltage non-linear resistor whose range is 1st or 2nd. 4 The borosilicate jelly thread glass is a high single point glass with a baking temperature between 850°C and the blowing temperature θ of the sintered body, which is the range I of Patent 111! l Any one of the first to third terms ffi@C) Voltage nonlinear resistor. - A ceramic high-resistance layer is formed on the side surface of a compact mainly composed of zinc oxide, and on top of that, zinc oxide is 50 to 11%, boron oxide is 5 to 20%, and silicon oxide is 10 to 10%. 5
0 heavy weight, except 1 group 5-23 turtle ji- and tantalum oxide 2-4N6%? A paste consisting of a zinc borosilicate glass component and a binder having 100% is heated to a temperature of 850°C or higher in a halogen-containing gas atmosphere, and the temperature is lower than the temperature determined by Glass C and the softening temperature of Glass C. The sintered body O is baked at a temperature between 100 and 200 mm to form a glass plate, and one end surface of the sintered body O is rc.
A floor made of a non-voltage abrasive material characterized by forming a magnetic field.