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An Introduction to Superconductivity

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therefore conductivity Perry in a large number of metal alloys and comply there seems to be no simple definition "quotation mark conduct it rather we feel here with the complex world of phenomena the superconducting state doesn't even seem to be the same in all the material in which it has been filed with that that's the other so-called Type 1 and tied to superconductors not be possible that touch in this film on all matters of interest In our experiments we will be using superconductors but I 1 these are easier to describe and more completely understood at the present time then the other
however it to properties are common to all types of superconductors 1 is that the temperature the involved are low the transition temperatures between the normally conducting and the superconducting phases like close to the absolute Of course the transition temperatures vary from 1 material to the other yet in most cases linked with union is required as a call back to produce superconductivity the 2nd property shared by all superconductors is perhaps the most striking their electrical resistance suddenly dropped in value at the transition temperature not only does the resistance dropped in value we have every reason to believe that it go to exactly 0 we can now make a perfectly conducting searches asserted in which parents precedence over great length of time without measurable and requiring no external elect promoted for to maintain them
now the perfect conductivity is only 1 aspect of superconductivity superconductors exhibit striking magnetic properties as well 1 of the most significant of these the so-called my well studied in some detail later but to begin with Will major resistance in a piece of wire at very low temperatures I had here a length of chain wire while over a plastic cylinder the 2 companies
have been invited to each and ride upward through here we will
use this wire in brings series of experiments of increasing precision to show you that the resistance of Kenyans immeasurably small below a certain temperature the fire has been put into the inevitable of double vacuum bottles or doubled over the other around you 1 and is filled with liquid nitrogen Thursday contained in the that American jackets together with a layer of liquid nitrogen between them provide thermal insulation for for the liquid helium the normal
boiling temperature of liquid helium is about 4 . 2 degrees Kelvin that erupted 206 93 field centigrade cover over the inaccurate it credible and threw their support the pressure and there is an atmosphere of the temperature of liquid and therefore the wired tool is 4 . 2 degrees Kelvin the lead from the 10 wire right out of the were resembles sealed learning from each vendor is connected to a battery in series with which I really bad and an and the other pair rolled to a the parent
of 1 and creates a potential problem of about 25 million people across the meanwhile it's resistance is about 25 million or so sold at 4 . 2 degrees Kelvin Chan is a normal conductor 1st we
want to call application wire this window magic of over there it connects through valves called at present for the interviewers which will be vacuum tight as soon as I couldn't support the rubber stuff the can't carries up Union vapor the evaporation of local adequate remaining behind and therefore the wire to 2nd we want to keep a record of the potential drop across the chin wire as the cooling proceed to get such a record we remove them illegal and we will face it with this recording the cities so-called X Y
recorded by horizontal or x motion will be so you would have to indicate the lot of time and cooling precede the vertical or wide motion is connected to the voltage you from the June what it is calibrated in many we again that the current
1 and there is reflects the recorder 25 million people the temperature of the wire is still about 4 . 2 degrees Kelvin
next week set the recorder pen down on the graph paper and start the horizontal sweep indicating a lack of time the current in the wire
is held constant at 1 end the potential drop in medical and reasons in nearly all have therefore people numerical values the
2nd half of the year catch the the the the the the the the the
the the the the I think they actually
excess C. This is the superconducting transition during the day the
resistance has dropped to 0 within the limits of accuracy of this instrument the recording both
media had been used to just above the limit of its sensitivity we now replaced it with the hiding in the amplifier it will be
used at the Micro Vault me to the small to be measures are applied here the amplifier output is measured on a docile involve which is bi-directional but it's 0 at the center we select the range in which full-scale deflection to the right on that signifies an input voltage of 1 Michael the liquid helium
has been cooled in all the transition temperature of Canaan
led measures the liquid temperature To do so would use an accurate pressure gage which is connected to the vapor space above the liquid surface in the inner door the needle registers the pressure there is the saturated vapor pressure of liquid helium the corresponding liquid temperatures are well known and the gage is already calibrated in terms of the
liquid helium is at 1 . 6 degrees Kelvin the voltage leaves
from the between wire in the do work are connected to the microbe all media input the voltage across the is there is no current in the tin wire
and the current both 1 and here there is a sudden voltage surge of about one-tenth of a microphone and then the voltage returned to Europe this surge is a small electric promoted for you to the self inducted into party breaking the circuit causes the current collapse and then opposing South induced yet met at a church the result important to us here is that after this transient the potential property euro again filed current is 1 and I remember that at its current about the transition temperature the voltage was 25 million illegal it is not 0
within the estimated error of this scale which is about 1 in 100 of a Michael the voltage across the wire is not more than 100 over Michael Wiley superconducting above the transition temperature it was 25 million therefore the resistance of the water has dropped on transition by a factor of 2 and a half million or more we have fashioned a ring out of 10 and placed it into liquid the ring hangs from a bread which provides little caught by a simple electromagnetic method we have endured the current around the ring to prove the existence of this current bring a magnet to the the current in the rain gives it a magnetic moment and it off today the external fuel of the bar magnet removes the magnet and the oscillation disappeared reversed the bar magnets and the ring turned around such current
carrying ring the kids on there have been kept in a cold that we know that transition temperature for months and even years in all this time the current kept flowing by measuring the magnetic field of the current periodically it was possible to show that the current does not became measurably at all for period years in this way and in other ways it has been proved set during the superconducting transition resistance dropped by a factor of 10 the 20 1 more we now leading the resistance becomes Europe we call the state of the metal superconductivity the current range it's called a persistent current denominated experiment we measured the
temperature at which can become superconducting we will use our Michael needed to detect the transition because it is a very sensitive we need only a small amount of currently an alliance not be as long as the 1 which we use for 2 companies have again been
started to each on 1 pair carries the current and the other pair is connected to the ball
the inquiry is now in the liquid helium at a temperature of 4 . 2 degrees Kelvin here is a composite
picture of our equipment the current in the wire producer the potential given the wire is in the normal state we're calling the wire with the vacuum pump and before the voltage
is just now dropping to the world this is the transition we are marking the
corresponding temperature on the thermometer our value
for the transition temperature of skin is 3 . 7 1 degrees Kelvin more precise measurements put it at 3 . 7 2 to increase we've shown that superconductivity the state of 0 electrical resistance for 2 put it the other way around of infinite conductivity although that is remarkable enough by itself superconductivity is quite a bit more than that as I mentioned at the start to show this to we now go on a new experiment they would involve magnetic field to begin with we again major the transition temperature of kids using the short sample of invite but this time we put in parallel the magnetic field of the electromagnetic force
fields and which we shall be using the magnet it's flux is proportional to the core current there isn't any series with the court it is already calibrated in terms of the flux density prevailing in the central region between the poles of the feels that the wire is think he got the same current
which we used in the previous experiment is again flowing through the wire and it registers the same high art dropped on the micro-blogging clearly that in sample is in the normal state but notice that its temperature is already much lower than 3 . 7 1 degrees Kelvin this was marked off in the previous experiments that our value for the transition temperature of in the absence of an external magnetic field as you can see by the motion of the Government and people the temperature is falling we are pumping on the liquid you keep your eyes on the micro-blogging here there and
thinking about the transition temperature of 10 is 3 . 1 2 degrees Kelvin field is still that the top of the sample has been cool to 1 . 5 degrees Kelvin that sample is superconductors we are increasing the magnetic field while keeping the temperature picks In the neighborhood of 250 go the field destroyed the superconductivity of the sample when its temperature is 1 . 5 degrees we call it the critical field for the temperature in question at field exceeding the critical value the superconducting state cannot exist at a given temperature we have discovered an important properties it is this magnetic flux density is a state variable in superconductivity that plant the 3 transition temperatures and the located critical flux densities by measurements similar to those we have performed 1 finds that the critical flux density depends upon the transition temperature in roughly parabolic fashion a parabola headed verdicts at absolute 0 .period about this curve signified
normal I want to know where superconducting state fault superconductors show this type of behavior in magnetic fields the value of the transition temperature will vary from 1 material to another there is a maximum critical field for every superconducting materials that field whose values lie about Europe superconductivity cannot exist below the euro or the value of the deal strongly affect the transition temperature these results would seem to indicate that the superconducting transition might involve more then a change from finite to infinite conductivity the external magnetic field influences the temperature at which the transitional occurred therefore it seems natural to analyze whether the magnetic properties of the materials don't also undergo a change as a matter of fact the love of electric begin magnitude have something to to say about magnetic field in materials with infinite conductivity than explained in the following way as we have seen before Honourable Ministers there can be no potential difference between 2 points in a material with infinite conductivity even if the current is flowing through it so the potential has the same value at all .period this means that the electric field intensity is your it also means that no 1 had promoted .period what ever can exist in the material in the articular considered the induced elect promoted ports which would exist around circuit whenever the magnetic flux is changing with the times and we know from Tarrant of electromagnetic induction studying our material notes that young that may exist therefore the magnetic flux cannot change with time in the material to put it succinctly the magnetic flux density cannot change with time in a material of infinite conductivity any magnetic flux which resides in a while it has infinite conductivity is frozen to as long as it has in this country we now wish to find out by experiment but actually does have magnetic flux during the transition from the normal for the superconducting state for this purpose we wouldn't cylinder into the liquid helium With the taxes at right angles to the field of the electromagnet in its median
plane with conditions on the array of many small compass needles as you can see the needle of free tutoring and tend to align themselves parallel to the magnetic flux space authority
by that you were that can sample is normal and the flux density is the peak of the
needles are oriented parallel to the magnetic field this indicates that the magnetic flux is essentially unaffected by the presence of the cylinder and permeates the profits unaltered intensity as a matter of fact the permeability of the superconducting materials in their normal state there was very little from Munich the failure of some needles .period horizontally is not new to the magnetic properties of the cylinder but to their own small magnetic fluxes in mutual interaction our
next step is to call the 10th cylinder into the superconducting state while holding the magnetic field ,comma it so happens that this transition takes a considerable length of time with the experimental setup we have here follow large
cylinder in the field of 80 and 1 dealer he must be carried out before the transition is complete no we do this by pumping up union they and the heat of vaporization of you it's quite small in fact the transition proceeds so slowly that we're showing it to you at 6 times normal speed noticed that company like turning the direction are taking on show at the magnetic flux is coming out of the cylinder we are witnessing here undetected of fundamental importance in superconductivity magnetic flux ejected during the transition to the superconducting state it's called Meissner effect and the currency in all superconductors there's 1 special group of superconductors which under ideal conditions checked all the facts during this transition can and most of the other metallic elements exhibiting superconductivity belong to this group they are called type 1 superconductors the effect is then called the complete Meissner effect it should be emphasized that this demonstration does not in itself proves that the magnetic flux is completely excluded from the system and that it could be partially excluded and show roughly the same exterior flux configuration in Canada and in all other types 1 superconductors the flux density being is identically 0 under ideal conditions this could not be predicted from the knowledge that the material has infinite conductivity or that did say it another way the electric field is 0 in the material that we saw earlier the fact that the vanishes requires only that he may not change with time so we must consider the Meissner effect as a separate and independent phenomenon of superconductivity consistent with but not a consequence of infinite conductivity in our next
experiment we want to demonstrate that the Meissner effect in is complete we want some insulated copper wire into a "quotation mark several hundred turned
directly on a piece of him Socorro senses changes of magnetic flux in the 7th the
terminal will come out of the work through this to the cylinder is
placed in the work with its axis parallel to the field of the electromagnet by parodies of induction any time rate of change of magnetic flux through this Court induces unelected promoted for the rate of change reflects multiplied by the number and of coral turns is numerically equal to the idea that induced the therefore the time integral part this voltage is a measure of the change reflects during a given time interval In our experiment this voltage integral or mold which involves will be about four-tenths of a newly-built 2nd to
recorded the time integral induced voltage we use this instrument is a digital integrating bold meter of very high voltage sensitivity the time integral of input voltage will appear on the dial In revolt 2nd with the decimal point here the
cylinder and coral foreign liquidity about the transition temperature case Our plan is 1st turn on the magnetic field welded until injuries in the normal state and to recordable digital involved that while keeping the field constant we call it cylinder through the superconducting transition and again measurable we are slowly turning on the magnetic field up to blocks density of 40 and I'll recorded corresponding voltage involved which is a measure of the apartment blocks this final value last point for 1 Minnewil seconds we requires this valuable and
centimeter dialed back to the world now we begin pumping underneath with you as you can see by the increased rate of spoiling the whole of the cylinders during all this time the current in the coil of the electromagnets is being held on a negative voltage impulses piling up indicating the onset of the superconducting transition the final reading is a measure of the flux excluded from the cylinders during the transition it's costly equals a numerical value the reading which measured the plucked initially admitted by the cylinder in it's normal that we found that experiment 11 times the average of the measurement of flux admitted in the 11 runs exceeded the average reflecting rejected by several the average deviation of the measurement was such that the range of possible values overlap we have taken the liberty of showing you the best of these 11 runs Sweden draw on a
number of conclusions from the discovery of a complete Meissner effect in tight 1 superconductors pledges to let me emphasize 1 of them that if such a material is in the superconducting state electric current control only on its very if there were apparent in the interior it would have to be accompanied by magnetic flux yet in our 2 conducted private 1 the Mets manager at Interior .period of current control only in are actually however there is a layer of fine called the penetration depth in which the superpowers no it is of the order of several hundred extra he and demand condition 3 did not explain what happened when the cylinder :colon superconducting in an external magnetic field to
current form and long service in such a direction that their own blood samples that the external fuel inside the material they also expect the exterior in such a way that the net effect is Latics their called shielding the encounter no resistance and remain as long as the few remaining for as long as the sample remains to connect it if we have hindering its normal state and in the field of apartment near the magnetic flux penetrate through the rain as well as through the hole in rendering the school into the superconducting shielding currents are created which rejects all the blood from the clean itself as long as the
Rangers superconducting that flexible magnet which lasted through the whole cannot crop up recruiting is trapped here you see the ring in the new work it has been holding the president of a partner in the flux density at the rate is roughly 15 golf but the temperature is about 1 . 5 degrees Kelvin or in superconducting since the
plastic trash removal of a magnet into appearance on certain such that this current now account for all the plants that cannot meet its sense and in many ways consistent with lenses and territories lost current encounters personal reasons it remained the persistent current everything once before small vomited brought down Porter led have whose temperature is 4 . 2 degrees Kelvin the superconducting transition of led occurred at 7 . 2 3 in 0 magnetic field even in the field of a magnet the bullring superconducting at 4 4 . 2 degrees supra currents are set up on the surface of the polls which shielded led from the blocks they are in use electromagnetic green and finances lost the magnet is the of the superconductors we're dealing with a quantum-mechanical state which dominates the electrical and magnetic properties of the solid in the vault our next experiment will shed some light on the nature of this state it turned out that the conduction electrons become bound to each other during the superconducting transition I have on this last flight what is known as an electron tunneling junctions metallic cross
was laid down in 3 steps 1st of thin-film aluminum was deposited on the last by the technique of evaporating metal a hot wire on the back of this film was then restored to the atmosphere at room temperature is produced at the film of aluminum oxide although the unleashed finally another middle film with evaporated crosswise over the oxide layer the 2nd medal amid the upside of aluminum is not conducting such a dielectric at the junction where the 2 men don't lie about each other it separated we are justified in thinking of a parallel plate condenser if we applied voltage between the mentally ill or condensed place we would expect in terms of elementary classical physics the little condenser to become charge and that is that only if the voltage were high enough to produce a breakdown in the dielectric should we expect an appreciable floor current across the oxide we have a lot of sorts of variable voltage across the junction of jury connected across the junction from the other side what we wish to test is whether current when voltage is applied in the voltage produces current across the junction it could be detected differing were inserted like this then if there is a current BII dropped between terminal agent we will be proportional to its we shall apply these terminals to the Y axis of an XY require the junction voltage will be applied to the expected to junctions in the lower the temperature of liquid helium is about 4 . 2 degrees Kelvin the junction is above the surface of the liquid at temperatures sufficiently about 4 4 . 2 degrees Kelvin so that gold medal films are normally conducted in 0 magnetic field the transition temperature of LED lies at 7 . 2 degree for aluminum it lies at 1 . 2 5 including the Walter Jackson's is
calibrated in nearly we set the recording them down on paper and now apply slowly increasing voltage to the junction by sliding the potent Jonathan on the potential dividing sister as you can see the application of the voltage is accompanied by current impact the relation of voltage to current new unit this curve which over things slow the junction does not act like a condenser at all how does the current get across the oxide the explanation for is quantum-mechanical electrons are free to move around in each of the 2 layers of metal but the oxide layer is not conducting an intensive area thinking in terms of classical physics electrons headed for the oxide layer are reflected if they don't possess enough energy to overcome the height of the potential of the area which the oxide represent they can cross the oxide layer at the energy exceeds the barrier and we might call this dielectric breakdown but quantum mechanically apply Article may cross the potential barrier even if energy is less than the height of the various this process is called funding it is due to the fact that I'm moving electron like all particles and waves character we may consider a group of electrons in the medley here heading for the oxide at the way because the broken line represent electrons over and energy the wavelength is given by the moment the amplitude represent the number of electrons at that energy of the disturbance of this kind in the electrons may penetrate the oxide layer but since their energy is less than the barrier potential they cease to be like in this layer rather the disturbance to carry out the other side of low amplitude disturbance awaiting again of the same length and energy can be joined to the line the low amplitude signifies that only a fraction of the electrons can come through the thin barrier the more will come keep in mind that in our junction the oxide layer is very thin now has been told before there are electron in each of the 2 metallic layers which are free to move around these electrons occupy a certain set of closely spaced energy level if the temperature is close to the absolute material there to a 1st approximation these levels are occupied only up to a certain maximum energy level called the Fermi energy from the energy is not in general the saying in different metals considered separately but in the junction we have here the metals are in contact with the oxide layer by tunneling process planning that take place and so on a number of electrons move from 1 side to the other until the highest occupied level equalized at this point no further exchange of handling electrons will occur a consequence of this transfer of electrons is to raise the electrical potential 1 middle layer and lower than that of the other the difference in potential of this call the contact potential next let us suppose that we apply are slowly increasing voltage across the junction from the outside with the help of a battery this is exactly what we did in our experiments it further raises the energy of each electrons on 1 side while lowering the energy of the electrons on the other now we can see why in our last experiment current flowed through the oxide layer when the voltage was applied across the junction tunneling and continues as long as the voltage is applied electrons Connell and occupied and the excited levels on the other side of the external battery meeting them again another stumbled into their places and so on we now repeat the tunneling experiment but this time we will do it at 1 and a half degrees Kelvin with junction submerged in liquid helium at 1 and a half degrees that led film is superconducting the aluminum Phil however remains a normal conducted at this temperature as a matter of fact we're using it here only as an ordinary conductor aluminum was chosen because it is easy to evaporated from the hot water in vacuo and because it follows a strong and thin aluminum oxide layer when restored to their again
we applied a slowly increasing voltage to the junction which is now at 1 . 5 agreeing it is that now hardly any electron tunnel through the junction until at about 1 and a quarter million below a sharp rise current workers are we to believe that the oxide layer becomes perfect dielectric allowing no funding when it was schools from about 7 to 1 . 5 degrees Kelvin and that it broke down when the condenser was charged to a value of about 1 and a quarter million people the answer is of course no 1 can show in fact that the properties of the oxide were not appreciably altered by this cooling whereas we do know that the letter will change from a normal with superconductors and we've shown earlier this is the condition of the energy levels on either side of the junction when the ladies normally conducted however when the lectern superconducting an increasing external voltage is applied there is no tumbling until this voltage reaches about 1 and a quarter million people can we conclude in the letter later there are no energy levels available 1st that some energy levels are forbidden to the electron In the letter after it made its transition to the superconducting state to put it another way there is a gap in the electron energy levels for superconducting that when we applied increasing external voltage now there will be at 1st no possibility for channeling until the voltage is large enough to bring occupied levels opposite unoccupied level then common current flows and will continue to do so as long as we applied external voltage of sufficient size as we have seen in our experiment the minimum required voltage is of the order of municipal such energy that have also been found in most of the other superconductors in all cases they're small and the order of newly elected from people in the normally conducting state of a metal the conduction electrons behave like free particles that means they behave as if they were freed from each other the strong Ghulam repulsion between any 2 2 of them is screened out by the aggregate of all the other in the behavior like free particles also means that the electrons are not bound by the positive ions of the embattled critical matters in the superconducting state of a mental we find gap In the electron energy level now this indicates that the conduction electrons bound state the smallness of the gap signified that the attractive for which binds them if we we may rule out of binding force which hold the the electrons to the public black designer is that were the case the material world like have higher rates of we know the superconductor that you're always so we conclude that the electrons are bound to each other all last
word may not yet have been booked the most successful model proposed robot assumed that the electrons become bound pair is heading back been experimentally proved unless and until end it was shown that the magnetic flux tracked by superconducting is quantized during views were very small and the quantum of trapped flux was actually measured at magnitude clearly indicated that carrier is an electron pair now
the electron has been quantum number 1 half considerate quantum mechanical system of particles with have introduced into the family exclusion principle then requires that each particle of the system must be in a different quantum-mechanical state this is just what we need to be the case for the conduction electrons in the normal manner on the other hand the requirement does not apply to a system of particle pollution in quantum number is an integer in that case there is no limit on the occupation number of each possible states in his theory of superconductivity based on bound electron pairs it is shown that the most likely appear to form is that 1 in which the paired electrons have offices self of has 0 Spain and even 0 is an integer in other words pair In some ways behaves like a particle indigenous in this line of reasoning makes the pairing of electrons a plausible model for superconductivity let me put it this way I so it follows that a piece of metal that conduction electrons tend to become bound in such areas then the the temperature of the metal is sufficiently low all these bonds pared may form in macroscopic numbers into a giant quantum-mechanical many particle state of low energy which may extend over the whole sample from this point of view the superconducting phenomena some of which we showed you earlier become the observable properties of this thing 1 proposal for the
nature of the force which tends to bind conduction electrons to each other in a medal in the following that involves the lattice of positive now this letter it is not perfectly Richard the negative charge on the passing electrons start a vibration in nearby politicking on the electron moved swiftly by the ion is heavy and vibrates relatively slowly a little later a 2nd electrons passing the vibrating is affected by this local elected distortion of positive charge if it arrives at the right pace of the vibrations the forced on it will be in effect an attraction toward the 1st elected from the theory based on this model has had considerable success in explaining the properties of superconductors
Magnetisches Dipolmoment
Satz <Drucktechnik>
Gigant <Bagger>
Magnetische Kraft
Angeregtes Atom
Flüssiger Brennstoff
Astronomisches Fenster
Magic <Funkaufklärung>
Maßstab <Messtechnik>
Discovery <Raumtransporter>
Relativistische Mechanik
Elektrische Ladung
Negativer Widerstand
Schwarzes Loch
Verzerrung <Elektrotechnik>
Elektrische Spannung
Thermodynamische Temperaturskala
Schwache Lokalisation
Kette <Zugmittel>
Negativ <Photographie>
Elektronische Medien
Elektrischer Leiter
Dünne Schicht
Elektrolytische Leitfähigkeit
Direkte Messung
Elektrischer Strom
Proof <Graphische Technik>
Scheinbare Helligkeit
Dipol <Nachrichtentechnik>
Widerstand <Elektrotechnik>


Formale Metadaten

Titel An Introduction to Superconductivity
Alternativer Titel Einführung in die Supraleitung
Autor Leitner, Alfred
Lizenz CC-Namensnennung 3.0 Unported:
Sie dürfen das Werk bzw. den Inhalt zu jedem legalen Zweck nutzen, verändern und in unveränderter oder veränderter Form vervielfältigen, verbreiten und öffentlich zugänglich machen, sofern Sie den Namen des Autors/Rechteinhabers in der von ihm festgelegten Weise nennen.
DOI 10.5446/17738
IWF-Signatur W 931
Herausgeber Michigan State University
Erscheinungsjahr 1965
Sprache Englisch
Produzent G. William Hughes

Technische Metadaten

IWF-Filmdaten Film, 16 mm, LT, 528 m ; SW, 48 1/2 min

Inhaltliche Metadaten

Fachgebiet Physik, Technik
Abstract Demonstration und Erläuterung zur Supraleitung Typ I.: spontanes Absinken des elektrischen Widerstandes eines Zinndrahtes beim Abkühlen (Unterschreiten der Übergangstemperatur); persistierender Strom in einem Zinnring. Magnetische Effekte, erniedrigte Übergangstemperatur durch starkes Magnetfeld, Meißner-Ochsenfeld-Effekt. Quantenmechanische Effekte (Energie-Defekt).
Looks at measurements indicating the disappearance of electrical resistance in the transition to the superconducting state. Considers the temperature of transition, the exclusion of magnetic flux (Meissner effect), shielding currents, the "floating magnet," and persistent currents. Includes a discussion of the tunnel effect in connection with experiments involving an electron tunneling junction.
Schlagwörter Zinn / Supraleiter
Meissner-Ochsenfeld effect
tin / superconductor

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