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Some experiments on a supraconductive alloy in a magnetic field
SOME E X P E R I M E N T S ON A SUPRACONDUCT.IVE ALLOY IN A MAGNETIC F I E L D b y J. M. C A S I M I R - J O N K E R and W. J. D E H A A S Communication No. 237c from the Kamerlingh Onnes Laboratory Leiden
Summary A new method for detecting electrical resistance of a supraconductor in a magnetic field is described. The method has been applied to the study of the magnetic transition curve of a Pb Tl-alloy.
§ 1. Introduction. In a recent p a p e r 1) the authors have studied the magnetic b e h a v i o u r of s u p r a c o n d u c t i v e alloys. I t was found, t h a t at a r e l a t i v e l y low value of an e x t e r n a l magnetic field, lines of force begin to p e n e t r a t e into the alloy. T h e first trace of electric resistance of the same specimen, however, is observed at a m u c h higher value of the magnetic field. These results were confirmed a n d e x t e n d e d b y o t h e r investigators ~). Since a resistance less t h a n 10-6 ~ is not easily d e t e c t e d when m e a s u r e d in the usual w a y with a D i e s s e 1 h o r s t c o m p e n s a t i o n a p p a r a t u s and a Z e r n i k e g a l v a n o m e t e r , we tried to work out a more sensitive m e t h o d for detecting electrical resistance. T h e main idea is to s t u d y the dying out of a persisting c u r r e n t in a ring consisting p a r t l y of Pb-wire p a r t l y of an alloy, when the alloy is placed in a longitudinal magnetic field. § 2. The apparatus. Fig. l is a schematical drawing of the apparatus. A persisting c u r r e n t can be p r o d u c e d in the closed circuit consisting of a ring of Pb-wire a n d a cylindrical rod (a) of a P b Tl-alloy. T h e alloy is s u r r o u n d e d b y a coil (c) of Pb-wire (having 4 layers of a p p r o x i m a t e l y 40 windings per cm), which produces a longitudinal field of 216 G per A. T h e coil and the alloy are enclosed in a leaden b o x (b). - -
9 3 5
- -
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j . M . CASIMIR-JONKER AND W. J. DE HAAS
Since in the neighbourhood of the transition point of the alloy the magnetic threshold value of Pb is much larger than the magnetic threshold value of the alloy, no heat is produced by the current in the coil c, even when the field of the coil is well above the threshold value of the alloy, while the leaden box prevents any disturbing action of the field of the coil. Actually the wires connecting the ring and the alloy do not pass through the bottom of the box, but through the same opening as the leads C and D. The leads of the coil c are not shown in the figure. The current flowing through the Pb-ring is measured by means of a magnetometer outside the D e w a r
11°
vese
I
The magnetometer was of a very simple
cii IIB c b_ °
PU
type: two magnetic needles are mounted on a thin glass rod 30 cm apart; a small mirror was attached to the rod and this system was hung on a cocoon thread. The system was approximately astatic. The whole magnetometer was surrounded by a glass tube, placed as close as possible to the D e w a r vessel. The upper needle faced the Pb-ring, the lower one was placed in the middle of a H e 1mh o 1 t z coil. The position of the H e lmh o 1 t z coil was adjusted in such a way t h a t the direction of its field made an
Fig. 1. Schematical drawing angle of about 170 ° with the direction of of the apparatus, the field produced by the current of the Pb-ring. The position of the magnetometer is read in the ordinary way by means of scale and telescope. The current in the Pb-ring is measured by detelTnining the current which has to be sent through the H elmh o 1 t z coil in order to bring back the mirror to its original position. The sensitivity of the measurement depends on the angle between the direction of the fields produced by the Pb-ring and the H e 1 mh o 1 t z coil. In our case a change in the current of the Pb-ring of two milliamperes could easily be observed (the absolute accuracy, however, is somewhat less).
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§ 3. The measurements. The arrangement described can be used in various ways. a) D e t e r m i n a t i o n of the tl~resholdvalue of the current in zero magnetic field. When the whole circuit is cooled down to a temperature below the transition point of the alloy (3.78°K) and a current in the direction from C to D is switched on, the distribution of current will be such that the total magnetic flux through the circuit remains zero. Since the part of the circuit A B contributed far less to the self-induction of the circuit than the Pb-ring, the main part of the current will passthrough the alloy. When the current is increased the field measured by the magnetometer will at first increase proportionally to the total current until the current through the alloy reaches its critical value. From then on no further increase of the current through the alloy will take place and the magnetic field of the Pb-ring will increase more rapidly than before. The same method m a y of course be used for determining the critical current in a constant magnetic field produced by the coil c. It m a y be pointed out that our way of determining the critical current has certain advantages above the usual procedure since a production of J o u 1 e heat (which often leads to melting of the wire) is avoided. b) D e t e r m i n a t i o n of the thresholdvalue of t h e m a g n e t i c f i e 1 d. At a temperature below the transition point of the alloy a current in the direction from C to D is switched on and kept constant. The magnetic field produced by the coil c is then gradually increased; as long as this field remains below the threshold value of the alloy no change in the distribution of current will take place, but when the supraconductivity of the alloy is disturbed the current through the Pb-ring will increase. c) Though method b was used in some of our experiments another method for determining the magnetic threshold value was found to be more efficient. In this method a persisting current is started in the closed circuit. This m a y be done in two different ways. 1. At a temperature above the transition point of the alloy a current is switched on; this current will then pass completely through the Pb-ring and the distribution of current will not change when the temperature is decreased to below the transition point. If then the external current is switched off the total flux through the supra-
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M. CASIMIR-JONKER AND W. J. DE HAAS
conductive circuit will have to remain constant and a persisting current will be produced. The geometrical conditions of our circuit were such that this persisting current is about 85% of the initial external current. 2. At a temperature below the transition point of the alloy, a persisting current m a y be started b y successively disturbing the supraconductivity of the alloy b y a magnetic field (produced b y c), sending a current from C to D, removing the magnetic field and finally, switching off the external current. This procedure is much more convenient than that described above, the only disadvantage being that the alloy is not in a virgin state. We shall see in the next section that in most cases this is of no importance. Once a persisting current has been started the current through the coil c is gradually increased. At first this does not appreciably affect the magnetic field of the Pb-ring b u t when the magnetic field is so strong that the supraconductivity of the alloy is disturbed, the persisting current will die out. If R is the resistance of the circuit, L the self-induction and t the time in seconds, then the current i will decrease according to the formula i -~ i 0 e-R/L't
We have not measured L, but it m a y be estimated from the dimensions of the circuit; it is certainly < 10--~ henry. If now e.g. the current is not changed b y more than 10°/0 during 100 sec. it follows that R < 10-9~ For strong persisting currents (> 100 mA) in general, the accuracy was considerably greater than that assumed in this example. § 4. The results. All our measurements were carried out with the same cylindrical rod of Pb Tl-alloy, diameter 0.35 cm, containing 64.8% T1 which was used in our former experiments. a. We have tried to determine the threshold value of the current at a temperature of3.74°K which is 0.04 degrees below the transition point of the alloy. It results from our measurements that the threshold value lies between 1.05 and 1.20 A, the most probable value being I. 14 A. In order to avoid overloading of the leads the measurements
SOME E X P E R I M E N T S ON A S U P R A C O N D U C T I V E ALLOY
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had to be carried out very quickly and are therefore not very accurate. It is of interest to remark t h a t the magnetic threshold value at this temperature is about 15 G, whereas the field produced by the current is 1.3 G. The field of the current is of the same order of magnitude as the "penetration value" of the field. K e e s o m 3) and R j a b i n i n and S c h u b n i k o w 4 ) found similar results. b. This method was used at a temperature of 3.75°K. The total current had the values 56, 120, 200 and 300 mA of which 7/8 passed through the alloy. The strength of the magnetic field of the coil that gives rise to an appreciable change in the distribution of current was found to depend very strongly on the value of the current. In a large region of t h e m a g n e t i c field there corresponds to every value of the field a current which can pass through the alloy without producing a potential difference. The current distribution adjusts itself in such a way that the current through the alloy does not surpass the critical value and the current through the Pb-ring, which is found after waiting several minutes, is a continuous function of the field of the coil and not a discontinuous function as would be the case if the threshold value did not depend on the current. The threshold value foi zero current was found to be 44 G 4- 2 G, whereas for a current of 200 mA this value is 20 G q- 2 G. The penetration value at this temperature is of the order of magnitude of 2G. c. The most reliable results were obtained by method c. The chief advantage of this method is that the current through the Pb-ring which is measured by means of the magnetometer is equal to the current through the alloy. The current through the alloy is therefore proportional to the compensation current and the measurements give directly the threshold value of the field as a function of the current through the alloy. A first series of measurements was carried out at a temperature of 3.36°K. The persisting current was produced by method 1. It was found that the persisting current was not affected by a magnetic field of about three times the penetration value; the current remained constant within 1% during 6 minutes; it follows that the resistance was < 3 × 10 - l l f~. Disturbance of the persisting current took place at a field above 200 G but this field was too strong to be determined accurately.
940
j.M. CASIMIR-JONKER AND W. J. DE HAAS
It follows from these experiments t h a t it does not m a t t e r whether the alloy is in a virgin state or not, since for fields well above the penetration value there is no difference between virginal and nonvirginal states. Later on the persisting currents were therefore started by means of method 2. Fig. 9. shows the results of two series of measurements at a temperature of 3.65°K. In weak fields the persisting current remains practically constant (the slight increase of the current is probably due to a magnetic field of the leads of the coil or to incomplete 600
A
i .
H---
60
_
_
.
.
.
.
.
.
.
.
.
.
.
.
too
.
.
.
.
.
,
Is0
ooc
Fig. 2. Persisting current as a function of the magnetic field. screening off of the leaden box). At a field > 60 G for the upper curve (current o,~ 500 mA) and at a field > 95 G for the lower curve (current c,o 200 mA) the current decreases and adjusts itself to a lower value at which it remains constant. The points given in fig. 2 are the values of the current in the Pb-ring, after waiting for several minutes, as a function of the magnetic field. The currents plotted did not change by more than 2% during a period of 3 minutes. They correspond to a resistance < 2 x l0 - l ° .~. Only for the very weak measuring currents the upper limit of the resistance is somewhat higher but certainly < l0 -9 ~.
SOME EXPERIMENTS
ON A SUPRACONDUCTIVE
ALLOY
-941
The values of the current plotted in the figure are not the true values but it was assumed that the persisting current was equal to the external current by means of which it was produced. In order to obtain the true values the values of the curve must be multiplied by a factor 7/8. §5. The magnetic transition curve. We have determined the magnetic transition curve in the ordinary way at a temperature of 3.55°K for different measuringcurrents.The accuracy of the measurement was 10-6 f2. The results were as follows: with a measuring current of 300 mA the first trace of resistance was found at a field below 195 G. This resistance increased very slowly until the field reached the value of 265 G; from then on the increase was much faster. For 10 mA measuring current no resistance was found below 270 G, from then on the transition curve is almost identical with the steep part of the curve for 300 mA measuring current. This is in agreement with the results described above. It must be kept in mind, however, t h a t a resistance of 10-6 f~. would not have been found with a measuring current of 10 mA. Our measurements with the persisting current show, however, that no such resistance is present. The magnetic threshold values for measuring current zero--being the values of the magnetic field at which the first trace of resistance appears - - are given in the following table. TABLE
I
T
H
method
3.55 s 3.65 3.75
275 4- 3 G 160 4- 3 G 44 4- 2 G
Comp. apparatus c b
The corresponding points in a H vs. T diagram lie rather accurately on a straight line through the thermal transition point. § 6. Discussion o/ the results. Though our experiments are of a preliminary nature it seems to us t h a t the following results have been established with certainty: a) In a magnetic field slightly above the penetration value there is no electrical resistance > 5 × 10-11 f2.
942
SOME E X P E R I M E N T S
ON A S U P R A C O N D U C T I V E A L L O Y
b) The magnetic threshold value depends very strongly on the measuring current. We m a y arrive at a qualitative understanding of the influence of the measuring current if we assume that in a supraconductive alloy or at any rate in a supraconductive alloy in a magnetic field above -the penetration value only part of the materialisin a supraconductive state, so that the current has to follow definite paths.This assumption has often been discussed in this laboratory in the past s) and several authors have recently tried to work out a description of the properties of a supraconductive alloy based on this idea 6). If we assume that the cross section of the supraconductive paths is small compared with the total cross section of the alloy then the current density along these paths will be large even for weak measuring currents and the maximum strength of the total current that can be conducted without producing a resistance will be very low. The curve of fig. 2 m a y be explained if we assume that the total cross section of available paths decreases with increasing field. Further experiments will have to show whether this explanation is correct. We intend to apply the method described in this paper to the investigation of several other problems, for example of the question whether or not the transverse magnetic threshold value for tin depends on the strength of the measuring current. Received J u l y 29, 1935.
LITERATURE l) 2) 3) 4) 5) 6)
W.J. de Haas and J. M. C a s i m i r - J o n k e r , C o m m u n . L e i d e n N o . 233c;: Proc. roy. Acad. A m s t e r d a m 38, 2, 1935. J.N. Rjabinin a n d L . W. S c h u b n i k o w , Nature L o n d o n 1 3 5 , 5 8 1 , 1 9 3 5 . . K. M e n d e l s s o h n and J. R. M o o r e , Nature London 135, 826, 1935. W . H . K e e s o m , C o m m u n . Leiden No. 2341; Physica 's-Gray. 2, 35, 1935. J.N. Rjabinin and L. W. S c h u b n i k o w , 1.c. Compare for example W. J. de H a a s a n d H. B r e m m e r , Pr0c. roy. Acad.. A m s t e r d a m 35, 323, 1932 and W. J. d e H a a s , Leipziger Vortr~ge, 1933. IC M e n d e l s s o h n and J. R. M o o r e , 1.c. C . J . G o r t e r, Physica 's-Grav. 2, 449, 1935. F. L o n d o n . in ~rint.
Summary A new method for detecting electrical resistance of a supraconductor in a magnetic field is described. The method has been applied to the study of the magnetic transition curve of a Pb Tl-alloy.
§ 1. Introduction. In a recent p a p e r 1) the authors have studied the magnetic b e h a v i o u r of s u p r a c o n d u c t i v e alloys. I t was found, t h a t at a r e l a t i v e l y low value of an e x t e r n a l magnetic field, lines of force begin to p e n e t r a t e into the alloy. T h e first trace of electric resistance of the same specimen, however, is observed at a m u c h higher value of the magnetic field. These results were confirmed a n d e x t e n d e d b y o t h e r investigators ~). Since a resistance less t h a n 10-6 ~ is not easily d e t e c t e d when m e a s u r e d in the usual w a y with a D i e s s e 1 h o r s t c o m p e n s a t i o n a p p a r a t u s and a Z e r n i k e g a l v a n o m e t e r , we tried to work out a more sensitive m e t h o d for detecting electrical resistance. T h e main idea is to s t u d y the dying out of a persisting c u r r e n t in a ring consisting p a r t l y of Pb-wire p a r t l y of an alloy, when the alloy is placed in a longitudinal magnetic field. § 2. The apparatus. Fig. l is a schematical drawing of the apparatus. A persisting c u r r e n t can be p r o d u c e d in the closed circuit consisting of a ring of Pb-wire a n d a cylindrical rod (a) of a P b Tl-alloy. T h e alloy is s u r r o u n d e d b y a coil (c) of Pb-wire (having 4 layers of a p p r o x i m a t e l y 40 windings per cm), which produces a longitudinal field of 216 G per A. T h e coil and the alloy are enclosed in a leaden b o x (b). - -
9 3 5
- -
936
j . M . CASIMIR-JONKER AND W. J. DE HAAS
Since in the neighbourhood of the transition point of the alloy the magnetic threshold value of Pb is much larger than the magnetic threshold value of the alloy, no heat is produced by the current in the coil c, even when the field of the coil is well above the threshold value of the alloy, while the leaden box prevents any disturbing action of the field of the coil. Actually the wires connecting the ring and the alloy do not pass through the bottom of the box, but through the same opening as the leads C and D. The leads of the coil c are not shown in the figure. The current flowing through the Pb-ring is measured by means of a magnetometer outside the D e w a r
11°
vese
I
The magnetometer was of a very simple
cii IIB c b_ °
PU
type: two magnetic needles are mounted on a thin glass rod 30 cm apart; a small mirror was attached to the rod and this system was hung on a cocoon thread. The system was approximately astatic. The whole magnetometer was surrounded by a glass tube, placed as close as possible to the D e w a r vessel. The upper needle faced the Pb-ring, the lower one was placed in the middle of a H e 1mh o 1 t z coil. The position of the H e lmh o 1 t z coil was adjusted in such a way t h a t the direction of its field made an
Fig. 1. Schematical drawing angle of about 170 ° with the direction of of the apparatus, the field produced by the current of the Pb-ring. The position of the magnetometer is read in the ordinary way by means of scale and telescope. The current in the Pb-ring is measured by detelTnining the current which has to be sent through the H elmh o 1 t z coil in order to bring back the mirror to its original position. The sensitivity of the measurement depends on the angle between the direction of the fields produced by the Pb-ring and the H e 1 mh o 1 t z coil. In our case a change in the current of the Pb-ring of two milliamperes could easily be observed (the absolute accuracy, however, is somewhat less).
SOME E X P E R I M E N T S O N A SUPRACONDUCTIVE ALLOY
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§ 3. The measurements. The arrangement described can be used in various ways. a) D e t e r m i n a t i o n of the tl~resholdvalue of the current in zero magnetic field. When the whole circuit is cooled down to a temperature below the transition point of the alloy (3.78°K) and a current in the direction from C to D is switched on, the distribution of current will be such that the total magnetic flux through the circuit remains zero. Since the part of the circuit A B contributed far less to the self-induction of the circuit than the Pb-ring, the main part of the current will passthrough the alloy. When the current is increased the field measured by the magnetometer will at first increase proportionally to the total current until the current through the alloy reaches its critical value. From then on no further increase of the current through the alloy will take place and the magnetic field of the Pb-ring will increase more rapidly than before. The same method m a y of course be used for determining the critical current in a constant magnetic field produced by the coil c. It m a y be pointed out that our way of determining the critical current has certain advantages above the usual procedure since a production of J o u 1 e heat (which often leads to melting of the wire) is avoided. b) D e t e r m i n a t i o n of the thresholdvalue of t h e m a g n e t i c f i e 1 d. At a temperature below the transition point of the alloy a current in the direction from C to D is switched on and kept constant. The magnetic field produced by the coil c is then gradually increased; as long as this field remains below the threshold value of the alloy no change in the distribution of current will take place, but when the supraconductivity of the alloy is disturbed the current through the Pb-ring will increase. c) Though method b was used in some of our experiments another method for determining the magnetic threshold value was found to be more efficient. In this method a persisting current is started in the closed circuit. This m a y be done in two different ways. 1. At a temperature above the transition point of the alloy a current is switched on; this current will then pass completely through the Pb-ring and the distribution of current will not change when the temperature is decreased to below the transition point. If then the external current is switched off the total flux through the supra-
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M. CASIMIR-JONKER AND W. J. DE HAAS
conductive circuit will have to remain constant and a persisting current will be produced. The geometrical conditions of our circuit were such that this persisting current is about 85% of the initial external current. 2. At a temperature below the transition point of the alloy, a persisting current m a y be started b y successively disturbing the supraconductivity of the alloy b y a magnetic field (produced b y c), sending a current from C to D, removing the magnetic field and finally, switching off the external current. This procedure is much more convenient than that described above, the only disadvantage being that the alloy is not in a virgin state. We shall see in the next section that in most cases this is of no importance. Once a persisting current has been started the current through the coil c is gradually increased. At first this does not appreciably affect the magnetic field of the Pb-ring b u t when the magnetic field is so strong that the supraconductivity of the alloy is disturbed, the persisting current will die out. If R is the resistance of the circuit, L the self-induction and t the time in seconds, then the current i will decrease according to the formula i -~ i 0 e-R/L't
We have not measured L, but it m a y be estimated from the dimensions of the circuit; it is certainly < 10--~ henry. If now e.g. the current is not changed b y more than 10°/0 during 100 sec. it follows that R < 10-9~ For strong persisting currents (> 100 mA) in general, the accuracy was considerably greater than that assumed in this example. § 4. The results. All our measurements were carried out with the same cylindrical rod of Pb Tl-alloy, diameter 0.35 cm, containing 64.8% T1 which was used in our former experiments. a. We have tried to determine the threshold value of the current at a temperature of3.74°K which is 0.04 degrees below the transition point of the alloy. It results from our measurements that the threshold value lies between 1.05 and 1.20 A, the most probable value being I. 14 A. In order to avoid overloading of the leads the measurements
SOME E X P E R I M E N T S ON A S U P R A C O N D U C T I V E ALLOY
'939
had to be carried out very quickly and are therefore not very accurate. It is of interest to remark t h a t the magnetic threshold value at this temperature is about 15 G, whereas the field produced by the current is 1.3 G. The field of the current is of the same order of magnitude as the "penetration value" of the field. K e e s o m 3) and R j a b i n i n and S c h u b n i k o w 4 ) found similar results. b. This method was used at a temperature of 3.75°K. The total current had the values 56, 120, 200 and 300 mA of which 7/8 passed through the alloy. The strength of the magnetic field of the coil that gives rise to an appreciable change in the distribution of current was found to depend very strongly on the value of the current. In a large region of t h e m a g n e t i c field there corresponds to every value of the field a current which can pass through the alloy without producing a potential difference. The current distribution adjusts itself in such a way that the current through the alloy does not surpass the critical value and the current through the Pb-ring, which is found after waiting several minutes, is a continuous function of the field of the coil and not a discontinuous function as would be the case if the threshold value did not depend on the current. The threshold value foi zero current was found to be 44 G 4- 2 G, whereas for a current of 200 mA this value is 20 G q- 2 G. The penetration value at this temperature is of the order of magnitude of 2G. c. The most reliable results were obtained by method c. The chief advantage of this method is that the current through the Pb-ring which is measured by means of the magnetometer is equal to the current through the alloy. The current through the alloy is therefore proportional to the compensation current and the measurements give directly the threshold value of the field as a function of the current through the alloy. A first series of measurements was carried out at a temperature of 3.36°K. The persisting current was produced by method 1. It was found that the persisting current was not affected by a magnetic field of about three times the penetration value; the current remained constant within 1% during 6 minutes; it follows that the resistance was < 3 × 10 - l l f~. Disturbance of the persisting current took place at a field above 200 G but this field was too strong to be determined accurately.
940
j.M. CASIMIR-JONKER AND W. J. DE HAAS
It follows from these experiments t h a t it does not m a t t e r whether the alloy is in a virgin state or not, since for fields well above the penetration value there is no difference between virginal and nonvirginal states. Later on the persisting currents were therefore started by means of method 2. Fig. 9. shows the results of two series of measurements at a temperature of 3.65°K. In weak fields the persisting current remains practically constant (the slight increase of the current is probably due to a magnetic field of the leads of the coil or to incomplete 600
A
i .
H---
60
_
_
.
.
.
.
.
.
.
.
.
.
.
.
too
.
.
.
.
.
,
Is0
ooc
Fig. 2. Persisting current as a function of the magnetic field. screening off of the leaden box). At a field > 60 G for the upper curve (current o,~ 500 mA) and at a field > 95 G for the lower curve (current c,o 200 mA) the current decreases and adjusts itself to a lower value at which it remains constant. The points given in fig. 2 are the values of the current in the Pb-ring, after waiting for several minutes, as a function of the magnetic field. The currents plotted did not change by more than 2% during a period of 3 minutes. They correspond to a resistance < 2 x l0 - l ° .~. Only for the very weak measuring currents the upper limit of the resistance is somewhat higher but certainly < l0 -9 ~.
SOME EXPERIMENTS
ON A SUPRACONDUCTIVE
ALLOY
-941
The values of the current plotted in the figure are not the true values but it was assumed that the persisting current was equal to the external current by means of which it was produced. In order to obtain the true values the values of the curve must be multiplied by a factor 7/8. §5. The magnetic transition curve. We have determined the magnetic transition curve in the ordinary way at a temperature of 3.55°K for different measuringcurrents.The accuracy of the measurement was 10-6 f2. The results were as follows: with a measuring current of 300 mA the first trace of resistance was found at a field below 195 G. This resistance increased very slowly until the field reached the value of 265 G; from then on the increase was much faster. For 10 mA measuring current no resistance was found below 270 G, from then on the transition curve is almost identical with the steep part of the curve for 300 mA measuring current. This is in agreement with the results described above. It must be kept in mind, however, t h a t a resistance of 10-6 f~. would not have been found with a measuring current of 10 mA. Our measurements with the persisting current show, however, that no such resistance is present. The magnetic threshold values for measuring current zero--being the values of the magnetic field at which the first trace of resistance appears - - are given in the following table. TABLE
I
T
H
method
3.55 s 3.65 3.75
275 4- 3 G 160 4- 3 G 44 4- 2 G
Comp. apparatus c b
The corresponding points in a H vs. T diagram lie rather accurately on a straight line through the thermal transition point. § 6. Discussion o/ the results. Though our experiments are of a preliminary nature it seems to us t h a t the following results have been established with certainty: a) In a magnetic field slightly above the penetration value there is no electrical resistance > 5 × 10-11 f2.
942
SOME E X P E R I M E N T S
ON A S U P R A C O N D U C T I V E A L L O Y
b) The magnetic threshold value depends very strongly on the measuring current. We m a y arrive at a qualitative understanding of the influence of the measuring current if we assume that in a supraconductive alloy or at any rate in a supraconductive alloy in a magnetic field above -the penetration value only part of the materialisin a supraconductive state, so that the current has to follow definite paths.This assumption has often been discussed in this laboratory in the past s) and several authors have recently tried to work out a description of the properties of a supraconductive alloy based on this idea 6). If we assume that the cross section of the supraconductive paths is small compared with the total cross section of the alloy then the current density along these paths will be large even for weak measuring currents and the maximum strength of the total current that can be conducted without producing a resistance will be very low. The curve of fig. 2 m a y be explained if we assume that the total cross section of available paths decreases with increasing field. Further experiments will have to show whether this explanation is correct. We intend to apply the method described in this paper to the investigation of several other problems, for example of the question whether or not the transverse magnetic threshold value for tin depends on the strength of the measuring current. Received J u l y 29, 1935.
LITERATURE l) 2) 3) 4) 5) 6)
W.J. de Haas and J. M. C a s i m i r - J o n k e r , C o m m u n . L e i d e n N o . 233c;: Proc. roy. Acad. A m s t e r d a m 38, 2, 1935. J.N. Rjabinin a n d L . W. S c h u b n i k o w , Nature L o n d o n 1 3 5 , 5 8 1 , 1 9 3 5 . . K. M e n d e l s s o h n and J. R. M o o r e , Nature London 135, 826, 1935. W . H . K e e s o m , C o m m u n . Leiden No. 2341; Physica 's-Gray. 2, 35, 1935. J.N. Rjabinin and L. W. S c h u b n i k o w , 1.c. Compare for example W. J. de H a a s a n d H. B r e m m e r , Pr0c. roy. Acad.. A m s t e r d a m 35, 323, 1932 and W. J. d e H a a s , Leipziger Vortr~ge, 1933. IC M e n d e l s s o h n and J. R. M o o r e , 1.c. C . J . G o r t e r, Physica 's-Grav. 2, 449, 1935. F. L o n d o n . in ~rint.