A Simple Demonstration of High TcSuperconductive Pow* .. Roger Baker 1303 Bentwood, Austin, TX 76722 James C. Thompson Department of Physics, University of Texas, Austin, TX 78712 Expulsion of a magnetic field (Meissner effect) and the conseauent diamaenetism is one manifestation of superconducti\,'ity. Since the new copper oxide-tlased superfondurtorsare produrrd in the formof powders it turnsout that the .Mris~nereffect is much more easily demonstrated than is h i ~ hrundurtivity. This note descrit~esa simple demonstration that also provides a way to determine if a given sample contains even a small frartion of superconductinp material. The rmulsion ofthe oowder from a maenetic field is inrlica" tive of superconductivity. The procedure follows the standard elementary demonstration used to map out a magnetic field. In the usual demonstration, one sprinkles iron filings onto a nonmagnetic surface over a magnet. The filings are magnetized and oriented by the field. The pattern of filings makes the regions of high and low field visible. In the present case, one sprinkles the superconductor powder into a small Dewar flask containing liquid nitrogen and a permanent magnet. In contrast to iron, the superconducting powder avoids high field regions leaving a bare spot close to the magnet. For a minimum display, it is possible to use a piece of white paper on top of an ordinary "refrigerator" ceramic magnet. Only a little refinement yields a cleaner result.
The photos were obtained with a magnet constructed of 16 small ceramic magnets glued between two steel sheets with silicone rubber c i m e n t r ~2 X 40-mm gap was formed by cutting one sheet in the center. The bottom of a soft drink can was used to contain the powder. The liquid level need he only a few mm above the bottom of the can. The can bottom is the circular obiect in each ~ h o t oIn . Fieure 1.the diaeonal white stripe is t i e high-fieldAregionand ;he grky smudie on either side is the powder. In Figure 2, the two irregular dark areas are formed from mixed powder after swirling the Dewar for several seconds;. again - the highest field is between them.
Refinements
First, i t is better to observe the powder pattern on a metal surface to avoid electrostatic charges. Second, i t is better to use a concave surface so as to concentrate the powder over the magnet. This also allows the suspension in liquid nitrogen to he gently rocked from side to side, much as gold is panned. Two percent of superconducting powder ground together with 98% of inert copper oxide can be detected in this way. Finally, using an arrangement yielding a gap hetween two poles increases the field. Powdered material is snrinkled into the dish from ahove the liquid surface. If the powder is totally superconductive, then it will arrange cleanlv on o.~.o o s i t esides of the aaD .. . (Fig. 1). I f the powderk .illghtl; contaminated with foreign material. then the nmsu~erconductiveort ti on will fall inside the gapwith the remaiider clearly delineated a t thesides. If the powder contains only a small percentage of superconducting material (2% in the photos in Fig. 2) no separation is achieved. but the Dewar mag then be rocked from side to side, causing the powder toshbw aslight tendency toarrange itself in two parallel lines to either side of the gap. This work was supported in part by the US NSF through MRG Gram No. DMR8418086. The photos were taken by K. I. Trappe. H. Steinfink provided the superconductive powder.
Figure 1. The superconductor demonstration performed using the bonom of a soda can as a Dewar.
Figure 2.
The demonstration after swirling the container.
Volume 64
Number 10 October 1987
853
