ABSOLUTE ZERO — SIMON 251 



balloon filled, the temperature fell to about 4°. I can also show 

 you another proof that we have reached the temperature region of 

 liquid helium. 



You have all heard of supraconductivity. This is the name given 

 to the phenomenon discovered by Kamerlingh Onnes, that at certain 

 temperatures some metals lose their electrical resistance altogether. 

 The temperatures at which this happens are very low; they all lie 

 below 10°. In a normal substance a current once induced would 

 disappear very quickly, as its energy would be absorbed within 

 about 1/1,000 of a second by its resistance. In the case of a supra- 

 conductor, however, there is no resistance, so that the current goes 

 on flowing indefinitely. 



In this lecture room. Professor McLennan ^ showed you this phe- 

 nomenon of a persistent current in a lead ring, which was brought 

 from Leiden immersed in liquid helium, so I need not speak about it 

 in detail now. For the moment we will only use it to show that we 

 really have the temperature of liquid helium. For this purpose a 

 lead ring is fixed within the apparatus. Lead becomes supraconduc- 

 tive at about 7°, so that now at 4° it is already in the supraconduct- 

 ing state. We will now induce a current within this ring.^ 



Before the current was induced a magnetic needle was not affected. 

 Now, you see that when I hold it a little above the ring it points 

 with its N. pole toward it. This indicates that there is a S. pole 

 of the magnetic dipole corresponding to the persistent current. 

 Bringing it below the ring, it naturally changes its direction. 



I can show you the existence of this current in yet another way. 

 I have here a small coil connected with a galvanometer. When I 

 bring it up to the apparatus, you see a ballistic deflection of the 

 galvanometer. This is due to the cutting of the magnetic lines of 

 force originating in the persistent current, and the magnitude of 

 this ballistic deflection is a measure of the intensity of the current. 

 So we have here a quantitative measure for the current, and we will 

 verify at the end of the lecture that it is steady. 



You see the great difference in outlay for the two experiments. 

 For generating 2,500° you have only to connect an electric lamp to 

 the mains; for generating —270° you have this decidedly compli- 

 cated apparatus — and yet this is certainly the most simple one in 

 existence. And we have not yet even taken into account that in the 

 second experiment we did not start at room temperature, but at 

 — 260°. This temperature was obtained by hydrogen which we 

 liquefied in the plant in the Clarendon Laboratory in Oxford, and 



2Proc. Roy. Inst., vol. 27, p. 446, 1932. 



' This was performed by switching on a magnetic field, higher than the threshold value 

 of lead at this temperature, so, with the field on, no persistent current flowed ; reducing 

 the field, a current is induced, which now persists as the substance is in the field zero. 



