Kerry Kuehn

A High-Resolution Thermometer for the Range 1.6 to 5 K

This paper presents a detailed design, a theoretical analysis, and experimental tests of a high-resolution thermometer for use in the temperature range from 1.6 to 5 K. The device uses a dc-SQUID magnetometer to determine the change in magnetization with temperature of a paramagnetic salt in a magnetic field. The field is provided by a small permanent magnet attached to the thermometer. Measurements of the sensitivity of the device agree well with the theoretical analysis. Near 2.17 K (the superfluid transition of4He at saturated vapor pressure) the thermometer has a specific sensitivity of 4000φ0/K Gauss. There it achieves a temperature resolution better than 10−9K when it is charged with a field of about 300 Gauss. At 4.2 K, the specific sensitivity is smaller by a factor of 50, but should still allow temperature measurements with a resolution better than 10−7K. Near 2.17 K, drifts of the device are below the level of 10−13K/s. The thermometer has a small mass of about 7 g (excluding the magnet), and thus the advantage of relatively small cosmic radiation heating during microgravity experiments in Earth orbit.

Singularity in the thermal boundary resistance between superfluid (4)He and a solid surface.

We report new measurements in four cells of the thermal boundary resistance R between copper and (4)He below but near the superfluid-transition temperature T(lambda). For 10(-7)< or =t identical to 1-T/T(lambda))< or =10(-4) fits of R = R(0)t(-x(b))+R(B) to the data yielded x(b) approximately equal to 0.18, whereas a fit to theoretical values based on the renormalization-group theory yielded x(b) = 0.23. Alternatively, a good fit of the theory to the data could be obtained if the amplitude of the prediction was reduced by a factor close to 2. The results raise the question whether the boundary conditions used in the theory should be modified.

Thermal resistance of 4He below but very near the superfluid transition

We report high-resolution measurements at saturated vapor pressure of the thermal resistivity R of superfluid 4He over the reduced-temperature range 3×10−7<t≡1−T/Tλ<3×10−5 (Tλ is the transition temperature for Q=0) and heat-current-density range 4<Q<200 μW/cm2. For smaller Q and t, no thermal resistance was detectable below a transition at Tc(Q)<Tλ. For Q≳10 μW/cm2 we find that the results can be described well by R=(t/t0)−2.8 K cm/W with t0=(Q/Q0)0.904 and Q0=393 W/cm2. Thus R has an incipient divergence at Tλ which is, however, supplanted by Tc(Q) where R remains finite. The results imply R∝Q(m−1) with m=3.53±0.02. This differs from the original assumption m=3 of Gorter and Mellink, and from experimental results obtained well below Tλ. However, it agrees with measurements by Leiderer and Pobell at larger currents and further below but still close to Tλ. Our measurements could not resolve a critical heat current for the onset of resistance.

Faraday’s Law

In his Explication of Arago’s Magnetic Phenomena (§4 in the First Series of Experimental Researches in Electricity), Faraday described how he was able to generate a sustained electric current using only a bar magnet and a spinning copper disk. This he did by first touching the ends of a conducting wire to the disk, one to its axis and one to its periphery. When the pole of a magnet was aimed at a point on the disk between the two wire contacts, an electrical current flowed thought the wire when the disk was spun. This curious observation had profound consequences; it led directly to the development of the practical electric generator. For example, a constant electric current can be generated by a magnet placed before the face of a conducting disk being spun by a water- or wind-mill. But why does this work? What is the physical law governing this phenomenon? To answer this questions, Faraday considers a simpler system: a single conducting wire passing before the face of a bar magnet.

The boundary resistance between superfluid 4He near Tλ and a solid surface

We report high-resolution measurements of the singular contribution Rb to the thermal boundary resistance between a solid surface and superfluid helium near the superfluid-transition temperature Tλ. The results confirm the observation by Murphy and Meyer that a gap between the cell end and the sidewall leads to an apparent finite-current contribution to Rb. In the absence of such a gap, overall agreement of Rb with theoretical predictions is very good. Remaining small differences require further investigations. Without a sidewall gap and within our resolution we found no finite-current effects over the range 3.9 μW/cm2

The Primeval Atom

In Chap. IV of The Primeval Atom, Georges Lemaitre outlined his own theory of cosmogony, contrasting it with those previously conceived by thinkers such as Buffon, Kant and Laplace. Now, in Chap. V, he presents his theory “in deductive form”, explaining in detail its points of contact with modern cosmological observations. As you study this text, consider the question: is Lemaitre’s theory plausible?

Carnot’s Cycle

How might a heat engine attain the highest possible efficiency? Does a heat-engine’s efficiency depend on its construction? On the type of intermediate substance employed? On the temperature of the boiler? Of the condenser? Carnot argued that all these factors may, in fact, affect the efficiency of a heat-engine. But in principle, there exists a special class of heat-engines which possess the highest possible efficiency. These special heat-engines are the reversible heat-engines—those in which there is (essentially) no useless heat flow between various internal components, and thus in which all of the heat flow from the hot to the cold reservoir provides useful work.

Apparatus for real-time acoustic imaging of Rayleigh-Bénard convection

We have designed and built an apparatus for real-time acoustic imaging of convective flow patterns in optically opaque fluids. This apparatus takes advantage of recent advances in two-dimensional ultrasound transducer array technology; it employs a modified version of a commercially available ultrasound camera, similar to those employed in nondestructive testing of solids. Images of convection patterns are generated by observing the lateral variation of the temperature dependent speed of sound via refraction of acoustic plane waves passing vertically through the fluid layer. The apparatus has been validated by observing convection rolls in both silicone oil and ferrofluid.

Boundary effects near the superfluid transition (BEST), an experiment proposed for the ISS

We review the opportunities for microgravity measurements of the thermal resistivity R(L,t) near the bulk superfluid-transition line Tλ(P) of 4He confined in cylindrical geometries with axial heat flow. A scaling function is used to estimate R as a function of the cylinder radius L and of the pressure P. These predictions are used to assess the effect of gravity on potential Earth-based measurements. Radii L≳8 μm can only be investigated fully in micro-gravity. At higher pressures the gravity effect is larger. At 30 bar samples with L≳4 μm require microgravity. Modern thermometry has sufficient resolution to permit quantitative measurements of the finite-size effect for L as large as 50 μm, the size chosen for the proposed ISS experiment BEST.

The Particle Theory of Light

what, according to Newton, is a ray of light? And why do rays of light obey the laws of optics which he laid out so succinctly in Book I of his Optics? The next two chapters are drawn from Newton’s Queries included in Book III of his Opticks. These Queries take the form of rhetorical questions by which Newton articulates his theory of light. He begins slowly, exploring the relationship between light, heat and matter. In particular, in the first 24 Queries Newton suggests that when a body vibrates—whether due to heat, friction, percussion, or chemical action—it emits light. Conversely, when light strikes a substance it initiates vibrations within the substance. For instance, when light strikes the eye, vibrations arise in the vitreous humor; these vibrations, in turn, cause the visual perception of color. Likewise, when light strikes a transparent medium—such as water, air or even aether—vibrations arise in the medium; these vibrations, in turn, affect the trajectory of the light within the medium. Indeed, far from rejecting the existence of aether, Newton actually argued that its existence (or rather, inhomogeneities in its density) are responsible for the bending of light rays. Newton, however, carefully distinguishes light itself from vibrations of the aether. This is critical to Newton: vibrations of the aether can certainly affect the trajectory of a ray of light, but light is not a vibration of the aether. We join the discussion at Query 25, where Newton begins to articulate the specific properties that a ray of light must have in order to explain the curious phenomenon of birefringence observed in Iceland crystal.