Sensitive temperature gradiometer for use at low temperatures.
A device capable of very sensitive temperature gradient measurements in the presence of relatively large ambient temperature fluctuations is described. The device uses a pair of ''matched'' superconducting thin films in a bridge arrangement, with a SQUID magnetometer probe as the null detector.
A fast superconducting bolometer.
A superconducting bolometer has been developed for the fast detection of infra-red signals. It has a compound response in which the primary response time is about 5 × 10−4 sec. and the secondary response time about 5 × 10−2 sec. In the primary range a signal of 5 × 10−4μ watt is equivalent to the noise of the apparatus. The frequency at which the response is down 6 db from the primary maximum value is 3000 c.p.s. and from the secondary maximum value about 140 c.p.s. The bolometer consists of a ribbon of CbN, 200 × 10 × 0.25 mils, attached to the surface of a copper base-plate by means of a coating of Bakelite cement which covers the lower face of the ribbon and is about 0.1 mil thick. This unit is placed beneath a rocksalt window in the vacuum chamber of a cylindrical cryostat, 12″ long × 6″ diameter, which maintains operating temperature for about twelve hours when filled with 1000 cm3 each of liquid nitrogen, and liquid hydrogen kept at the triple point. For best operation this temperature must lie within a certain region of the super-conducting transition zone at about 15°K where dR/dT = 10 ohm deg.−1 and R = 0.2 ohm. The noise level varies with temperature in the transition zone and passes through several maxima and minima with variations in level of more than tenfold. Even at the minima it is at least ten times higher than in either the normal or superconducting states immediately above or below. For most sensitive operation, the temperature is kept constant at one of these minima by means of a precisely adjusted current which passes through a heating coil wound on the copper rod supporting the base plate. Radiation signals produce variations in resistance of the ribbon, observed by means of a bridge circuit, transformer, and amplifier.
The phase transition in superconductors - I. Nucleation
A detailed study of ‘supercooling' in tin rods has confirmed the hypothesis that this is caused by the difficulty of forming a nucleus of the superconducting phase. The experiments show that when nucleation does occur it only happens at certain flaws in the metal, which have been proved to lie at the surface and to be between 10-4 and 10-3 cm in size. These flaws do not appear to be necessarily associated with surface conditions, impurity content, or crystal boundaries. Any handling of the specimen affects them, but simply warming it to room temperature often does not, in which case the supercooling observed in separate experiments is reproducible. The degree of supercooling ϕl defined as (H2c - H2l)/H2c, where Hl is the field at which nucleation is first possible, varies in magnitude from flaw to flaw (0⋅8 is the largest value found), but it always depends on temperature in the same way, rising as T → Tc, The supercooling can sometimes be increased by applying a high field to the specimen beforehand, indicating that some of the flaws can be temporarily destroyed by such treatment. The behaviour of the flaws can be accounted for if they are assumed to be domains where the interphase surface tension has become negative, perhaps as the result of local lattice distortion produced by dislocations. Using a simple model based on this picture the magnitude and temperature dependence of ϕl have been explained quantitatively in terms of the characteristic flaw size and the positive surface tension in the undistorted metal (as estimated from work on the intermediate state). Slight superheating (up to 1⋅5% of Hc) has also been observed.