Proceedings of the Technical Session of Institute of Physics, Sri Lanka 5 (1989) c1

Higher Education: Criteria for Selection and the Competition for Places in the University in Sri Lanka
G.A. Dissanayake
Department of Physics, University of Peradeniya, Peradeniya

Due to the limited number of places in the University in Sri Lanka, the standard of entry has to be raised appreciably. As a result, a very large proportion of candidates has been rejected, many of whom may have been successful at the degree had they been admitted to the Universities.

The most suitable candidates for admission would perhaps be those who have the highest probability of obtaining a good degree. Thus, any Entrance Test must have the potential for predicting future degree performance. The test must have a high predictive validity. In Sri Lanka, the A-level aggregate is used as the criterion for selection to the Universities. An important question that arises is to what extent the A-level can be considered a valid educational signal for predicting future Degree performance.

Extensive studies of the correlation’s between A-level grades and the degree performance, across subject groups and across Universities, have been carried out in the U.K. All correlation’s have been found to be statistically significant, though weak. Such detailed correlation studies have yet to be made in Sri Lanka.

Assuming a hypotecnical form of correlation between A-level performance and degree results, the paper will discuss briefly the effect of raising the standard of entry on (a) the proportion of degree successes among those admitted, and (b) the percentage of candidates rejected who may have been successful had they been admitted. The present selection criteria, governed by the limited places in the Universities, Deny admission to a significant proportion of candidates with a potential for doing well at the degree. There is therefore a strong argument for providing alternative avenues of tertiary education, with a re-structuring of the education system.

Proceedings of the Technical Session of Institute of Physics, Sri Lanka 5 (1989) c2

Sulphate based Solid Electrolytes and their Applications
M. A. Careem
Department of Physics, University of Peradeniya, Peradeniya

Sulphate based solid electrolytes can have ionic conductivity as high as 3^-1cm^-1 as in the case of pure Li₂SO₄ at a temperature of 800 ⁰C. By mixing one or more of other sulphates in correct proportions, highly conducting phases can be found at relatively low temperatures. The conductivity is due to the cation mobility, and in contrast to the solid electrolytes such as Balumina and Ag-I based systems both mono-and divalent cations are mobile. The high conductivity in the fcc Li2SO4 and in certain other sulphate systems are caused by an unusual transport mechanism called 'paddle wheel mechanism' in which the mobility of the cations is enhanced by a rotational motion of the transitionaly static, sulphate ions. Sulphate based solid electrolytes can be used in high energy density primary cells and in heat storage system.

Proceedings of the Technical Session of Institute of Physics, Sri Lanka 5 (1989) c3

Fabrication Techniques of Crystal Orientation to Obtain a High Critical Current Desity in Ceramic Superconductors
B.S.B. Karunaratne
Department of Physics, University of Peradeniya, Peradeniya

The practical use of a superconductor is mainly determined by the ability of the material to conduct a high critical current at the superconducting stage. Although the new Y-Ba-Cu-O ceramic superconductor has a high transition temperature (Y_c), the critical current density (J_c) in the bulk sintered samples is considerably low for practical applications. In this communication we report some fabrication techniques of crystal current. These techniques include hot-pressing, hot-extrusion, thin film deposition, tape casting and gradient sintering. The microstructural development during fabrication is also discussed.

Proceedings of the Technical Session of Institute of Physics, Sri Lanka 5 (1989) c4

Development of a Phase Sensitive, Frequency Selective Detector
H.L.R.V. Nadanayake and Ravi Kulasekara
Department of Physics, University of Colombo, Colombo 3

Accurate measurement of thermometer resistance, at given temperatures, is of prime importance especially in the field of thermometry. The Inductively Coupled Double Ratio Bridge (which is analogues to the Kelvin Double Bridge) is an instrument which is commonly used for such precise measurement of resistance.

The Double Ratio Bridge uses an alternating current technique carried to obtain out at low frequencies. The bridge arms consist of inductive dividers to obtain a ratio read-out, the accuracy of being about one part in ten million or so. The use of alternating currents and inductive dividers introduces unwanted noise and phase factors into the measurement which causes null balancing of the bridge exceedingly difficult, using only frequency selective detection, as the resolution of the ratio readout is increased. This in turn introduces errors in measurement resulting in the decrease of the high precision required.

The Detector used by the bridge for null balance is of the frequency selective type (which selectively picks out the input signal from the noise superimposed on it ) having a transformer input and a positive deflection meter read-out. This arrangement is insensitive to phase, maintains a fairly high noise level at the output and also does not indicate whether the bridge is over or under balanced, all of which are disadvantage to the user. The detector was reconstructed to show and eliminate the introduced phase factors and to greatly reduce the noise level of the output.

This was achieved by incorporating a circuit, after the frequency selective detection, which synchronously detects both the in-phase and in-quadrature components of the output relative to the input signal to the bridge. These signals are later independently integrated over a period of time and the read-out is taken on a centre-zero moving coil instrument. Here in-quadrature component indicates the unwanted phases factors which could be effectively read and eliminated. Integrating the output signal before the read-out greatly reduces the noise level, since the integration of random noise over a period of time usually results in very small output. The use of a centre-zero instrument also has an added advantage of being able to indicate whether the bridge is under or over balanced. Therefore, phase Sensitive Frequency Selective Detection supplies more information about the measurement, is easier to perform and is also rid of most phase related and noise errors, thus, enabling signals as small as a few nanovolts to be detected.

Institute of Physics, Sri Lanka