Very tight control of the speed of a d.c. motor drive may be achieved by incorporating the machine within a closed loop system. The employed feedback signal is proportional to the motor speed for comparison against a reference quantity. For armature-controlled d.c. motors the difference or error signal provides appropriate adjustment to the armature supply voltage. There are varieties of methods of achieving a feedback signal proportional to the speed of the motor. This paper describes a technical application in which the armature of a d.c. motor is connected as one of the arms of a Wheatstone bridge, now becoming an active Wheatstone bridge. By analyzing the operation of the active Wheatstone bridge, its transfer function is derived. The active Wheatstone bridge is incorporated in a feedback control system. Applying a variable torque to the motor proves the achievement of a very tight speed control. Experiments are performed on a motor with low rated power, although the implementation of active Wheatstone bridge can be applied for d.c. motors with high rated power as well.

Possible effects of botswana's telephone number change on design of telephone call charging systems, J.S.J. Daka, D.D. Mbewe, I. Zibani

The Botswana Telecommunications Corporation recently changed the structure of the national telephone numbers to seven digits. The change introduces two digit area codes in Botswana. The change also provides an increased exchange numbering range, and an effective and efficient management system for the growing customer base. This change not only affects all telephone subscribers but also the design of telephone call charging systems and billing software. The design of call charging systems is based on the structure of the national and international telephone numbering scheme in a country. A change in this structure therefore affects the design of call charging systems. This paper examines the effects of this change on the design of call charging and billing systems.

In this paper analyses have been done on the climate data for various climatic regions in North Cyprus to obtain physical architectural building design specification with a view to develop passive solar energy-efficient building. It utilizes a computer program, ARCHIPAK, together with climate data (for 25 year period) to get "comfort zones", and "control potential zones", for four stations representing North Cyprus fairly well by virtue of their geographical locations. Our main aim is to provide appropriate information on physical architectural pre-design guidelines at strategic stages to make better use of passive solar energy in urban planning and building design for better indoor "comfort" climate. The use of the control potential zones (CPZ) in building design and their objectives are discussed. Opportunities and limitations of the pre-design guidelines are also discussed.

Coffee husks: - A possible wood substitute in the particleboard industry, W.O.Ogola, J.V.Tesha^*.and E.T.N.Bisanda

Coffee husks, a waste by-product from coffee processing has been used as particles with hydrolysed tannin and urea formaldehyde as binders to make homogenous coffee husks particleboards. The resulting particleboards were subjected to physical and mechanical tests and a comparison made with the existing agricultural waste particleboards and the Standards. Results indicate that coffee husks/tannin particleboards had higher physical and mechanical properties compared to coffee husks/urea formaldehyde particleboards and that their properties compared well with other agricultural waste/residue based particleboards.

The present experimental study was conducted to monitor the aerosol deposition on plant leaves, when exposed to open atmosphere, due to biomass burning in winter months, in particular. The number concentration of aerosols was measured with automatic particle counter which were capable of monitoring aerosols in the range of 0.1 — 5 : m. The deposited aerosol mass on a variety of plant leaves was found to vary from 0.1 to 1 mg cm^-2. The plant leaves with a rough, hairy and dorsiventral character, in general, have greater deposition. The experimental results are almost in agreement with theoretical estimates.

Secondary wastewater treatment by microalgae isolated from Gaborone oxidation ponds, D. D Shushu and P. Chipeta

Microalgae play a fundamental role in primary and secondary wastewater treatment. In this work the growth, photosynthetic activity and removal of phosphorus from wastewater effluents by indigenous blue-green algal species, Spirulina and Oscillatoria, isolated from Gaborone oxidation ponds was studied. Oscillatoria and Spirulina cultured in wastewater effluents grew as much as the algae cultured in specific medium. The photosynthetic activity for Oscillatoria and Spirulina cultured in wastewater effluents was 91 and 122 mole of O₂ evolved mg ^—1 chlorophyll a h ^—1 respectively compared to 52 and 92 mole of O₂ evolved mg ^—1 chlorophyll a h ^—1 respectively when cultured in defined medium, BG11₀. Oscillatoria removed 100% orthophosphate within six days of culturing while Spirulina removed 86% after eleven days of culturing. The results indicate that these blue-green algal species can grow in wastewater effluents for biomass production and for further reduction of excess nutrients from wastewater effluents before discharge into the receiving streams.

A proposed flat roof variant for the low technology tropical environment., A. N. Aniekwu

Reports on the failure of built-up bituminous roofs over concrete decks exposed to tropical conditions indicate a worse than below average performance, and thus an urgent need for a re-assessment of the use of this material within this environment. This paper reviews the modes of failure of this roof system under tropical exposure with a view to ascertaining the reasons for the very high rates of failure recorded. It also reviews the trends in this roof type and proposes a system that will be more durable under low technology tropical exposure.

Unsteady free convection and mass transfer flow of a non-newtonian fluid with thermal diffusion effect, A Ogulu; K D Alagoa and D D S Bawo

We study the unsteady free convection and mass transfer of a non-Newtonian fluid past an infinite vertical plate in the presence of thermal diffusion. Closed form analytical solutions are obtained for the concentration and the temperature distributions by means of the Laplace transform technique on the assumption that the thermal conductivity, k, of the fluid is small. Consequences of the thermal diffusion effect (the Dufour term D_f) are then discussed quantitatively.

Investigation of regional trends and seasonal shifts in the river flow regimes of Southern Africa, B.F. Alemaw and T.R.Chaoka

The historical trend and variability of river is investigated in 502 river flow gauging in 9 countries of the Southern African region with a view to document the special variability of the river flow regimes. The study identifies regions with strong evidence of declining or increasing trend in annual runoff and those with strong backward or forward seasonal movement in the monthly river runoff across Southern Africa. Of the 502 rivers considered decline in annual runoff has occurred in 250 rivers that constitute 57% of the area considered. Backward seasonal movements have occurred in 180 rivers (42% area studied). The seasonal movement seems to explain the background decline in the trend of the annual discharge of most of the rivers. The decline in runoff is clustered in parts of Zambia, Namibia, Mozambique and the Veld in South Africa.