Salford City College

We set out the theory and discretization scheme for a new finite-element computer code, written specifically for the simulation of maser sources. The code was used to compute fractional inversions at each node of a 3-D domain for a range of optical thicknesses. Saturation behaviour of the nodes with regard to location and optical depth were broadly as expected. We have demonstrated via formal solutions of the radiative transfer equation that the apparent size of the model maser cloud decreases as expected with optical depth as viewed by a distant observer. Simulations of rotation of the cloud allowed the construction of light-curves for a number of observable quantities. Rotation of the model cloud may be a reasonable model for quasi-periodic variability, but cannot explain periodic flaring.

The interpretation of last year's examination results in your news story “Exam results reveal ups and downs” (Physics World, September p5) cannot be left unchallenged. As an A-level teacher for the last 12 years and one of the people involved in the Institute of Physics initiative to the examining boards, it seems clear to me that the steady drop in physics candidates over the last six years has been largely due to the loss of those at the lower end of the range.

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A generalized microcomputer based flight simulator has been developed, one part of this simulator is a numerical solution of the equations of flight. For accurate solution, a simple numerical scheme would demand a very small timestep: A modified scheme is therefore developed which allows a much larger timestep to be used by introducing certain scaling factors. These factors are themselves functions of the timestep and of the characteristic response time of the equations being solved. The modified numerical scheme is shown to be adequate when applied to simultaneous solutions of all the components of an aircraft motion. Within limits, the simulation is independent of the aircraft being simulated. Relevant performance and control parameters are entered before the commencement of solution.