Abstract: Sommerfeld's 1931 presentation of his derivation of the Thomas precession is set in its historical context. Some aspects of his particular geometrical approach are also discussed.

Abstract: An equation is derived which expresses the conservation of intrinsic angular momentum of the radiation field. The intrinsic angular momentum density is shown to be D*A- epsilon mu phi H in an isotropic non-conducting medium. Application of the conservation law to a circularly-polarised plane wave shows that angular momentum in this wave can be interpreted as being transmitted over the entire wavefront, in contrast with conventional ideas. The angular momentum of the field from a rotating electric dipole is considered.

Abstract: It is argued that the reduction of the wavepacket following a measurement is no more than a computational convenience to which no meaning should be attached. In a strict application of quantum mechanics all measuring instruments must be included in a single wavefunction, along the lines of an early important paper of Mott. Thus the activity of physics is treated as the analysis of public information, as conveyed by instruments, with quantum mechanics the accepted analytical procedure rather than a model of objective reality. Finally the classical world of particle trajectories that can be agreed on by all observers is shown to be a natural corollary. Only very simple examples are employed, and all derivations are segregated in appendices, so that the argument (which contains no revolutionary features) may be readily accessible.

Abstract: The Nice Exploratoire exhibits a curious experiment: a ball rolling without slipping on a horizontal table arrives on a circular rotating disc. The ball is deflected, but nevertheless comes out of the rotating disc in the exact projection of its initial rectilinear trajectory! The phenomenon, elementary although non-trivial, is analysed and generalised.

Abstract: The total entropy of an information storage system can be decomposed into independent terms, i.e. into functions which have no independent variables in common. One of these terms represents the information (=entropy) in which the user of a computer is interested. This decomposition corresponds to a break-up of the entire system into non-interacting subsystems and is analogous to the decomposition of the total energy of a system into independent terms commonly referred to as energy forms. In both decompositions, the term usually of interest is many orders of magnitude smaller than the rest.

Abstract: An elementary formalism is developed for latent heat in such a way as to demonstrate its physical significance and to obtain expressions by which its value may be determined and its properties studied. Software has been written to enable students to investigate these properties in a flexible manner using a standard experimental set-up. Data files in the menu enable standard data to be called upon for analysis in a comparable way with experimental data. By the use of transducers students are led to appreciate how thermometric parameters can be automatically monitored such as during a change of state.

Abstract: The Compton formula is obtained in a semiclassical approximation due to Schrodinger. X-rays are treated within classical electrodynamics and electrons within quantum mechanics. The superposition of the wavefunctions of the incident and recoiling electron sets up a moving periodic probability density. Incident X-rays are specularly reflected from this pattern. The results are transformed back into the original rest frame of the electron by the Doppler shift equation. Finally, some remarks pertaining to the philosophy of physics are added.

Abstract: Different formulations of Newton's first law are compared. A proposal for a real body-fixed inertial reference frame and for a corresponding separate formulation of the law of inertia is given.

Abstract: An elementary proof is given for the fact that a field of forces in which all orbits are plane curves must be a central field. A proof by calculation is also given for comparison and contrast.

Abstract: In the process of electromagnetic induction from a long solenoid, an induced EMF occurs in a circuit where the magnetic flux density is effectively zero. It is argued that the crucial magnetic quantity in electromagnetic induction is not the flux density, but the vector potential. The problem of energy flow from the solenoid to the circuit is considered by calculation of the Poynting vector.

Abstract: It is an old question whether an energy current can be imagined as energy moving with a well defined velocity. It is shown that in two important systems, namely in the electromagnetic field and in moving matter under stress, the energy current can be decomposed into two parts of opposite directions. Each part can be imagined as energy moving with the velocity of light or with the velocity of sound, respectively.

Abstract: A didactic apparatus is described which can be used by students to study both linear and nonlinear oscillations. The transitions from linear to nonlinear oscillations are considered both theoretically and experimentally.

Abstract: The elementary method of locating and classifying the stationary points of a function of two variables is applied to the potential energy arising from an experiment in which two inflated balloons are connected to form a closed system. It is shown that if the amount of gas in the system is less than that which is required to inflate both balloons to twice their initial radius, then in the position of equilibrium the radii of the balloons are equal. However, if the amount of gas is greater than this value, then in the equilibrium position one balloon will be larger than twice the initial radius while the other is smaller.

Abstract: It is well known that when heat degrades in temperature the amount of work that can be extracted from it decreases. The lost work dW, increase in entropy of the heat dS and lowest available temperature T0 are related by dW=T0 dS. If the heat source for an ideal heat engine consists of two vessels each containing a different perfect gas, no loss of thermal efficiency occurs when the gases mix, but there is an increase in entropy of the heat source. It is shown, however, that the relationship between entropy and available work still holds. It is further pointed out that the contention that entropy increases specifically as a consequence of mixing is false.

Abstract: A model experiment for teaching in order to obtain an equation of state for a two-dimensional system of hard particles is presented The simulation has been performed using a developed molecular dynamics method The virial equation of state is calculated and compared with the statistical equation of state obtained by the partition function The behaviour of the model in different density ranges is discussed Experimental results are compared with kinetic theory results in the low density approximation

Abstract: Indoor concentrations of alpha -emitting radon daughters can be measured by a high-voltage method The alpha -active atoms usually stick to charged dust particles which can be collected using a long wire (21 m) given a negative potential (-5 kV) Such an electrostatic method will influence the air streams in the room less than if the collection were arranged by pumping air through a filter The alpha counting rate can by calibration be transformed to a radon daughter level in Bq m-3 A high radon daughter level in a dwelling is caused by the emission of an inert gas, radon, either from building materials or from the ground The contributions of 'wall radon' and 'ground radon' can be estimated by measuring both the gamma radiation and the radon daughter level

Abstract: A demonstration experiment was developed which is able to simulate the mode of operation of X-ray computer tomography (CT). A HeNe laser is substituted for the X-ray tube and a selenium cell for the detector. Different optical filters represent the absorbing structures of the human body. By the use of a personal computer with monitor it is possible to demonstrate the image of the structures resulting from different adjustments of centre and window.

Abstract: The purpose of this note is to point out a deficiency in the account of the image forming process to be found in many introductory optics textbooks (such as Hecht and Zajac 1974, Ditchburn 1976) and to give an alternative derivation of the resolution limit. Advanced texts (for example, Cathey 1974) normally give a correct description of the process. Figure 1 shows a widely-used illustration. Here an object (usually a ‘square-wave’ grating) is illuminated by a coherent beam of parallel light (usually from a laser). Such an illustration correctly identifies the object plane S,, the Fourier transform plane S, (the plane of the Fraunhofer diffraction pattern of the grating) and the image plane S, . Where it fails is to suggest that certain plane waves do not enter the lens. To quote from Hecht and Zajac (1974): ‘unless the objective lens has an infinite aperture it functions as a low-pass filter rejecting spatial frequencies. . . . which extend beyond the physical boundary of the lens’. As will presently be explained, all the spatial frequencies do enfer the objective lens but-contrary to what is implied in figure l-these frequencies may fail to overlap in the image plane. To start with, it is worth recalling the arguments first advanced by Porter (1906). He pointed out that the graph of amplitude transmittance against position J along an (essentially) one-dimensional object-such as the familiar grating composed of alternate clear and opaque strips-could be Fourier analysed into a series of cosinusoidal curves plus a constant-in-space component (‘an imperfectly transparent surface’ to quote Porter). Thus the archetypal object is a ‘cosinusoidal grating’ of period d. When such a grating is illuminated with a normal-incident laser beam the E field on the output side of the grating will have the

Abstract: The authors systematically bring together the standard mathematical properties of Fourier transforms and the experimental characteristics of diffraction phenomena. They display in detail an experimental realisation of a particular case of the convolution theorem.

Abstract: Band-gap measurements based upon the temperature dependence of the electrical conductivity of a forward biased p-n junction are described Results are given for germanium, silicon, gallium arsenide and gallium phosphide

Abstract: The quasilinear diffusion equation of a particle moving in a field of many waves with random phases has been calculated classically and related to the quantum picture of Cerenkov scattering of particles by wavepackets.

Abstract: The depth V and width b of the square-well potential representing the neutron-proton interaction are determined by analytical and numerical methods from measured values of the deuteron binding energy Bd and the root-mean square radius of the neutron electric charge density, Rd. New graphical methods are described to find V for a given b, using the value Bd, and to calculate the variation of the energy E* of the (unbound) first excited state as a function of the V and B values set by Bd. Common misunderstandings in the interpretation of the radial wavefunctions are discussed.

Abstract: New trends in physics teaching, such as research into students' conceptions and concept names and physics of everyday life, are considered and their inherent limitations discussed. Overemphasising the qualitative aspects may reduce the scope of physics teaching, which is based on quantitative calculations. Physics teaching may benefit from these trends if they are taken as a complement rather than as an alternative to the existing approach.

Abstract: In modern texts the roots of the MKSA system of units are increasingly ignored. This has led to some mysteries in the definitions and equations of electromagnetic theory, particularly for the student. Examples are the peculiar definition of the ampere and the question of how Maxwell could know, in the pre-SI era, that the phase velocity given by his wave equation was the velocity of light. By retracing Maxwell's derivation of the relevant expressions the authors try to resolve some of these mysteries.

Abstract: Recent articles in this journal have proposed that certain rather specialised mathematical formalisms (quaternions, or alternatively dyads) should be employed in the derivation of the fundamental equations of rigid-body kinematics. Here, to the contrary, the merits are argued of a description based simply on the properties of rotation matrices. One particular advantage of such a mainstream approach is that the construction of the algebra of angular velocity vectors from the group of rotation matrices is a basic example of a general construction, that of a Lie algebra from a Lie group. It therefore offers the possibility of making the acquaintance, in a familiar context, of ideas which are well known to be of major importance in several branches of theoretical physics. The distinctive features of the rotation group case, as well as the pointers it gives to general theory, are identified in the course of a full, but it is hoped not too technical, discussion of angular velocity in this vein.

Abstract: A way (involving the use of line delta functions) of formulating and depicting convolutions, autocorrelation and cross correlation is discussed.

Abstract: It is well known that if a body is dropped from a tower situated at the equator it will not strike the ground directly below the tower, but at a distance to the east. This is a consequence of the Earth's rotation, and is normally explained with reference to the Coriolis force. The author illustrates how the same result may be derived by considering the body's path as part of an elliptical orbit about the Earth's centre, and derives the exact result for the time of fall and eastwards deflection.

Abstract: Shortly before his death in January 1984 Professor Alfred Kastler, who won the 1966 Nobel physics prize for his work on optical pumping, wrote an extensive book review of Masers and Lasers by Mario Bertolotti (1983 Bristo1:Adam Hilger). A shortened version appeared in Nature (1985 316 307-9). This is the full version.