A method is proposed of converting the frequency of an electromagnetic wave to a higher frequency by reflection from an electron cloud moving with relativistic velocity. Such an electron cloud can be reaized by compressing all or part of the electron beam of an electron accelerator into one or more groups. It is shown that there is a gain of wave energy arising from the relativistic law of reflection of a wave which is reflected from such a cloud. It is also shown on the basis of Bailey's relativistic electro-magneto-ionic theory that under certain circumstances the reflection from a moving slab of electrons maybe increased considerably if the slab moves through a longitudinal or transverse magnetic field. It is estimated that a wave of length 1 mm and of a power at least one milliwatt can be generated by reflecting a wave of length 3 cm from the beam of a small betatron. Equipment is being designed to test this prediction experimentally.T HE purpose of this note is to point out that through the development of electron accelerators (such as the van de GraafT machine and the betatron) the generation of very short electromagnetic waves by means of some relativistic effects has come within the range of possibility. 1
The cooling of a single thermoelectric junction is limited to a certain temperature difference by the inherent properties of the elements used. Altenkirch gave a theoretical treatment of the single couple and showed a way to overcome the limitation on cooling by using a number of thermocouples in thermal cascade. Such an arrangement was constructed by Turrettini and shown to work. A simplified form of Altenkirch's theory is presented and a form of cascade which is simpler and more efficient than that of Turettini is described and experimental results are given. A two-stage cascade of elements limited to 11°C cooling as a single couple reached 15°C cooling while rather heavily loaded by lack of adequate thermal insulation. A good agreement with theory is obtained for the behavior of this cascade. It is shown that such a cascade is in theory more efficient than a single couple under all circumstances.
It was shown in a previous article by Landecker and Findlay that when transient currents are passed through a cooling Peltier junction the resulting transient temperature depends strongly on the current waveform. In particular, an extremal of the current as a function of time exists for which the resulting transient temperature is much lower than for any other current waveform. With the aid of an experimental method developed for the purpose, temperatures well below 100°K were in fact observed with bismuth telluride junctions. Since the occurrence of temperature transients producing such low junction temperatures is rather difficult to reconcile with some notions of solid-state theory, these results have been reexamined for current densities of up to ∼4000 A/cm2 using a modified technique that eliminates possible sources of systematic errors in the previous method. The results may be summarized as follows. Up to the highest current densities that could be produced a monotonic increase of the temperature drop with final current density was observed. The final temperature drops ranged from 205° to 120° measured from a reference temperature of ∼20°C (room temperature) depending on the efficiencies of the junctions. These efficiencies were difficult to predict from the performance of the junctions with stationary currents. For the highest currents and the very low transient temperatures an error of up to 150° may arise from imperfect heat transfer from the hot junctions to the water-cooled heat sinks unless special probes are provided to measure the Seebeck voltage. It is very likely that the accuracy of the measurements cannot be expected to be improved much further as long as grown crystalline n and p branches are used. It is hoped to produce sintered junctions of more suitable shape and greater mechanical strength for further experiments.
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