Electrical leads carrying currents into cryogenic apparatus also introduce heat. Even with an ideal Carnot cycle, the mechanical power needed to remove this heat can be one hundred or more times the heat flow itself. If the currents and hence the input leads are heavy, a very sizeable refrigerator may be required. In this article the configuration of the leads which minimizes the influx of heat is derived theoretically, taking variations in thermal and electrical conductivity into account. Graphs are given for the minimum heat flow and optimum cross section of a copper input lead carrying arbitrary current I. The optimum is found to be fairly sharp. If the diameter of the lead differs by a factor of two from the optimum, the influx of heat is increased by over 100%.
The currents set up in the chest by the electromotive forces of the heart produce magnetic fields at the chest surface which have a peak intensity of about one microgauss. To detect these fields, an optimized coil assembly has been constructed which yields an rms noise level of about 10−8 G in a 1- to 40-cycle band. The design of the coil assembly and the interpretation of its output are based on an analysis employing an unusual form of the reciprocity theorem. The theory shows that magnetic detection is fundamentally different from its electric counterpart and may reveal new clinical information. It also shows how immunity to magnetic interference can be achieved without resorting to bulky and expensive magnetic shields.
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