Quantitative measurement on the human in the so-called evaporative, vasomotor, and metabolic control regimes has revealed frequency spectrum of sustained thermal power oscillations with approximate periods of 2, 7, 35 min, and 3 1/2 hr independent of the regime. Step function adjustments take place with a time constant of about 7 min. It is believed that the 3 1/2 hr cycle represents the shortest equilibrium cycle. The hypothesis that it might be possible to measure the resistance of clothing as an ohmic relation among time-averaged equilibrium values, and for a specific mode of operation of the system has now been put in rational context in the time domain. Two equilibrium modes of the human system were explored. The active mode of operation of the system, to which the resistance concept of clothes is most applicable, is as a feedback system in which the extremities are used as error indicators of deviations from a comfort level set point. In response to deviations, the human feeds back a signal to generate an activity level in which only internal work—immediately degraded into heat—is done to maintain the comfort level. This is referred to as the comfort mode of operation of the system. Another “survival” mode of operation of the system is also described.

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