In this paper a new parameter is suggested as a criterion for selecting the optimal value of the copper thermal resistance in temperature measurements in the atmosphere under normal conditions. The proposed parameter is directly proportional to the sensitivity of thermal resistance and inversely proportional to the product of ‘dead’ resistance and ‘dead’ temperature (the product of errors of temperature and resistance when their determination). It has been shown through the example of 4 samples that this criterion is convenient. For one of the thermal resistance this parameter is higher in the investigated range of temperatures.