The methodological bases for the study of the systematic error of temperature measurement by liquid-in-tube thermometers and microthermometers with diminution of the capillary diameter, that is, when passing from thermometers to microthermometers, in the field of predominance of gravitational forces over surface tension forces, are given. The existence of components of the systematic total error of temperature measurement with help of such thermometers is established with different thermometric liquids (water, mercury, alcohol). Their relative weight varies depending on the conditions of calibration and application, the peculiarities of manufacturing and used structural materials, their compatibility and wetting. It is shown that the change in the linear dimensions of the thermometers, in particular their reduction in micro size, leads to the strengthening of some components and the weakening of others. The equation of motion of a thermometric fluid in a capillary with a number of parameters is derived. It is shown that the geometry of the inner surface of the capillary can become a determining factor for the formation of the component of the systematic error of the so-called dead passage. The content of the two components of the systematic error due to the deterioration of the conditions of movement of the liquid in the capillary due to changes in its cross section is revealed. The value of the component of the systematic error, determined by the drop in the internal diameter of the capillary due to technological deviations in its manufacture, is estimated at the level of up to 1 %.
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