I. INTRODUCTION

It is a matte of experience that when two bodies are in thermal equibrium with a third body, they are in thermal equilibrium with one another. Tjis statement, which is some times called the zeroth law of thermodynamics, is tacitly assumed in every measurement of temperature. Thus, to know if two bodies are at the same temperature, it is necessary only to see if they are individually in thermal equilibrium with a third body. The third body is usually a thermometer (Burghardt and harbach 1999 and Moran and Shapiro, 2000) A body with at least one measurable property that changes as its temperature changes can be used as thermometer. Such property is called a Thermometric property, The particular substance that exhibitis changes in the thermometric property is known as a thermometric substance. A familiar device for temperature measurement is the liquid-in-glass thermometer, which consists capillary tube connected to a bulb filled with a liquid. As temperature increase, the liquid expands in volume and rises in capillary. The length L of the liquid in the capillary depends on the temperature. Accordangly, the liquid is thermometric substance and L is the thermometric property. Although this type of thermometr is commonly use for ordinary temperature measurements, it is not well suited for application where extreme accuary is required. The contstant-volume gas thermometer is so exceptionally as the standard instruments for calibrating onther thermometers. The thermometric cubstance is the gas (normaly hydrogen or helium), and the thermometric property is the pressure exerted by the gas. As shown in the figure, the gas is contained in a bulb, and the pressure exerted by it is measured by an open-tube mercury manometer. As temperature increase, the gas expands, forcing mercury up in the open tube. The gas is kept at a constant volume by raising or lowering the reservoir. The gas thermometer is used as standard worldwide by bureaus of stnandards and research laboratories. However, because gas thermometer require elaboratw apparatus, have a large size, give a slow respond that demand painstaking experimental procedures, smaller, more rapidly responding thermometer are use for most temperature measurements and they are calibrated (directly or indirecly) against the gas thermometer.