Measuring Instruments

MEASURING INSTRUMENTS. By measurement is meant the comparison of a quantity being measured with another homogeneous quantity that serves as a unit of measurement. One can compare length with length, surface with surface, force with force, etc. In physics, it is shown that all quantities studied in physical processes can be reduced to measurements of length, mass, and time. Thus, for basic measurements in the field of physics, it is necessary to measure the three basic quantities mentioned above, from which all others can be derived. For measuring length, a ruler divided into centimeters or millimeters is used, and the length being measured is compared with the ruler by placing one end of the measured length at zero on the ruler. If special accuracy is not required, as is usually the case in medical measurements, then visual estimation is sufficient. If precise measurement is required, then a vernier is used, which allows determining tenths (and generally fractional) parts of the quantity being measured. For measuring the length of curved lines, either a flexible tape divided into millimeters or centimeters is used, or a special instrument connected with a wheel that allows directly counting the length when it is moved along the given line. Measurements of time are usually made with clocks and stopwatches, which are compared with instruments that give absolute time derived from astronomical observations; comparisons are made by starting the stopwatch or marking the time on the clocks at the moment considered to be the beginning of time, and stopping the stopwatch or marking the position of the clock hand when the phenomenon ends. For measuring mass, scales are used, which are one of the most precise physical instruments. On modern analytical scales, masses can be weighed with accuracy to tenths of a milligram. But there are instruments that allow for comparisons with even greater precision. For measuring volumes, graduated measuring cylinders are used, into which water is poured to a certain level, and then, by immersing the body being measured, the volume of the immersed body is observed by the rise of the water. Measurements of other quantities encountered in physical and medical practice are made on the basis of studying either length, or mass, or time. Thus, when measuring electric current, the measuring instrument is a galvanometer, in which a movable magnetic needle is affected by the electric current passing through a coil, and this effect, as a certain force, is measured in comparison with another force that is known. Thus, it becomes possible to express the magnitude of the current's effect on the magnet in mechanical units and therefore to measure the strength of the current, since its action produces a certain mechanical effect. When measuring the amount of electricity, an electroscope with gold leaves is used, in which a charge causes the leaves connected to a metal rod to diverge. Here, the leaves are charged with electricity and repelled, which can be measured by the divergence of the leaves. Thus, the magnitude of the charge can be related to the magnitude of the repulsive force caused by a certain electric charge that charges the leaves. Thus, in this case too, the measurement of the magnitude of the electric charge is reduced to a certain mechanical unit. Temperature measurements are reduced to measurements of body volumes and thus ultimately to measurements of the length of a mercury column emerging from a heated reservoir. Measurements of light intensity are made by comparing the light intensity of a given source with the light intensity of some source taken as the unit of measurement, using the law relating the brightness of illumination to distance, and ultimately the comparison of the brightness of sources leads to measuring certain distances. Thus, it is clear that in all cases where it is necessary to make various measurements of quantities encountered in physics, their measurements are reduced to the basic methods of measuring LENGTH, MASS AND TIME.