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Comparison is a commonly used and basic measuring method in physical experiments. It aims to obtain the magnitude by comparing the measured quantity with its standard value. According to their differences, the methods can be divided into two types, direct comparison and indirect comparison.
In general, direct comparison is a measuring method which directly determines the magnitude of a quantity by comparing to its standard quantity. Direct reading is the main characteristic of the instrument used in this method. The physical quantities measured by this method are generally basic ones. For example, one can measure length with rulers, vernier calipers or spiral micrometers. One may measure time with a stopwatch or computer counter, or measure voltage with voltmeter. The scale of instrument is graded and calibrated by a standard instrument in advance. Therefore, one can simply get the value of an objective by multiplying the constant or rate of the measuring instru ment without additional operation or calculation. For direct comparison, its measuring accuracy is limited by the accuracy of the instrument being used.
Indirect comparison is used to determine the quantity through an intermediate quantity or calcu lation by formulas. For example, one can get the unknown resistance R x by indirect comparison to an adjustable standard resistance R s , as shown in Figure 2.1.1. If the applied voltage U remains constant and the switch K is respectively connected to position 1 and position 2, we can obtain the value of R x by adjusting R s to the same current.
Figure 2.1.1 Schematic diagram of indirect comparison method