2.1.1　Direct-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 instrument without additional operation or calculation.For direct comparison,its measuring accuracy is limited by the accuracy of the instrument being used.(https://www.daowen.com)