Among all kinds of soft pencils, 2B pencil has good effect and low cost, so 2B pencil is chosen.
(3B~6B pencils are too easy to break and expensive)
Pencils are divided into 10B, 9B...3B, 2B, b, HB, h, 2H, 3H... 10H.
B is the depth of soft ink, 10B the softest ink is the deepest, h the hardest ink is the shallowest, and 10H the hardest ink is the shallowest. According to the actual needs, the depth of 2B pencil ink is suitable for machine-readable inspection chart, which is convenient for writing.
Since the introduction of "microcomputer card reading" technology from Britain in the 1980s, China has provided a very practical, fast and powerful solution for marking objective questions. The "microcomputer card reader" and cards are also updated quickly, and the machine card reading technology has now developed to carbon (graphite) technology using infrared induction!
At present, the domestic card reading equipment is designed according to the graphite concentration of 2B pencil. Don't talk so much nonsense, to be more direct, that is:
Indicator 1: concentration of carbon (graphite) in pencil.
Indicator 2: the area of blackened area.
Supplementary note: carbon (graphite, the main component of 2B pencil)
Therefore, to measure whether your answer is correct, draw it on the answer sheet. "Computer card reading equipment" is to use the electrical sensitivity of carbon to infrared rays. Through the sensitive reaction of carbon to infrared rays, two main data are obtained: the "concentration" and "area" of the coating area on the answer sheet! !
So we can easily explain the following phenomenon.
Why can't we use pencils with high carbon content, such as 3B and 5B?
First of all, I can tell you that the answer sheets 3B and 5B are recognizable!
However, due to the high graphite content in pencils 3B and 5B, they are easy to adhere to other answer sheets with paper, which will make carbon (graphite) everywhere on other answer sheets, which may affect others' grades.
2H, why can't you use a hard pencil?
The main reason is that the concentration is difficult to identify, because the carbon concentration is low! From Baidu: michaelee 372 1