The pencil is actually lead-free and the main component is graphite. 
Since lead is written on rocks or walls, there are black marks, so in ancient Greece people used lead as a drawing and writing tool. In ancient China, it was written on enamel and paper. If there were mistakes, it was changed with lead powder. Therefore, the ancient Chinese text does have the word "pencil", but it is different from modern pencils.
In the fifteenth century, graphite ore was discovered. At that time, people did not know the composition of graphite, and graphite was called blacklead. Around 1492, the United Kingdom began to use a pen made of graphite, called a pencil. It was not until 1779 that scientists knew that graphite was a form of carbon. After being ground into graphite, it is mixed with clay and then poured into two semi-circular wood tubes. In recent years, there are also plastic pens.
In order to meet various needs, the graphite and clay in the pencil refill can be mixed in various proportions, the mixing ratio is different, and the nature and use of the refill are also different. For example, the refill used for sketching needs to be softer and darker; the refills used by students for homework need to be harder and lighter in color. When buying a pencil, pay attention to the markings on the pen, where H stands for hard, F stands for firm, and B stands for bold.
According to the information provided by the manufacturer, the order of the various pencils from soft to hard is the code between (2) and between 2B and 9B or between 2H and 9H.
Teaching Interactive Whiteboard
The use of high-efficiency multimedia teaching Interactive Whiteboard in education will not only improve the teaching efficiency of the school, but also improve the teacher's ability to guide students correctly, and teaching smart whiteboard can let teacher spend time in effective places instead of wasting ineffective things. The intelligent touch teaching all-in-one machine gives teachers more room for expression. The teaching smart digital board belongs to the teaching market, and the teaching interactive smart board market has been growing rapidly since the end of 2015.
Teaching Interactive Smart Board,Teaching All-In-One Machine,Teaching Smart Digital Board,Teaching Smart Whiteboard
Guangdong Zecheng Intelligent Technology Co., Ltd , https://www.szzcsecurity.com