period meter for reactor

例句：

1. 1.The period meter is used to measure the length of a period.
   周期计用于测量一个周期的长度。
2. 2. The periodic table is a tool used by scientists to organize and study elements.
   周期表是科学家用来组织和研究元素的工具。
3. 3. The period meter was invented by a scientist to help people better understand the natural world.
   周期计是由一位科学家发明的，帮助人们更好地理解自然世界。