The follicular phase begins with an increase in follicle-stimulation hormone (FSH), which causes increases in luteinizing hormone (LH) and gonadotropin-releasing hormone (GnRH). Concurrent increases in estrogen levels cause increases in progesterone, stimulating proliferation of the endometrium.
Examples of insulators include plastics, Styrofoam, paper, rubber, glass and dry air. The division of materials into the categories of conductors and insulators is a somewhat artificial division. It is more appropriate to think of materials as being placed somewhere along a continuum.
In an earlier section of Lesson 1, it was stated that an electrical attraction would be observed between a charged object and a neutral object. If a charged plastic tube is held near to neutral paper bits, the attraction between the paper and the plastic would be sufficient to raise the paper off the table. If a rubber balloon is charged by rubbing it with animal fur, the balloon can subsequently be stuck to the surface of a wooden cabinet or a whiteboard.