[The master and his school - on the biography and action of instrument developer Heinrich Geissler].

This contribution focuses on the work and development of a notable personality in physics and technology: Heinrich Geissler, a glassblower, educator, inventor, and experimentalist, influenced the development of physics in the second half of the 19th century with lasting effect. He initiated and contributed to research on electric discharge in gases, created the first actually working vacuum pump, and continuously improved its design. His students significantly contributed to research on the photoelectric effect, and early vacuum electronics. They were involved in the development of the electric light bulb, and in the extremely quick practical application of X-rays in medical and other contexts. All these activities contributed to the fundamental change in physical thinking that took place at the beginning of the 20th century. It is an aim of this article to describe the momentum, as well as the centrifugal forces, at work in this critical phase of physics research.