Exhibit 10 - an X-ray tube
The electron beam in early X-ray tubes was generated by an electrical discharge flowing through a gas. A common problem was that the glass of the tube, and metal sputtered on to the glass, absorbed some of the gas thus lowering the pressure so that unacceptably high voltages were needed to maintain the discharge. A solution was to place an external spark gap in series with a fine wire heater in a block of ceramic material within the X-ray tube, so that when the voltage across the tube rose, the current which resulted from the sparking across the gap heated the ceramic which gave off gas and restored the pressure in the tube.
The tube shown above dates from 1912 and is attributed to Lindemann although he published nothing on the subject. Similar X-ray tubes were common and were made commercially until eventually they were superceded by hot-cathode vacuum tubes.