Jackson X-ray tube, 1896.

This is a very early example of an X-ray tube, through which electricity was passed. It features a cup-shaped cathode which serves to focus the cathode rays onto the target (a platinum anode). When the rays hit the anode, their energy changed into invisible X-rays, which passed out through the glass. Experiments revealed that these rays penetrated some substances more easily than others, and also fogged photographic plates. The fact that X-rays could produce images differentiating between the densities of body tissues, was a discovery which the medical profession was keen to exploit.