We will not describe details of construction, since those  vary somewhat, and are dealt with in a more or less lucid way  to suit the resistance of different X-ray tubes than does a  valve-tube.

By screwing the point home till it touches the plate the  spark-gap may be abolished; by screwing back the point the  resistance can be increased gradually, till the light in the  tube indicates that the inverse current has ceased.  Where tubes of different hardness are used in succession,  this ready means of regulation is of great value, and its effect  on the appearance of a tube is often very striking.  in most catalogues of electro-medical instruments, as well as  in works on theoretical electricity.  The external appearance, doubtless familiar to all our  readers, is recalled by Figs. 29, 87, and 45.  Fig. 41 shews diagrammatically the arrangement of  essential parts.

Considering the primary current as entering at (A +), it may  be traced up the metal pillar (G), across platinum points at  (H) to the hammer (J), down the spring (D), and thence to the  primary winding of the coil (bb). By that it is led round the  core (aa), then back to the other terminal (A-).  The core (aa), consisting of a bundle of wires or thin sheets  of soft iron, becomes rapidly magnetised by influence of the  current circulating round it.

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