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RONTGEN-RAY PHOTOGRAPHY 345
electrode of the tube to the negative secondary terminal when using a Rontgen-ray tube.
E is a switch by means of which the primary circuit can be broken or reversed at will. This switch should always be kept open until all the connections are made and the work is ready to be started. In the diagram it is shown in the " cut off " position.
D is the hammer by means of which the alternate make and break of the primary current is arranged.
The screw C is for the purpose of regulating the distance of the hammer from the soft iron core, so as to ensure the most easy and uniform action of the coil.
The screw B alters the pressure upon the point of the screw C of the spring to which the hammer D is attached. When turned to the right, this screw lessens the pressure, and so makes the amount of current passing through the primary circuit less. On this account, if it is desired to obtain a brighter glow in the tube, increase the current in the primary circuit by turning screw B slightly to the left.
When working properly the coil should make a minimum amount of noise. The sparks should be sharp and snappy when the coil is used for radiographic purposes. Great care must always be taken not to touch any part of the secondary circuit while the coil is in action, otherwise a very unpleasant shock may be experienced. The kind of coil which is very extensively used by Rontgenray workers in our hospitals and elsewhere is shown in fig. 153.
Coils of this type are made in several different sizes, which are capable of giving sparks from four inches to eighteen inches in length in air at ordinary atmospheric pressure. It requires a coil capable of giving at least an 8-inch spark to get satisfactory results in X-ray work.
When a discharge of electricity is sent through a vacuum tube, the appearance of the discharge depends