1. Continuous current from the main, where available, should certainly be used direct for any permanent installation. If at a voltage of 100 or less, the current may be sent unaltered to a suitably-designed interrupter and coil. If at a higher voltage (as in West London, where voltage is 240), some form of modifying apparatus must be interposed. In this country the usual supply from the mains is continuous, at a pressure of 200 to 250 volts.

(a) A rheostat in some form may be introduced, whereby current at any voltage in a regular series may be used as desired. The usual form consists of a number of open coils of thick iron wire, suitably fixed to a slab of slate or marble, and connected, as shewn in Fig. 18, so that any length may be introduced as resistance in the circuit. This piece of apparatus, from its function, is often termed a * volt selector.’ Its principle may be understood from the annexed diagrams (Figs 13 and 14). In « direct’ resistances the current is directly ‘ choked off’ before reaching the further parts of the installation, as in the smaller coil shown in Fig. 14. In a ‘ shunt * arrangement, as shown in Fig. 13, and in the larger coil of Fig. 14, an alternative path is offered to the current, and more or less of the available current passes by this alternative circuit as more or less resistance is intro- duced into the installation circuit, which receives current inversely proportional to the resistance. This latter form is preferable, as the transition is rendered more smooth and gradual between adjoining steps of the selector. In both instances the residue of current disappears in the resistance, being spent in the production of heat in its coils. Thus such an arrangement may appear wasteful of current; but it is really more economical than an individual supply for the relatively small quantities usually required, and current is supplied from the main for such purposes at reduced rates.

Fig. 14 shows diagrammatically a very convenient arrangement, whereby a high voltage (say 250) is first adapted by a shunt resistance to 100 volts, then current selected as desired from a series resistance giving a range from 100 to 50 volts.

(b) A motor-transformer is probably more economical where larger quantities of current are likely to be used, as in regular hospital work. This piece of apparatus, as shown in Fig. 15, consists of a motor constructed to run at the voltage of the main supply, and connected direct to a dynamo wound to give off current at the voltage desired for use. In our opinion, 50 volts is the ideal strength for general X-ray work, since it is sufficient for all practical purposes; and disturbance in the induction-coil is obviated from the inverse currents, which at higher voltages become of sufficient magnitude to be troublesome.

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