A motor-transformer is extremely easy of manipulation, and, beyond ordinary care of lubrication and attention to brushes of the generator, requires no attention, expert or otherwise.

The starting-switch is usually made of a form to send the driving current into the motor gradually, since the full current, suddenly switched on to the motor winding, might readily do serious damage to it. In ordering this piece of apparatus, the voltage of the supply must be noted, as well as the nature of the current desired to be generated for use.

2. Alternating current is unsuitable for direct use, and must of necessity pass through some modifying apparatus before reaching the X-ray installation.

(a) A motor transformer is, in our opinion, the most satisfactory means of adaptation, the motor being wound to drive off the alternating circuit, and being coupled direct to a dynamo designed to supply continuous current as desired. In the West London Hospital the current supplied from the main is alternating, at 110 volts, with a periodicity of 50 per second. This is led to a 2J horse-power motor-transformer, from which continuous current is derived at 50 volts, with a maxi- mum of 20 amperes, for use in the X-ray depart- ment.

The above note and figure on motor-transformers for converting the voltage of continuous current from the main apply equally to those designed for converting alternating current. In this case it is necessary, in ordering, to state the voltage of supply, its period, and its phase—single, triphase, etc.

(b) The Gaiffe-Blondel mercury jet break, shewn in Fig. 16, is an example of a type of machine designed to work synchronously with the period of an alternating current, and so transmit the impulses in one direction, while arresting, or diverting, those in the other. This action produces an interrupted unidirectional current suitable for supply to an induction-coil. But, in our experience, though this arrangement is good for high- frequency or for therapeutic work, the synchronicity may occasionally fail. Thereby the polarity of the resultant current may become reversed, and the irregular rays thus produced in an X-ray tube spoil an exposure for radiographic purposes. Along with a Villard’s valve- tube to protect the X-ray tube from such inverse currents this break may, however, form a satisfactory means of utilizing an alternating current.

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