For radioscopy or screen examination we wish a steady fluorescence; hence rapidity of interruption will be the criterion. For radiography the same high rate is not essential, but will lessen the requisite length of exposure. Since those two classes of work are usually combined, we may say that for such work a fairly high rate of interruption is essential, up to 8,000 per minute.

In radio-therapy, on the other hand, there is no especial call for rapid interruption, and a rate of 1,000 per minute will be more than sufficient for such work. The duration of exposure will here frequently be much in excess of those employed in the other classes of work, varying from five to twenty-five minutes, so that unless the apparatus be designed to stand such prolonged runs, it may be unable to withstand the strain.

Where one or other class of work distinctly preponderates, the installation should be designed to suit that work; where neither preponderates, a compromise must be struck, unless Intermediate Apparatus 5′ the installation may be duplicated. Some recent coils are made with arrangements for adjustment to suit varying conditions, and with an interrupter of wide margin of rate may be made to suit the work in hand, but never so efficiently a coil designed specially for specified conditions.

With a knowledge, then, of the nature of the work to be done, the radiologist may settle what rate of interruption will be most suitable, and he will have a coil built to suit; but first he will settle on the interrupter likely to fulfil the conditions.

Interrupters or breaks are in type many and various. They may be classified as—
1. Vibrating or platinum.

2. Motor mercury, including—

(a) Dipper type;

(b) Turbine or jet type.

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