These two pieces of apparatus—coil and interrupter—should always be considered jointly, and designed mutually to suit each other. Different breaks produce very different rates of interruption, and no single coil can be expected to work efficiently with widely varying rate of interruption. Thus, a coil wound to suit a low rate cannot be ‘ saturated’ by each fractional current sent to it by a break giving a much higher rate of interruption; and, conversely, a coil wound to suit a high rate of interruption cannot respond efficiently to the longer periods of excitation allowed by a more slowly acting break. The duration of each contact, or * make,’ during which current is allowed to pass to the coil is also determined by the interrupter, and may be varied according to the result desired. For this also the coil should be adapted. An important point in the action of an interrupter may be here noted—namely, the break of the current must be as sharp and sudden as it can possibly be made. Many workers have made the mistake of applying to the same coil interrupters of vary- ing construction and rate of interruption, and have been prone to judge the interrupter according to the result obtained, possibly ascribing failure to the construction of an interrupter rather than to the true reason of inco-ordination between it and the coil employed.

A coil may be badly damaged also by using with it a break different from that with which it was designed to work. Thus, by substituting a break which passed a much heavier current than the break originally used, we had a coil seriously injured, and we know of at least one London hospital where several coils were rendered useless by such a change injudiciously made.

The choice of coil and interrupter depends upon the demand likely to be made on them—that is, upon the nature of the work to be done.

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