Convert the E.M.F. of the current supplied to a voltage suit-  able for the X-ray tube. The degree of this function may be  appreciated when we note that to produce a 12-inch spark  across the discharging points of a coil requires a potential of  about 148,000 volts. This increase in voltage is obtained at a  corresponding loss in amperage, and the product of the two  factors—technically expressed as Watts—in the current  derived from the coil should approach that in the current  supplied, a slight loss being inevitable. No satisfactory  method exists, however, whereby the output of a coil may be  accurately measured.

The action of a coil is commonly expressed in terms of the  length of spark which it is able to send across the terminals  of its secondary when the primary is supplied with a suitable  exciting current, but this expression is misleading.  Formerly, indeed, it was the custom to consider the spark-  length as synonymous with the power of a coil to do good  X-ray work, and a spark of from 16 to 20 inches was  considered a desideratum for a good X-ray coil. But we now  recognise that a coil with a maximum spark of 10 to 12 inches  may be capable of satisfying all our requirements, and we pay  more attention to the nature or ‘ thickness’ of the spark  emitted. Thus, in our hospital installation, the coil gives us  the full spark-length with a current of 4 amperes, but we  commonly use 6 to 8 amperes, and may pass as much as  12 amperes at an odd time. The additional current does not  lengthen the spark, but increases its intensity or fullness.  This coil, we may say, is designed to work nominally with  interruptions at the rate of 600 per minute, this being  considered a mean rate for usual work. There is, of course,  a margin of reasonable efficiency above and below this rate.

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