With 8 amperes passing, the break works on steadily with  a regular ‘ dick-dick’ bruit, but on approaching 12 amperes  the sound becomes lower-pitched and laboured, denoting that  the coil cannot absorb or utilise the surge of current supplied  to it.

We have said that a great potential spark-length is not  essential for a good coil for X-ray work; it is, indeed, un-  desirable. From a coil emitting very long sparks it is difficult  to obtain more than 1 milliampere of current, whereas with  the shorter and coarser winding suitable for shorter sparks  we may obtain a current of 10 to 15 milliamperes. No tube  presently made could stand that current for more than a few  seconds, so that alteration of coils in that direction is for the  present limited in its utility. Long sparks are correspondingly  ‘ thin’ and thready. What is now wanted for general use is  a spark of moderate length, but ‘ fatter.

Some recent coils have their primary windings made in  separate sections, so that a greater or less length of wire may  be put in circuit according to the strength of current supplied  to the coil, a shorter length, as represented by a less number  of sections, being employed for heavier currents.

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