An adjustable spark-gap, as illustrated in Fig. 39, may  be interposed in the secondary circuit for the same purpose of  cutting out inverse currents, and is so used commonly with  static machines. For heavier currents, as from coils, this is  a somewhat noisy working arrangement; but such an arrange-  ment as is shewn in Fig. 39 allows much more easy regulation  and the valve-tube is interposed on that side. Fig. 38 shews  the arrangement diagrammatically, but in practice the valve-  tube should be interposed, as in Fig. 40, between the coil and  the sparking pillars, otherwise the alternative spark will  measure the resistance of the valve-tube in addition to that of  the X-ray tube, and thereby convey a false idea of the condition of the latter.  Readings on a milliamperemeter placed in the secondary  circuit show that this action is more than theoretical, and

The use of the spintermeter and milliamperemeter in  the secondary circuit of the coil has already been explained,  and the annexed diagram (Fig. 40) shews their arrangement  during operation.

Hammer Break.

Since the efficient working of an induction-coil depends in  so large a degree upon an intelligent understanding of its  principle and construction, we have on request decided to add  here, as in the case of accumulators, some more theoretical  and detailed instructions to workers.

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