Cylinder 1 did not affect the maximum spark length. Cylinders 2-4 reduced the maximum spark to ⅒th of its former value; 7 and 8 to ¹/₁₀₀th, and No. 9 to ¹/₂₀₀th of its former value (viz., from 9 millimetres to ·05 millimetre). Nos. 10 and 11 had but a feeble action, and reduced the spark from 8 to 7 millimetres. Tube No. 12, filled with distilled water, scarcely affected the spark length; the period of the secondary increases a little, but the maximum spark is the same as before, once syntony is re-established. Filled, however, with dilute sulphuric acid, containing 10, 20, or 30 per cent., the tube reduced the spark considerably, in each case about the same, viz., from 9 to 1·3 about. (Currents induced by Maxwellian radiation in electrolytes had been already observed by J. J. Thomson.) While trying to re-establish syntony between primary and secondary, I found that the period of the resonator was considerably increased by the cylinders 2-4, but that the maximum spark length was much diminished. With the cylinders Nos. 5-9 in the spiral, it was no longer possible to establish syntony, “a fact which is certainly due to their considerable absorption of energy. Take, for example, cylinder 9: electromagnetic energy must converge rapidly towards it in order to be transformed, and the space finds itself empty of energy as air is exhausted of vapour in presence of an absorbing substance.” “This absorption is probably due to hysteresis in the ferruginous cylinders; the development of Joulian heat, so typically shown by cylinder 12, being undoubtedly of the same order in cylinders 3-9 as in Nos. 10, 11. “It is probably by reason of this absorption that I have not succeeded in establishing stationary magnetic waves in a circuit of ferro-paraffin.” If one of the cylinders 2-9, is wrapped in tinned paper before introducing it into the spiral A, its action is completely stopped. (These conducting cores _diminish_ the period of the resonator; it is much as if the spiral A were partially shunted out; but the maximum spark returns as soon as syntony is re-established.) To examine this further he enclosed the cylinder in drums of cardboard having fine wires either along generating lines, or along circular parallels. The latter suspended the action of an interior ferruginous cylinder, the former did not. To find to what depths the magnetism penetrated, Birkeland inserted hollow ferruginous drums into A, measured their effect, and then plunged solid cylinders into them to see whether the effect increased. He thus found that the magnetisation easily traversed 7 millimetres thickness of the 10 per cent. ferro-paraffin, and 5 millimetres of the 25 per cent. The substance is comparable to a dieletric on the theory of Poisson-Mossotti.