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INVENTIONS 01' NIKOLA TESLA.

In these London lectures, among the many notable points madewas first, the difficulty of constructing the alternators to obtainthe very high frequencies needed. To obtain the high fre-quencies it was necessary to provide several hundred polar pro-jections, which were necessarily small and offered many draw-backs, and this the more as exceedingly high peripheral speedshad to be resorted to. In some of the first machines both arma-ture and field had polar projections. These machines produceda curious noise, especially when the armature was started fromthe state of rest, the field being charged. The most efficientmachine was found to be one with a drum armature, the ironbody of which consisted of very thin wire annealed with specialcare. It was, of course, desirable to avoid the employment ofiron in the armature, and several machines of this kind, withmoving or stationary conductors were constructed, but the re-sults obtained were not quite satisfactory, on account of thegreat mechanical and other difficulties encountered.

The study of the properties of the high frequency currentsobtained from these machines is very interesting, as nearly everyexperiment discloses something new. Two coils traversed bysuch a current attract or repel each other with a force which,owing to the imperfection of our sense of touch, seems contin-uous. An interesting observation, already noted under anotherform, is that a piece of iron, surrounded by a coil through whichthe current is passing appears to be continuously magnetized.This apparent continuity might be ascribed to the deficiency ofthe sense of touch, but there is evidence that in currents of suchhigh frequencies one of the impulses preponderates over theother.

As might be expected, conductors traversed by such currentsare rapidly heated, owing to the increase of the resistance, andthe heating effects are relatively much greater in the iron.The hysteresis losses in iron are so great that an iron core,even if finely subdivided, is heated in an incredibly short time.To give an idea of this, an ordinary iron wire inch indiameter inserted within a coil having 250 turns, with a currentestimated to be five amperes passing through the coil, becomeswithin two seconds’ time so hot as to scorch wood. Beyond acertain frequency, an iron core, no matter bow finely subdivided,exercises a dampening effect, and it was easy to find a point at