HIGH FREQ UENCY AND HIGH POTENTIAL CURRENTS. 385

pass uniformly through the space between the washers, is di-verted through the wire. But if the tube be inclined sufficientlyto that line, it will light up in spite of the washers. Also, if ametal plate be fastened upon a glass rod and held at right anglesto the line joining the binding posts, and nearer to one of them,a tube held more or less parallel with the line will light up in-stantly when one of the terminals touches the plate, and will go-out when separated from the plate. The greater the surface ofthe plate, up to a certain limit, the easier the tube will light up.When a tube is placed at right angles to the straight line joiningthe binding posts, and then rotated, its luminosity steadily in-creases until it is parallel with that line. The writer must state,however, that he does not favor the idea of a leakage or currentthrough the space any more than as a suitable explanation, for heis convinced that all these experiments could not be performed witha static machine yielding a constant difference of potential, andthat condenser action is largely concerned in these phenomena.

It is well to take certain precautions when operating a Ruhm-korff coil with very rapidly alternating currents. The primarycurrent should not be turned on too long, else the core may getso hot as to melt the gutta-percha or paraffin, or otherwise injurethe insulation, and this may occur in a surprisingly short time,considering the current’s strength. The primary current beingturned on, the fine wire terminals may be joined without greatrisk, the impedance being so great that it is difficult to forceenough current through the fine wire so as to injure it, and infact the coil may be on the whole much safer when the terminalsof the fine wire are connected than when they are insulated;but special care should be taken when the terminals are con-nected to the coatings of a Leyden jar, for with anywhere nearthe critical capacity, which just counteracts the self-induction atthe existing frequency, the coil might meet the fate of St. Poly-carpus. If an expensive vacuum pump is lighted up by beingnear to the coil or touched with a wire connected to one of theterminals, the current should be left on no more than a fewmoments, else the glass will be cracked by the heating of therarefied gas in one of the narrow passages—in the writer’s ownexperience quod erat demonstrandum}

1. It is thought necessary to remark that, although the induction coil maygive quite a good result when operated with such rapidly alternating currents,,yet its construction, quite irrespective of the iron core, makes it very unfit forsuch high frequencies, and to obtain the best results the construction should begreatly modified.