HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 357

through a section near the ends. Furthermore, the current passesprincipally through the outer portions of the conductor, but thiseffect is to be distinguished from the skin effect as ordinarily in-terpreted, for the latter would, or should, occur also in a continu-ous incompressible medium. If a great many incandescent lampsare connected in series to a source of such currents, the lamps atthe ends may burn brightly, whereas those in the middle may re-main entirely dark. This is due principally to bombardment, asbefore stated. But even if the currents be steady, provided thedifference of potential is very great, the lamps at the end willburn more brightly than those in the middle. In such case thereis no rhythmical bombardment, and the result is produced en-tirely by leakage. This leakage or dissipation into space whenthe tension is high, is considerable when incandescent lamps areused, and still more considerable with arcs, for the latter act likeflames. Generally, of course, the dissipation is much smallerwith steady, than with varying, currents.

I have contrived an experiment which illustrates in an inter-esting manner the effect of lateral diffusion. If a very long tubeis attached to the terminal of a high frequency coil, the luminos-ity is greatest near the terminal and falls off gradually towardsthe remote end. This is more marked if the tube is narrow.

A small tube about one-half inch in diameter and twelveinches long (Fig. 189), has one of its ends drawn out into a finefibre/’ nearly three feet long. The tube is placed in a brass sockett which can be screwed on the terminal t, of the induction coil.The discharge passing through the tube first illuminates the bot-tom of the same, which is of comparatively large section; butthrough the long glass fibre the discharge cannot pass. Butgradually the rarefied gas inside becomes warmed and more con-ducting and the discharge spreads into the glass fibre. This spread-ing is so slow, that it may take half a minute or more until thedischarge has worked through up to the top of the glass fibre,then presenting the appearance of a strongly luminous thinthread. By adjusting the potential at the terminal the light maybe made to travel upwards at any speed. Once, however, theglass fibre is heated, the discharge breaks through its entirelength instantly. The interesting point to be noted is that, thehigher the frequency of the currents, or in other words, thegreater relatively the lateral dissipation, at a slower rate may thelight be made to propagate through the fibre. This experiment