KNOWLEDGE & SCIENTIFIC NEWS. 



[Al'gust, 1905. 



however, occurs, which must be g-ot over if the machine 

 is to be of practical service — the tendency to a reversal 

 of polarity during- working-. The remedy is easily pro- 

 vided by placing- additional brushes to touch the sectors 

 .-ifter they have passed the collector brushes; that is, at 

 the top of one disc and at the bottom of the other. The 

 reason for this will appear presently when reversal has 

 been explained. 



The defect of reversal is one that occurs more or less 

 in nearly all influence machines except the W'imshurst, 

 and the cause, though extremely simple, seems to have 

 escaped general detection, so that much more mystery 

 has been attached to it than is necessary. 



Reversal is an obvious consequence of the induction 

 which each charged disc exercises on its own earthed 

 sector — the "self-induction " of the disc as it may be 

 called. Each sector, at earthing, is under the influence 

 not only of induction from the opposite disc, but also 

 of induction from the contiguous surface of its own disc, 

 and as the charge increases this inductive influence 

 from its own disc at last overpowers that from the 

 opposite disc, and .so effects a charge of reversed sign. 

 Holtz's memorable observation that when a metal comb 

 is drawn over a highly charged glass plate a charge of 

 opposite sign is left on the glass is only a special case of 

 rif-induction with consequent reversal. 



The most satisfactory wav to prevent this self-induc- 

 tive influence would obviously be to keep the electrici- 

 ties of each disc " bound,'' except at the collectors. 

 " Bound " electricity does not induce a charge in 

 adjacent bodies. It is only when electricity is free that 

 It is competent to do this. Now, as has already been 

 pointed out, in the Wimshurst machine the electricities 

 of ea6h disc are held " bound " except at the collectors, 

 and that is why self-induction, with consequent reversal, 

 does not occur in the Wimshurst except under vei-;- 

 strained conditions, while with the \'oss and most other 

 influence machines reversal is a constant source of 

 trouble. 



In the induction pump, which has been described 

 above, the sectors of the overlapping portions of tho 

 disc have their charges " bound," and these are, there- 

 fore, incompetent to produce self-induction, but th: 

 sectors that precede the induction brusiies are charged 

 with free electricity, and it is when their charge is great 

 that they are able to induce a reversed charge. To 

 prevent this the additional brushes have to be provided 

 as already described in order to neutralise the sectors 

 at these portions of the discs' orbits. 



There is one possibility with regard to self-induction 

 that is worth considering, viz., the possibility of render- 

 ing it of .<.er\'ice instead of preventing it as a hindrance. 

 It is well known that Holtz turned it to account by 

 utilising the charge of opposite sign which he found 

 was left on a charged plate after passing a metal comb. 

 A suggestive parallel to the self-induction of a charged 

 di.sc occurs in dynamical electricity in the phenomenon 

 named by Faraday the " self-induction " of the coil, i.e., 

 the inductive influence of each winding of the coil on 

 the next winding. The question is whether, as in that 

 case the self-induction of the coil is made to produce 

 the " extra current," in some similar way the self-induc- 

 tion of the disc might not be made to produce "extra 

 charge," and so made advantageous to the output. To 

 effect this a machine totally different in construction 

 from any of the present influence machines would have 

 to be devised, but the problem is worth considering in 

 view of the advantages which influence machines offer 

 over the induction coil for X-ray work and even for 



vlrrlrcc tr-Vgraphy. 



The Great Meteorite of 

 WilloLmette. 



.\ FL'LL and interesting description is given in Cosmos 

 of this meteorite, found in the hills of Western Oregon 

 in 1902, and we reproduce one of the illustrations. 

 This clearly shows the peculiar honeycombing of the 

 base, the cause of which has been a matter of some 

 speculation. The conditions contributing to affect the 

 surface of the meteorite are peculiar. The air in front 

 of it, during its rapid progress to the earth, is com- 



pressed to such an extent that it becomes almost like 

 a solid body. The speed attained is calculated to be 

 something like 50 miles a second. The friction should 

 generate a temperature of about 5,000° Centigrade, 

 sufficient to melt any material of which the meteorite 

 is composed. This is mostly iron, with a small amount 

 of nickel. The deep holes and furrows in the stone 

 are, however, more probably the result of disintegra- 

 tion through chemical and atmospheric action on the 

 earth. The meteorite, which is 10 feet in greatest 

 length, weighs i^i tons. 



"A R^aised BeaLch irv Anglesey." 



To THE Editors oi- " Knowlf.dge." 



Dear Sirs, — In your issue for July last. Prof. Hryan, F.R.S., 

 refers to what he calls a " raised beach " restiiiK upon boulder 

 clay between Beaumaris and Penmon, AhkIcsi y. 



Reds of sand similar to that described l)y Prof. Bryan can 

 be seen at Llanddona. Ceinaes, and other places round the 

 coast of the island ; and at Towyn Trewan, Aberffraw and 

 Newborou{;h larfje tracts of land have been covered with sand. 

 But surely these sands cannot be called " raised beaches " in 

 any other sense than that of having been raised by the wind. 



.1 do not doubt the facts mentioned by Prof. Bryan, but w<! 

 must get stronger evidence than that of these "r.-iised 

 beaches" to establish the conclusion that "changes have 

 taken place in the level of \\u: earth," especially when we arc 

 dealing with a portion of the earth that has clearly been 

 remarkably stable all along the geological ages from Pre- 

 Cambrian limes to the present. 



Vours faithfully. 



W. Edwards, 



University College, Aberystwyth, July 24, 1905. 



