424 



NATURE 



\March 3, 1881 



Mr. Day in the Phil. Tramaciions for 1S76, showed that if we 

 assumed all the instances therein mentioned of sensibility of 

 selenium to light were due to an electromotive force set up, and 

 not to change of resistance at all, then on the whole all the 

 results would have been arrived at if this electromotive force set 

 up in different cells, for the same intensity of light, increased 

 more rapidly than the resistance of the cell, and was the s^reater, 

 the greater the electromotive force of the auxiliary battery 

 employed. They disagreed therefore from Mr. Bidwell in his 

 idea that the name "cell" was at all inappropriate. 



Professors Ayrton and Perry referred the Members to their 

 original letter in Nature for the account of their plan for seeing 

 by electricity. Shortly, it consistev.1 in projecting at the sending- 

 station an image on a screen consisting of a number of selenium 

 cells, the current flowing in each of which from an auxiliary 

 battery was controlled by the intensity of the light falling on it. 

 At the receiving-end of the line a light was thrown on a screen 

 intercepted more or less by little shutters, the opening or doling 

 of each of which %\ as controlled by the current allowed to pas^ 

 through the corresponding selenium cell at the sending tnd. 

 Hence on the receiving screen a picture in mosaics was cast cor- 

 respondiug w ith the image projected on the screen at the seuding- 

 end, ai.d ^•arying with every change in the 'image cast on the 

 sending-screen. 



The experiment they desired to show the Society was the suc- 

 cessful reproduction on the receiviug-.'^creen of every change of 

 illumination of one square of the sending-screen. The shutter 

 was an elhptical blackened aluminium disk suspended in a 

 blackened tube of a kind of galvanometer, and making an angle 

 of 45° with the tube when all the light tending to pass through 

 the tube was cut ofl". When this di^k was deflected through 45° 

 all the light passed thrjugh the tube and an image of a square 

 hole was formed by a small lens attached to the tube. For every 

 intermediate position of the shutter an image of the square hole 

 was formed on the screen, but varying in intensity of illumination. 

 Attached to the shutter was a small magnet making an angle of 

 671° with it, and the two were suspended by a silk fibre about 

 one-twentieth of an inch in length. These particular angles were 

 selected so that lirst all variation in intensity of the illumination 

 could be produced with a small motion of the shutter, and 

 secondly, so that the magnet should always be in its most sensi- 

 tive position in the coil through which passed the electric current 

 which traversed and w as controlled by the corresponding selenium 

 square at the receiving end of the line. [The apparatus was 

 then shown in action.] 



They explained ho«- their method of putting, say, thirty or 

 forty selenium cells on a revolving arm would enable them, 

 while dispensing with a large number of cells, to transmit elec- 

 trically a complete picture of even moving objects, and would in 

 addition obviate the difficulty arising from abnormal variations 

 of selenium. 



Instead of the apparatus exhibited to the meeting to show the 

 perfect feasibility of the scheme. Professors Ayrton and Perry men- 

 tioned that they were also experimenting with a large thin mirror 

 with many thick ribs at the back crossing one another. Electro- 

 magnets fim\ly fixed behind the thin parts of the mirror produced 

 by their expansion and contraction very small convexities and 

 concavities on the mirrors face. From their experiments, pub- 

 lished in the Proc. Koy. Soc, on the so-called Japanese m^ic 

 mirrors, it was know n that excessively small convexities and con- 

 cavities of this kind might be made to show themselves in a very 

 decided way on a screen by a divergent beam of reflected light. 

 They proposed to have a circular mirror in rotation, but with only 

 a certain sectional space at the back fitted with electro -magnets as 

 described, and they anticipated that this in conjunction with the 

 rotating section of selenium cells at the other end of the line 

 would produce on a screen a picture over the whole area of the 

 mirror corresponding with the distant image projected on the 

 area traced out by the revolving sector of selenium celb. 



EARTH CURRENTS— ELECTRIC TIDES 

 ^T a meeting of the Society of Telegraph Engineers and of 

 Electricians on Thursday evening, February 10, Prof. G. 

 C. Foster in the chair, a communication was read by Mr. Alex. J. 

 S. Adams upon " Earth Currents— Electric Tides," in which the 

 author related that, from investigations he had carried on in 

 connection with earth currents since the year 1S66, he con- 

 sidered the globe we inhabit as an electrified sphere whose 

 normal electrical condition was liable to disturbance both from 



withiii and from without. Starting upon this theory as a basis, 

 and finding from the result of his observations no evidence that 

 the sun exerted sufticicnt influence to materially disturb the earth's 

 electricity, he undertook a series of systematic observations upon 

 the daily earth-current variations in strength, to elucidate the 

 question, and obtained consecutive observations every quarter of 

 an hour during the interval from April i to 21, 1S79, with a 

 result that the curves of those observations coincided throughout 

 with the curve of moon phages for the same period, and clearly 

 indicated that the chief disturbing power was the moon, and that 

 the earth current variations were strictly lunar-iiiiirnal. 



"But," said he, " there is a yet deeper meaning to the lunar- 

 diurnal current curve than at first sight appears, for an examina- 

 tion shows that the curve for each day represents four datrual 

 maxima, two of a kind, and that each maximum is divided fiom 

 the other by a zero or point of no current." He further explained 

 that whilst two of these maxima always exist upon the opposite 

 sides of the globe, which are in a line pi-rpeiijicular to the moon, 

 two other maxima were also found upon the sides of the globe 

 lying at right angles to the former maxima, and that from a long 

 and carefid consideration of these features of the phenomenon he 

 had arrived at the conclu-ion that whilst the earth's disturbed 

 electricity was, as it were, heaped up by the moon upon the sides 

 of the earth nearest to and farthest from her, much as are the 

 waters of the globe in forming the oceanic tides, the two 

 lateral maxima, upon the other hand, must be considered as parts 

 of a belt or band of electrical maximum that encircles the earth 

 in a position at right angles to a line drawn between the earth 

 and moon. Thus it a]>peared that there were zones of maxima 

 at the sides of the globe nearest to and farthest from the moon, 

 and a circle of maximum at right angles between them, but 

 divided from them by zones of no current. This arrangement of 

 the earth's elec'ricity by the moon the author termed the earth's 

 lunar elulric di.tribution ; the electric maximum facing the moon 

 he designated the major electric /ole, that farthest from the moon 

 the minor electric pole, and the belt of maximum that encircles 

 the earth the electric circle. Likewise the zone of no current that 

 divides the electric circle fiom the major pole he terms the major 

 zero circle, and that zero which separates the electric circle from 

 the minor pole, the minor zero circle. 



The earth's electricity, as thus arranged by the moon, followed 

 that orb in her course through the heaven?, and this motion of 

 the earth's disturbed electricity round the earth, yet irrespective 

 of the globe itself, was termed the lunar diurnctl electric circula- 

 tion, and the axis upon n hich it turned the lunar-diurnal axis. 



A due apprehension however that the moon's influence is in 

 proportion felt by the earth's electricity at rcery fart of the earth's 

 surface he considered necessary for the proper appreciation of 

 the reasonings which led to the foregoing deductions. 



It was then pointed out that there existed a regular retardation 

 or lagging of the earth-current variations behind the correspond- 

 ing phases of the moon to the extent of nearly three hours, this 

 curious phenomenon being in no way, so far as he could tiace, 

 attributable to solar influence. 



The magnetic variations were then considered, and a striking 

 coincidence between the electric and the magnetic lunar-diurnal 

 variation-cm-ves was shown to obtain. The author reasoned 

 that the earth's electric forces^as constituted in the ilectric distri- 

 bution revolved also about an axis parallel to a line pa?sing 

 through the centres of the earth and moon, i.e. a line drawn 

 between the major and minor electric poles — a motion of the 

 electric forces that agreed with the observed direction of the earth 

 current, and which appeared fully sufficient to account for the 

 effect of lunar-diurnal magnetic variation. 



In conclusion he said that a comprehensive consideration of 

 earth-current phenomena opens out a much wider sphere of 

 investigation than that simply embracing variations of strength : 

 it has to recognise directive influence which, applied to electricity, 

 means the production of magnetism, and that the electric circu- 

 lating systems that appear to obtain by reason of these three 

 motions, the earth's diurnal rotation, the lunar current circula- 

 tion, and the terrestrial current circulation — causes which result 

 in the apparently disconnected variations observable in the 

 movements of the magnetic-needle. 



UNIVERSITY AND EDUCATIONAL 



INTELLIGENCE 



Cambridge. — The examiners for the Natural Science Tripos 



during this year are Dr. W. H. Gaskell, Prof. Bonney, Mr. P. 



T. Main, Prof. Watson (Owens College), Prof. Lewis (recently 



