NA TURE 



[Januarv 12, 1899 



writer would point out that the resources of cable 

 engineers in devising direct metallic-circuits are by no 

 means exhausted. There is a singular lack of evidence 

 on this point in the Blue Books ; the Royal Commission 

 seems to have swung about a good deal on its own small 

 swivel, with something very like disregard for outsiders. 

 For some reason, perhaps unknown, they failed to call as 

 witnesses the very men whose experience would have 

 made for success. In departing from these older methods, 

 it must not be forgotten that we are departing from all 

 the advant.-iges offered by the telephone ; skilled oper- 

 ators will have to be requisitioned, and there will be need 

 for a delicate device for " callingup " the operator. 



The fourth Report (dated May 12, 1896) of the Royal 

 Commission concluded with a very hopeful note on a 

 contemplated trial of an " inductive method" suggested 

 by Mr. Evershed. "Under this system," it says, "the 

 cable running from the shore is laid in a circle on the 

 bottom of the sea immediately under the light-vessel, the 

 -circle having such an area that the vessel will always be 

 within the circumference of it. Round the deck of the 

 vessel a number of 'turns' of insulated wire are coiled, 

 which are in connection with a telephonic receiver on 

 board ship. This system has not yet (May 1896) been 

 subjected to a practical test at a light-ship." 



The fifth and final Report of that same Royal Com- 

 mission iSeptember 1897), referring to the trial, observes 

 that "the experiment was carried out in .August 1896 at 

 the (".oodwin (North -Sand Head) light-vessel, but after a 

 •careful trial it proved a failure. The apparatus had been 

 tested on shore with satisfactory results, but when it was 

 tried at the light-vessel, which is moored in ten fathoms 

 of water, it was found almost impossible to eft'ect com- 

 munication by means of it, the electric energy being 

 almost entirely lost in the sea. The difficulties ex- 

 perienced were entirely electrical." 



In the following year a mathematical investigation as 

 to the cause of this failure was undertaken by Mr. C. S. 

 Whitehead {Proc. Pliys. Soc, vol. xv. pt. -xi. pp. 188-200, 

 1897) ; it was communicated to the Physical Society in a 

 paper read June 11, 1897. His theoretical results show 

 that if his calculations are correct, the normal magnetic 

 induction of the primary coil loses 79 per cent, of its 

 mitial value in passing to the secondary through ten 

 fathoms of sea-water. These figures as to the absorption- 

 factor of sea-water have been criticised by Mr. Oliver 

 Heaviside and by Dr. Oliver Lodge. In the meantime 

 Mr. Evershed has turned his attention to improving his 

 apparatus. In his recent paper he regards the inductive 

 ■coils as a particular case of a dynamo-electric machine, 

 he neglects the absorption-factor, and gives an ex- 

 pression for the power available at the receiving station 

 for ultimate conversion into motion at the receiving 

 instrument. The result arrived at is that the mechanical 

 power is independent of the number of turns into which 

 the total volumes of copper are divided on the primary 

 and secondary circuits, and that the two circuits should 

 have equal volumes of copper. His assumption at the 

 outset, that the use of two horizontal circuits implies a 

 loss of one-half the mutual induction between them, may 

 have to be modified. Perhaps the most important part 

 of Mr. Evershed's paper is his description of a "call" or 

 receiving apparatus, which enables exceedingly ;ninute 

 currents to be detected. Erom a remark in his paper, he 

 does not appear to be aware of the work that has been 

 done in America by Lucien Blake and Eugene Caldwell 

 in their attempts to provide an instrument to replace the 

 telephone. In the .Annual Report of the Lighthouse 

 Board of the United .Slates (June 30, 1895, p. 37) will be 

 found an account of a calling device. This Report says : 

 " The receiving apparatus or relay for this system would 

 be vibratory in character and tuned to a frequency of 

 vibration to correspond with the period of the calling 

 ■current. In addition to this mechanical adjustment, the 



NO. 1524, VOL. 59] 



electrical circuits might be adapted by the use of con- 

 densers and inductances to respond more readily to 

 alternate currents of the same period. The action of 

 such a system would be cumulative, i.e. each successive 

 impulse of current would arrive just in time to increase 

 the vibration in the relay until sufficient amplitude would 

 be obtained to operate a circuit-closing device." This 

 report also touches upon an interesting point raised by 

 Sir Henry Mance in the recent discussion. With 

 regard to the sensitiveness of " calls," it observes that " a 

 careful study of all kinds of instruments in which weak 

 alternating currents produce mechanical movement, 

 shows that by far the most efficient are those in which 

 the current does not produce an alternating magnetic 

 field, as in the electrodynamometer, but operates to 

 increase and decrease the strength of field of a permanent 

 magnet. The mechanical motion produced by such 

 polarised machines is always vibratory. Careful measure- 

 ments in the laboratory show that this relay will work 

 positively with a current representing ooooi watt, and 

 that it can be operated with much less energy than this." 

 This report seems to have escaped the attention of Mr. 

 Evershed ; he suggested in his recent paper that so far 

 as he was aware the ordinary Bell telephone has been 

 exclusively used as the receiving device in this class of 

 experiments. There is, of course, no question as to 

 priority ; Mr. Evershed was using a vibratory indicator 

 on this synchronous principle in 1892, the American 

 report refers to an instrument designed in 1895. In his 

 latest (1896) form of apparatus, Mr. Evershed dupli- 

 cates the vibratory metallic rectangles ; they are con- 

 nected to two separate secondary circuits in such a 

 direction that they oscillate in opposite phases in a strong 

 magnetic field. This arrangement has the advantage 

 that when the twin rectangles are in unison it is almost 

 impossible to bring them into contact by shaking the 

 instrument. 



Mr. Preece's paper is a history of the experiments 

 made by himself and his staff, since 1885, on "the 

 electromagnetic form of ;ctheric telegraphy," i.e., on the 

 method of signalling between one alternate current 

 circuit and another. This work has been more or less 

 familiarised to us by the newspapers. He used hori- 

 zontal coils of large diameter at the sending and receiving 

 stations, but they were regarded as " impractical things " 

 and they were replaced by straight conductors, placed in 

 parallel planes, one at each station. Capacity and self- 

 induction were eliminated. An interesting series of 

 experiments is described, in which two earth-plates are 

 buried at a distance apart in the earth ; the lines of 

 electric " flow " are traced, and the locus of a hypothetical 

 resultant-conductor is plotted. Incidentally, Mr. Preece 

 makes the curious statement that " we know /ly 0/iin's 

 law that the resistance of a circuit increases with its 

 specific resistance and length, and diminishes with its 

 sectional area." By Boyle's law, this involves a miscon- 

 ception I Of the various arrangements of inductors for 

 his mode of signalling, Mr. Preece jirefers parallel wires 

 connected to earth at each extremity, the wires being 

 carried to a considerable height. The most s.atisfactory 

 results were obtained over a distance of y;^ miles-across 

 the Bristol Channel ; and when it is remembered that 

 the speed of signalling is practically as high, and that 

 the system is as certain and as efficient as the ordinary 

 metallic circuit system, this result must be regarded as 

 the best so far brought to notice. 



The question naturally occurs to us at this point, 

 What then has become of the " coherer " systems ? Mr. 

 Preece' h.is recently said that the Marconi system is 

 able to traverse a distance of twenty-five miles ; but on 

 the same occasion he mentions that there would be no 

 difficulty in communicating by the alternate-current 

 inductor system over a similar distance. Hence there 



' T/u KHxiHttr, November J5, 1S9S. 



