234 



KNOWLEDGE. 



[OCTOBEE 1, 1900. 



The Discovery and Development of the Coherer. 



Hertz's splendid results were all obtained witli only 

 the simple resonator sbown in Fig. 2 as a detector of 

 the presence of electric waves. This would, however, not 

 have been nearly sensitive enough for transmitting 

 signals over considerable distances, even with the most 

 perfect oscillators or transmitting instruments, and each 

 transmission, therefore, only advanced into the realm of 

 the possible with the discoveiy of the microphonic trans- 

 mitter, or, as Professor Lodge calls it, the cohertr, which 

 was not brought into general use for this pui-pose until 

 years latei-, although it had been discovered and actually 

 used for the detection of the presence of the Hertzian 

 waves by that great and patient experimentalist. Pro- 

 fessor Hughes (whose loss we have so recently had to 

 lament), as far back as 1880, some half-dozen yeai's 

 before Hertz began his investigations. 



His experiments were shown to Sir George Gabriel 

 Stokes and other physicists in 1879 and 1880, but owing 

 to their unfortunate failm-e to gra,sp the meaning of 

 them as Hughes himself certainly did, their publication 

 was deferred. The result was that Hughes found his 

 own discoveries as to the sensitiveness of the micro- 

 phonic contact, and its useful employment as a receiver 

 for electrical ether waves, remade by others. 



A capital historical sketch of the course of 

 this discovery was given by Professor Lodge in the 

 Electrician for November 12th, 1897, from which 

 much of what follows has been taken. In this article 

 he suggests that probably the earliest discovery of co- 

 hesion under electric influence was contained in a for- 

 gotten observation of Guitard in 1850, that when dusty 

 air was electrified from a point the dust pai-ticles tended 

 to cohere into strings or flakes, and points out that 

 the same thing occiu's in the formation of snowfiakes 

 under the influence of atmospheric electrification, and 

 in the cohesion of small drops into large ones in the 

 neighbourhood of a charged cloud forming the familiar 

 thunder shower. In 1879, Lord Rayleigh showed that 

 when a stick of rubbed sealing wax was brought within 

 a few yards of a small fountain which was scattering 

 its spray in all directions, the scattering ceased, the 

 broken jet rising and falling in large heavy drops. 



The next stage, with the exception of Professor 

 Hughes's work, which it must be borne in mind re- 

 mained unknown during the whole of the development 

 described in what follows, was the re-discovcry by Pro- 

 fessor Lodge and the late J. W. Clark of Giiitai-d's dust 

 phenomenon when experimenting on the cause of the 

 dust^free region of air discovered by Professor Tyndall 

 as existing over hot bodies, and erroneously ascribed 

 by him to the dust being burnt up, but which was shown 

 by Lodge, Osborne, Reynolds and others to be really 

 due to molecular bombardment, phenomena analogous 

 to those occurring within a Crookes' radiometer. 



Before, however, aiTiving at this explanation, experi- 

 ments were made to see if it was caused electrically, 

 and it was found that when the hot body was placed 

 in a thick smoky atmosphere and then chai'ged with 

 electricity the smoky atmosphere immediately became 

 clear. In 1889, Professor Lodge was investigating the 

 action of the lightning guards used for protecting tele- 

 graphic insti-uments from the effect of the sudden rushes 

 due to lightning dischai-ges. These were made by adding 

 as a shunt to the circuit containing the instrument, an 

 open circuit with a small air-gap, with te:Tninals con- 

 sisting of a pair of small brass balls, across which the 



discharge jumped, rather than flow round the coils of 

 the insti-ument, which had great self-induction, and 

 therefore offered much opposition to a sudden rush of 

 cun-ent. Lodge found tliat when the knobs were placed 

 too close together even a Leyden jar discharge would 

 often short circuit the gap, the knobs being found both 

 by electrical and mechanical tests to be feebly united 

 at a single point. When the knobs were in mechanical 

 contact, and separated only by an extremely thin film, 

 consisting probably of oxide, extremely feeble sparks 

 were found to be sufficient to produce this effect. The 

 adhesion of the two sui-faces were demonstrated by 

 means of an electric bell placed, together with a single 

 battei-y cell, in the circuit, and every time a spark 

 occurred the bell rang, and continued to ring until the 

 table on which the apparatus was standing, or some part 

 of the support of the knobs, was tapped, so as to shake 

 them asunder again. The arrangement was found to 

 form a convenient detector in the syntonic Leyden jar 

 experiment described at the beginning of this article. 



If the electric bell was placed on the same table as 

 the sparking knobs, or, better, were allowed to touch 

 them, its tremor was found to be quite sufficient to 

 effect this separation, unless the spark and, therefore, 

 the adhesion had been too strong. In the meantime. 

 Hertz's experiments had attracted general attention from 

 physicists. Professor Minchin, in 1891, when working 

 with some photo-electric cells, and especially some which 

 behaved abnormally, as it seemed to him at the time, 

 and which he called " impulsion cells," found that when 

 a Hertz oscillator was working in another part of the 

 r om the electrometer connected with liis cells re- 

 sponded, and by means of this detector, which certainly 

 depended on the coherer principle, he succeeded in 

 signalling without wires over a considerable number of 

 yards. 



Professor Boltzmann, about the same time, used a 

 charged gold-leaf electroscope for a like purpose, 

 arranging it ,so that the electroscope was just on the 

 l^oint of discharging across a minute air-gap, so that its 

 leaves were deilected by a definite amount. It was 

 found when in this condition to be extremely sensitive 

 to Hertz waves, which, if excited in any part of the 

 room, would bridge over the gap and dischai'ge the 

 instniment. 



This, as Professor Lodge points out, is not a detector 

 depending on the principle of cohesion, but it led him, 

 when repeating the experiment in a modified form, to 

 the conclusion that cohesion could be effected by the 

 surgings due to the regular Hertz waves. 



One of the modifications adopted by him was to make 

 the gajj of carbon, and to connect it, with its wave 

 collector, to the terminals of the 110-volt electric light 

 leads, so that whenever a Hertz oscillator was discharged 

 across the gap, the spark would close the circuit and set 

 up an ai-c. This method was suggested to him by the 

 observation of the behaviour of some incandescent 

 lamps used to light his lecture tables, which were shaded 

 on one side, and prevented from rotating, by means of 

 a pair of copper wires stretched across the lecture room. 

 As long as the wires were there the lamp fuzes used to 

 blow whenever a Hertz oscillator was worked in the 

 room, owing to these wires acting as collectors, and they 

 were therefore replaced by silk threads, when the fuzes 

 ceased to blow. 



In 1891, Professor Branly, of the Catholic Institute 

 in Parifl, published some experimental researches of the 

 greatest importance, in which he showed that metals 



