Willoughby SMITH, Letter to Latimer Clark, 4 février 1873.
of Light on Selenium during the passage of an Electric Current",
Willoughby Smith, un pionner du câble sous-marin, lance les recherches sur le caractère photo-sensible du sélénium.
Willoughby Smith (né à Great Yarmouth, Angleterre, 6 avril
1828, mort à Eastbourne, Angleterre, 17 juillet 1891) est un des
noms importants du développement de la télégraphie, à laquelle il
s'intéresse dès l'age de 20 ans. En 1848, il rejoint la Gutta-percha
Company de Londres, et peu après commence des expériences sur les questions
d'isolation, grâce au gutta-percha des fils s en fer et en cuivre. En 1849, son
entreprise s'est engagée à fournir 30 miles de fil isolé pour relier Douvres à Calais, et pendant l'année suivante Smith
supervise la fabrication et la pose de ce câble. Par la suite, il est
constamment engagé dans les questions relatives à la pose de câbles et
collabore aux expériences de Charles.
Letter to Latimer Clark
My Dear Latimer Clark
Being desirous of obtaining a more suitable high resistance for use at the Shore Station in connection with my system of testing and signalling during the submersion of long submarine cables, I was induced to experiment with bars of selenium - a known metal of very high resistance. I obtained several bars, varying in length from 5 cm to 10 cm, and of a diameter from 1.0 mm to 1.5 mm. Each bar was hermetically sealed in a glass tube, and a platinum wire projected from each end for the purpose of connection.
The early experiments did not place the selenium in a very favourable light for the purpose required, for although the resistance was all that could be desired - some of the bars giving 1400 M W absolute - yet there was a great discrepancy in the tests, and seldom did different operators obtain the same result. While investigating the cause of such great differences in the resistance of the bars, it was found that the resistance altered materially according to the intensity of light to which they were subjected. When the bars were fixed in a box with a sliding cover, so as to exclude all light, their resistance was at its highest, and remained very constant, fulfilling ail the conditions necessary to my requirements; but immediately the cover of the box was removed the conductivity increased from 15 to 100 per cent, according to the intensity of the light falling on the bar. Merely intercepting the light by passing the hand before an ordinary gas-burner, placed several feet from the bar, increased the resistance from 15 to 20 per cent. If the light be intercepted by glass of various colours, the resistance varies according to the amount of light passing through the glass.
To ensure that the temperature was in no way affecting the experiments, one of the bars was placed in a trough of water so that there was about an inch of water for the light to pass through, but the results were the same; and when a strong light from the ignition of a narrow band of magnesium was held about 9 in above the water the resistance immediately fell more than two-thirds, returning to its normal condition immediately the light was extinguished.
I am sorry that I shall not be able to attend the meeting of the Society of Telegraph Engineers tomorrow evening. If, however, you think this communication of sufficient interest, perhaps you will bring it before the meeting. I hope before the close of the session that I shall have an opportunity of bringing the subject more fully before the Society in the shape of a paper, when I shall be better able to give them full particulars of the results of the experiments which we have made during the last nine months.
Minerai de sélénium. Photo
by courtesy of
Publications de W. Smith
"Effect of Light on
Selenium during the passage of an Electric Current",
Willoughby Smith, Selenium : its electrical qualities and the effect of light thereon, being a paper read before the Society of Telegraph Engineers, 28th November 1877, Nathan Bros. and Lilly, printers, s.l., s.d. [London, 1877-1878?] (by courtesy of the The Bakken - A Library and Museum of Electricity in Life)
La lettre de W. Smith à L. Clark est reproduite dans BURNS, R. W., Television - an international history of the formative years, Peregrinus, London, 1998, pp. 36-37.
"Effect of Light on Selenium
during the passage of an Electric Current",
"Curious effect of Light o Selenium", Scientific American, 29 March 1873.