Shelford Bidwell
Shelford Bidwell (1848-1909) was an English physicist and inventor. He is best known for his work with "telephotography", a precursor to the modern fax machine.
In the late 1870s, Shelford Bidwell carried out a number of experiments with selenium photocells.
In one experiment, he duplicated the "photophone" originally created by Alexander Graham Bell. This device used sound to vibrate a mirror. Variations in light intensity reflected from the vibrating mirror were detected using a selenium photocell which was then connected to a telephone. This showed that voice communication could be transmitted using light beams with a photocell used to convert it to an electrical signal.
In another experiment, Bidwell placed a selenium photocell inside a rotating cylinder. A small hole in the cylinder allowed the photocell to scan an image on a brightly illuminated glass slide. Another rotating cylinder was covered with a paper treated with potassium iodide. The electrical signal from the photosignal was sent to a platinum wire that darkened the paper when current was applied.
Bidwell's device did not solve the problem of synchronizing the transmitter and receiver. Both cylinders were on the same shaft.
Similar devices for transmitting images had previously been demonstrated Alexander Bain and Frederick Bakewell, but Bidwell was the first to use a photocell to scan the image.
Bidwell reported his results in an article entitled "Tele-Photography" in the February 10, 1881 issue of Nature. His device is dispayed in the London Science Museum.
In the June 4, 1908 issue of Nature, article "Telegraphic Photography and Electric Vision", Bidwell reported on "telegraphic photography" experiments by other scientists and commented on the greater difficulties faced by those wishing to develop "distant electric vision".
He pointed out that besides the problem of synchronizing the transmitter and receiver, there was a much larger problem in transmitting the extremely large volume of data at a rate that would create a continuous image to be viewed. He estimated that each image would need to have 16,000 to 150,000 "elements" (pixels in modern terminology) and a minimum scan rate of 10 times per second would be required. He suggested that this high rate of data transmission would require a separate circuit for each line scanned or possibly a separate circuit for each point in the image. This large number of circuits would result in a large, complex and expensive device.
This article illustrates some of the problems in creating television and the possible solutions that were being considered in the first decade of the 20th century.
This article is also noteworthy because it prompted a response by Alan Archibald Campbell-Swinton, who suggested that the problems would be best solved by using cathode ray tubes instead of mechanical devices.
External links
References
- Nature February 10, 1881
- Nature June 4, 1908