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A television picture is a scene cut into strips, turned into an electric current, sent somewhere else and rebuilt fast enough to fool the eye. John Logie Baird, a Scottish engineer, was the first to show this working with moving faces in light and shade. This post sets out what he showed in 1926, how his apparatus worked, the scanning numbers you can calculate from it, and what a student can learn from how he worked.
What happened at 22 Frith Street on 26 January 1926?
The IEEE Milestone citation states that members of the Royal Institution of Great Britain "witnessed the world's first public demonstration of live television on 26 January 1926" at 22 Frith Street, London, where Baird had his workshop from 1924 to 1926. The BBC's history page says he gave it "before 50 scientists in an attic room in central London".
The Times reported the event two days later. Its account, reprinted on bairdtelevision.com, describes "a large wooden revolving disc containing lenses", a revolving shutter and "a light sensitive cell". The head of a ventriloquist's doll was used as the subject, "though the human face was also reproduced". The paper was honest about quality: "The image as transmitted was faint and often blurred", but it showed "the play of expression on the face".
Who was Baird?
The BBC records that Baird was born on 14 August 1888 in Helensburgh, Scotland, studied at the Glasgow and West of Scotland Technical College, and was rejected as unfit for the forces in the First World War. By 1924 he could send "a flickering image across a few feet". He died on 14 June 1946 in Bexhill-on-Sea.
How did Baird's television turn a face into a current?
In his own lecture at the 1927 British Association meeting in Leeds, Baird explained that the image is divided "into a great number of little sections like a mosaic", and each section is projected in turn onto a light-sensitive cell, which "emits a current directly proportional to the light intensity". At the receiver, that current controls a lamp whose spot sweeps a screen in step with the transmitter. It moves so fast that "the image appears as a whole instantaneously to the eye".
This is Class 12 physics. The NCERT chapter Dual Nature of Radiation and Matter states that, for a given material and frequency, "the photoelectric current is directly proportional to the intensity of incident light", and that emission starts in a time "of the order of 10^-9 s or less". Baird's problem was the other side of the trade-off. He said the photoelectric cell was "practically instantaneous" but "extremely insensitive", while the selenium cell gave far more current but was slow to respond. Our post on photoelectric effect problems works through the current-intensity relation in exam form.
Worked example: how fast did the picture have to be built?
Writing in the BBC Annual Report for 1933, Baird described the 30-line studio system that the BBC was then broadcasting: 30 strips of light built side by side, the whole process "carried out so rapidly, namely, 12 1/2 times per second".
Time for one complete picture: T = 1 / 12.5 = 0.08 s, or 80 ms.
Lines scanned each second: 30 × 12.5 = 375 lines per second.
Time for one line: 1 / 375 s = 2.67 × 10^-3 s, about 2.7 ms.
Compare with the photoelectric response time from NCERT: 2.67 × 10^-3 / 10^-9 ≈ 2.7 × 10^6. Photoemission itself is millions of times quicker than one line, so a photoelectric cell's limit was its weak current, not its speed.
Baird's son Malcolm Baird writes that by 1935 the company had a mechanical camera giving a 240-line picture at 25 frames per second. That is 240 × 25 = 6,000 lines per second, or 16 times the 375 of the 30-line system.
Common mistakes:
Mixing up lines per picture and lines per second. 30 is per picture; multiply by the picture rate to get per second.
Forgetting the reciprocal. 12.5 pictures per second means 0.08 s per picture, not 12.5 s.
Assuming these numbers describe the 1926 demonstration. A 2014 historical study notes that no details of the equipment were released at the time and published reports were vague, so treat 1926 line counts with care.
What came after 1926?
The BBC lists a 1927 transmission over 438 miles of telephone line between London and Glasgow and the first transatlantic television transmission in 1928. Baird's 1933 article dates the first official BBC television broadcast to September 1929. In 1935 a BBC committee set Marconi-EMI's all-electronic 405-line system against Baird's 240-line one; Marconi-EMI won, and Baird's system was dropped in 1937. Television travels as radio waves, the same waves Heinrich Hertz first produced in a lab.
What can a student take from Baird's way of working?
Start with what you have. The IEEE Milestone record describes his work as "an ingenious series of improvisations" with the primitive parts of the 1920s.
Show your work early. He invited outside scientists to judge it, accepting that a faint, blurred image was still evidence.
Know when a method has run its course. The mechanical system lost to electronic scanning. Being good at one method is not a reason to keep it.
Scanning, photocurrents and persistence of vision are easier to grasp when you can watch them happen. The Fermi Simulation Lab has interactive simulations for ideas that are easier to see than to read.
Try this today: work out the time per line for a 625-line picture at 25 frames per second, and compare it with Baird's 2.7 ms.
