For historical allocation purpose, the television age started in 1876 when it was discovered that the resistivity of a Selenium plate varied with the amount of light falling on it.
Thus, researchers began trying to develop   means to transmit images by electricity. In this way, while some of schemes involved the use of a mosaic of Selenium, others called for scanning the image mechanically with one or more Selenium points.
However, the development of the first practical television apparatus was the electrical telescope, patented by Paul Nipkow in 1884. It consisted basically of the well-known Nipkow disk that comprised 24 holes equally spaced along a spiral near the periphery of the disk. (a)
During its operation, the image to be transmitted was focused on a small area at the disk's periphery, while the disk was spinning at 600 RPM. Hence the disk rotated, the sequence of holes scanned the image in a straigh line. Allocated behind the image area, a lens collected the sequential light samples focusing them on a Selenium cell, which consequently produces a succession of currents each one proportional to the intensity of the light on a different element of the image. In order to reconstruct the image at the receiver end, Nipkow proposed to use a magneto-optical light modulator. In this way to form the image, it was necessary another identical disk rotating synchronously with the one located at the transmitter.
Unfortunately, Nipkow invention was restritect only to theoretical ideas, probably because the technology of that time would not have permitted him to build such a kind of television system.
However, his proposal for a mechanical scanning was the base for several later built television systems, among them the most notably was the one developed by the Bristish inventor John Logie Baird.
In 1908, Alan Archibald Campbell-Switon described the first all-electronic television system as known nowadays that was based on the cathode-ray tube.

(a) Various type of scanning disks where:
(a) Flat disk with two sets of scanning holes
(b) Flat disK with three sets of scanning holes
(c) Single disk with with two sets of holes arranged to receive signals from stations employing two differents numbers of lines per picture
(d) Scanning drum with the holes arranged in spiral form
(e) Flexible scanning belt. Radio Physics Course, 1933

(b) Ilustração do princípio de varredura mecânica originalmente inventado por Paul Nipkow em 1884.

b) Illustration of the principle of mechanical scanning originally invented by Nipkow.
Radio Physics Course, 1933

(d) Typical scanner for experimenters sold in 1930. Radio News<BR>
(e) Primitive American concept of image reproducing apparatus circa 1925. Radio News