Parametric techniques are also used to reduce noise in beam-type amplifiers, such as traveling-wave tubes. In the quadrupole amplifier, for example, a parametric Pump modulates the electron beam and eliminates most of the noise, while retaining the amplification. Parametric amplifiers are almost as good as masers at frequencies up to 3,000 me., but masers are superior at higher frequencies.
This invention is epochal. It was the first of those photomechanical techniques that were soon to revolutionize the graphic arts by eliminating the hand of man in the reproduction of pictures of all kinds. It is the most important of Niepce's contributions, for it involved a principle that became basic to future techniques: the differential hardening by light of a ground that would control the etching in exact counterpart of the image.
Other metal techniques include the hammering into shape of metal (especially wrought iron) which has been softened by heating; and an additive process of constructing a work of sculpture with blobs of molten metal using welding equipment and materials. Both of these techniques are most used for small works since the products are apt to be solid and therefore very heavy for their size.
Parametric techniques are also used to reduce noise in beam-type amplifiers, such as traveling-wave tubes. In the quadrupole amplifier, for example, a parametric