1. Components incapable of controlling current by means of another electrical signal are called passive devices. Resistors, capacitors, inductors, transformers, and even diodes are all considered passive devices. Active devices include, but are not limited to, vacuum tubes, transistors, silicon-controlled rectifiers (SCRs), and TRIACs.
2. Thyratron is an active device, a type of gas filled tube used as a high energy electrical switch and controlled rectifier.Unlike a vacuum tube, a thyratron cannot be used to amplify signals linearly.A thyratron is basically a "controlled gas rectifier".Gases used include mercuryvapor, xenon, neon, and (in special high-voltage applications or applications requiring very short switching times) hydrogen.
3. A reflex klystron oscillator is preferred as an ideal source in micro measurements as it provides low power output with multiple frequency components.The interaction between the electron beam and the radio frequency wave in the re-entrant cavity depends upon the average gap transit angle, this is the phase change due to finite time, the electron beam takes to cross the central gap of the re-entrant cavity.
4. A step recovery diode is also known as snap-off varactor.
5. Schottky diode is a unipolar device because it has electrons as majority carriers on both sides of the junction.It can be used for rectification of signals of frequencies even exceeding 300 MHz. It is also used in clipping and clamping circuits and in computer gating. Its low noise figure finds application in sensitive applications like radars.
-Electronic devices and circuits, by Salivahanan
6. The graded index optical fibre cable can be considered to have smaller
numerical aperture than step index cable.
7. There are about 25 superconducting metallic elements at low temperatures,
like tin and aluminium.All superconductors have exactly zero resistivity to low
applied currents when there is no magnetic field present.Conventional superconductors
usually have critical temperatures ranging from less than 1K to around 20K.
When a superconductor is placed in a weak external magnetic field H, the field penetrates for
only a short distance λ, called the penetration depth, after which it decays rapidly to zero.
This is called the Meissner effect. For most superconductors, the penetration depth is
on the order of a hundred nm.
8. Porcelain is used as an insulator.Porcelain insulators are made from clay,
quartz or alumina and feldspar, and are covered with a smooth glaze to shed
water. Glass has a higher dielectric strength than porcelain.