High impedance

There are two procedures for doing this. The first makes use of a metal probe coated with an emitter such as polonium or Am (around 1 mCi) and placed above the surface. The resulting air ionization makes the gap between the probe and the liquid sufficiently conducting that the potential difference can be measured by means of a high-impedance dc voltmeter that serves as a null indicator in a standard potentiometer circuit. A submerged reference electrode may be a silver-silver chloride electrode. One generally compares the potential of the film-covered surface with that of the film-free one [83, 84]. 

The potential of the working electrode, which changes as the composition of the electrochemical cell changes, is monitored by including a reference electrode and a high-impedance potentiometer. 

Because of the very large resistance of the glass membrane in a conventional pH electrode, an input amplifier of high impedance (usually 10 —10 Q) is required to avoid errors in the pH (or mV) readings. Most pH meters have field-effect transistor amplifiers that typically exhibit bias currents of only a pico-ampere (10 ampere), which, for an electrode resistance of 100 MQ, results in an emf error of only 0.1 mV (0.002 pH unit). 

The inductor on the input side also suppresses the harmonics in the incoming supply, as high Lj will provide a high impedance path to higher harmonics. For suppression of harmonics, where the supply system is already substtintially distorted, tiddilional L-C... 

We discuss below a high-impedance differential protection scheme to provide a detailed procedure to select PS Class CTs. 

Relay - High impedance three element differential protection relay IVp - Windings ol a power equipment or section ol a power system to be protected... 

Figure 15.27 Three-element high-impedance circulating current relay scheme...

Relay - High impedance differential protection reiay. It operates for the fault occurring within the protection zone /pi - Fault current through ground for fault on phase 8... 

Relay - High impedance single element ground fault differential protection relay... 

As this is a high-impedance scheme, it can result in very... 

At power frequency, the effect of electrostatic capacitances is almost negligible as they offer a very high impedance (X,. l//j/being too low ) to the system voltage. The... 

Differential G/F protection (high impedance differential protection is discussed in Section 15.6.6(1)). 

Another remedial measure is the use of blocking circuits by providing a high-impedance path in the ground circuit when a phase-to-ground circuit is used for the com-munication network, to block the entry of the third harmonic quantities into the system from other systems. 

These are parallel resonant L-C circuits, and are tuned to offer a high impedance to a particular harmonic frequency... 

It has low impedance at power frequency and high impedance at carrier frequency... 

These are meant to provide high impedance to harmonic currents and block their entry or reduce their amplitudes and are therefore also known as blocking reactors. They may have any of the magnetic characteristics shown in Figure 27.2 and have a fixed reactance, rated continuously. 

These guns were developed to attain much higher impact velocities than those available with powder guns (Crozier and Hume, 1957 Curtis, 1962). Peak velocities in the 7-8 km/s range can be routinely attained, with peak shock pressures approaching 1 TPa in high impedance materials. 

McQueen et al. (1982) demonstrated that by placing a series of high-impedance transparent fluids (called optical analyzers) over the sample at a series of thicknesses less than d in the target that the overtaking rarefaction (sound) velocity can be accurately obtained. Arrival of rarefaction waves rapidly reduce the shock pressure. These wave arrivals could be very readily detected by the change in light radiance caused by the onset of a decrease in shock amplitude when the rarefaction wave caught up to the shock front. The... 

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