Measuring bridge

The hot-wire anemometer sensor is a very fine wire with a diameter of few micrometers and length of few millimeters. This wire is connected to a measurement bridge and an electrical current is fed through the wire. The wire is heated to a temperature above the air temperature and the air velocity is determined by the cooling effect of the wire. The voltage over the wire, U, is a function not only of the velocity but also of the excess temperature and the fluid properties in the following way ... 

It is often of interest also to measure both the external and internal resistances of the galvanic circuit by the use of appropriate resistance-measurement bridges or by even more elaborate techniques such as have been described by Pearson . 

The electrical conductivity (or resistivity) of an aqueous pigment extract is determined from the electrical conductance (or resistance). For standards, see Table 1 ( Resistivity ). Apparatus centrifuge with glass containers, resistance measurement bridge, electrolytic cell. 

For a PRT with R, = 100 O, a temperature resolution of 1 mK requires that Rt be measured with a precision of 4 X lO"" fl i.e., a random error <4 X 10 percent. Thus the measurement bridge and digital voltmeter must be high-precision instalments. In the case of the DVM, one needs at least six-digit resolution. It should be noted that, owing to decreasing sensitivity as T approaches 0 K, PRTs do not make attractive practical thermometers at very low temperatures. 

Fig. 1. Circuit diagram of the measuring bridge. Tip H. 2 Wagner auxiliary arm tl 3 H 4 Rv equivalent circuit Tlx measuring cell.

Thermal ionization is observable in flames, so that the gap between celestial and terrestrial conditions is in a certain measure bridged. 

The series capacitance Cs diverges toward infinite capacitance values at low frequencies. This will, for instance, be experienced with a measuring bridge if an unknown parallel combination is to be zeroed by a series combination in the balancing arm (Section 8.3.6). 

The ac bridge is based on a classical Wheatstone (or Wien for ac measurements) bridge in which one part is replaced by an electrochemical cell and the other compensating part by a variable, R or C. The dc potential is supplied by a potentiometer in the center and ac by the external source. The double layer capacitance measurements were initially carried out on a dropping mercury electrode (DME), and the bridge compensation had to be carried out always at the same surface area of the DME, that is, after exactly the same time from the beginning of... 

Dielectric Constant and Dielectric Loss. Impedance measurement bridges are the most commonly used instruments for dielectric constant and dielectric loss measnrements for lie-quencies up to 10 MHz. A capacitor structure is built as a bottom electrode, top electrode, and two or more separation layers of dielectric material. Figure 8.44 shows a typical electrode pattern. The outside guard ring on the top electrode pattern prevents measurement errors due to fringe effects. Dielectric constant and dielectric loss measurements at frequencies... 

Various compact commercial sets are available which incorporate the Wheatstone bridge circuit, valve oscillator, amplifying circuit, decade resistance box and cathode tube indicator. Of these, we instance the Phillips Conductivity Measuring Bridge PR 9500, on the dial of which the resistance can be directly read and has only to be multiplied by the cell-constant to give the specific resistance. This arrangement eliminates the need to solve equation (5-3),... 

The Measuring Bridge.—This very commonly consists of a thin wire of platinum, platinum-iridium, or nickelin, stretched over a scale i metre long and graduated in millimetres (Fig. 59). A sliding contact, c, having a platinum... 

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