Direct-reading bridges

Fig. 18.1. Standard hydrogen electrode connected to a Zn/Zn " half-cell with a salt bridge and external direct reading potentiometer.

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),... 

In most operating manuals, it is claimed that the time to reach the equilibrium values must remain, for all measurements, constant and equal to that required to set the bridge to zero. However, for a given drop size, the direction and rate of approach to the equilibrium value were found to vary (Morris 1977). The use of a recorder to show the shape of the bridge imbalance-time trace is therefore strongly recommended. Readings must only be taken when the system has reached a steady state, rather than after a fixed time interval. 

Mycorrhizal fungi show varying degrees of specificity in relation to the host plants they can infect (Smith Read, 1997). Where there is low host specificity, it is possible that individual plants may be colonized by the same mycorrhizal network and thus become connected. This has been confirmed by direct observation both for ectomycorrhizal (Finlay Read, 1986a) and arbuscular mycorrhizal (Heap Newman, 1980) types. Such infection establishes potential pathways for direct transport of material between roots via mycelial bridges, even of different plant... 

The specific conductance of a water sample provides a simple method to determine the total dissolved ionic solids present in the sample. It is also an inexpensive technique, which lends itself to continuous monitoring of a river or waste stream for the total ion content (Fig. 4.1), and can be easily used to check the accuracy of analyses conducted for specific ions. Specific conductance is measured via a pair of carefully spaced platinum electrodes, which are placed either directly in the stream to be measured or in a sample withdrawn from it [22]. The water temperature should be 25 °C, or the result corrected to this temperature. Voltages in the 12 to 14 range, and frequencies of 60 to 1000 Hz AC are used, plus a Wheatstone bridge circuit to obtain a conductivity reading in xmho/cm or xS/cm (microsiemen/cm). The response obtained is linear with the total ion content over a wide range of concentrations (Fig. 4.1). Examples of the conductance ranges and seasonal variation of some typical Canadian rivers are... 

The experimental measurement which determines k is either the resistance of the solution or the conductance. Older textbooks will generally quote the raw data in terms of the resistance, but modern work uses high accuracy conductance bridges and will quote the conductance directly. The basic experimental measurement is in both cases the resistance of the solution, but the modern bridges are calibrated to read the conductance direct. It is well to be aware of this. The experimental set-up is based on a Wheatstone bridge type of apparatus. 

A special conductance bridge reads directly over the range of 1 to 12% H2SO4. The recommended cell constant is 50. Extrapolating from data in handbooks and critical tables, calculate the approximate resistance and micromho range involved at 18°C. 

The lower group of controls on the panel operate the internal variable standards. The right dial is calibrated directly in ohms resistance, this dial is marked R. The lever switches immediately above the dial are the resistance adder switches. When the adder svritches are in the 0 position they are inactive. When the switches are in other positions the values indicated a3 e added to the reading of the R dial to obtain the resistance of the load. The two adder switches may be used in any combination of settings to extend the resistance range of the bridge to 1000 ohms. 

Many bridges having different designs are available commercially, with more complicated circuits to give direct and even automatic readings of s and e", or the real and imaginary parts of the impedance of the sample. 

Jens Rasmussen is a visionary engineer, self-taught in human factors, who is capable of reading and bridging different streams of theory that are mutually unaware of each other. He refocused his career on technical and human reliability after the nuclear accident which occurred at Three Mile Island in the United States in March 1979. He was to become one of the pioneers of modern approaches to safety in complex systems and went on to have a profound influence on a whole generation of researchers who studied directly under him, such as James Reason, Erik Hollnagel, Dave Woods, and... the author of this book. 

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