THE WHEATSTONE BRIDGE

In its simplest form, the Wheatstone bridge is used on D.C. for the measurement of an unknown resistance in terms of three known resistors. Its accuracy depends on that of the known units and the sensitivity of the detector. It is also used for sensing the changes which occur in the output from resistance strain-gauge detectors. The latter instruments can be made portable and can detect variations of less than 0.05 per cent. 

A modification of the method of determining the resistance of a solid conductor by the Wheatstone bridge is used for determining the conductance of the solution of an electro-... 

Classical conductivity meters are forms of the Wheatstone bridge. The Wheatstone bridge is a null device. It produces an off-balance potential when the four resistances making up the bridge do not satisfy the null condition in the following equation. 

The conductivity change measured by the Wheatstone bridge arrangement Is directly related to the extent of reaction. The conductivity and change In conductivity following a pressure perturbation are given by... 

J) In the course of a thermometric titration, the thermister attached to the insulated-beaker is connected to one arm of the Wheatstone Bridge as displayed in Figure 11.5. The values of the circuit component listed are for a thermister having an approximate resistance of 2 KI2 and a sensitivity of 0.04 Q/Q/°C in the 25°C temperature range. Hence, an observed change of 1°C = an unbalanced potential of 15.7 mV, and... 

Thermal conductivity detectors usually are built as dual detectors because originally a reference filament formed one leg of the Wheatstone bridge. Today most TCD units manufactured still require a reference gas. In dual column compensation mode the compensating column becomes the reference side. 

While most laboratory conductivity bridges are manually balanced, the Wheatstone bridge circuit also finds use in a variety of conductivity monitors, controllers, and recorders where it is mechanically rebalanced by a servomechanism operated by the detector. Generally in these devices, advantage is taken of the phase shift, which occurs in the detected signal as the bridge is driven through balance by the servo motor. 

In a null comparison measurement of resistance, the effect of an unknown resistance must be compared with the effect of a variable standard resistance under conditions as identical as possible. Therefore, the unknown and standard resistances are placed in identical circuits in such a way that the resulting voltage or current in each circuit can be compared. Then the standard is varied until the difference in voltage or current between the two circuits is zero. Several methods for performing this comparison have been devised, of which the Wheatstone bridge is by far the most common. Comparison methods for resis-... 

The Wheatstone bridge shown in Figure 8.5 provides the most direct and best known circuit for comparison of unknown resistances against standard resistances. Resistances RA, RB, and R are standard resistance values that are used in the measurement of the unknown resistance R Resistance R is made variable and is adjusted until the null detector indicates that the bridge is balanced. 

When a flammable gas enters a catalytic cell, the gases are combusted on the active filament, causing an increase in temperature and a change in the balance of the Wheatstone bridge. The output signal for flammable gases in air in this type of cell is linear up to the lower explosive limit (LEL). 

An electrical diagram for a Pirani gauge is shown in Fig. 7.47, where V and D comprise the Pirani tube. D is the dummy filament tube that is sealed off, and V is the tube that is exposed to the vacuum system. The filaments in the V tube are connected to a bridge circuit called a Wheatstone bridge with two resistance units called Rj and R2. Power from the power supply passes across the Wheatstone bridge and is adjusted to the proper setting by R3, whose value is read on the mil-... 

The thermostat is an iron box about thirty centimeters square filled with molten lead. An iron blade keeps the lead stirred and a platinum resistance thermometer functions both as a thermometer and as a thermoregulator. The Wheatstone bridge, which contains the resistance thermometer as one arm, is adjusted for a given temperature, and the beam of light from the galvanometer strikes a zero reading on the scale. At this position is introduced a photoelectric cell connected with a radio tube and amplifier, which in turn actuates an electro-magnetic relay. When the temperature reaches the predetermined value, the beam of... 

Commercial Wheatstone bridges are available in a form in which RilRs, can be set by a step switch to accurate decimal ratios from 10 to 10, and R- is a precision four- or five-decade resistance box. The use of the Wheatstone bridge with platinum resistance thermometers is discussed in Chapter XVII. 

One Form of the Wheatstone Bridge Used with the Electrical Conductivity Detector... 

Design of the A.C. Bridge.— Strictly speaking the condition of balance of the Wheatstone bridge given by equation (7) is applicable for alternating current only if /2i, R, Rz and R are pure resistances. It is um... 

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