Wheatstone Bridge Principle

In the year 1843, Charles Wheatstone, the first Professor of Physics at Ring s College, London, invented one of the most accurate and commonly used methods of measuring resistance. It is known as Wheatstone bridge method. By this method the ratio of two resistances is determined and if the value of one of them is known, the value of the other is obtained (Fig 1.3) shows the circuit diagram of Wheatstone bridge. 

Four resistances PQR and S are connected to form a close network ABCD. A galvanometer G is connected between the junctions B of P and Q and D of R and S. A cell E is connected between the other two junctions viz. A of P and R and C of Q and S. AB, BC, AD and AC are called the 1 S 2 , 3 and 4 arm of the bridge respectively. AB and BC are also called the ratio arms. By properly adjusting the value of the resistances, the current through the galvanometer may be reduced to zero. This happens when point B and D are maintained at the same potential. The galvanometer then shows no deflection and the network is said to be balanced. It can be shown that the resistances in the four arms of the bridge then satisfy the relation. 

When the bridge is balanced, let the current through P be and through R be Since no current flows through the galvanometer, the current through Q and S must also be equal to ij and 2 respectively. Moreover, the potentials at B and D are equal 

if the value of R is miknown, it can be fomid finm a knowledge of S and the ratio P 

The balance condition may be written as — = —. This shows that the balance condition remains 

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Electrical resistance monitors use the fact that the resistance of a conductor varies inversely as its cross-sectional area. In principle, then, a wire or strip of the metal of interest is exposed to the corrodent and its resistance is measured at regular intervals. In practice, since the resistance also varies with temperature, the resistance of the exposed element is compared in a Wheatstone bridge circuit to that of a similar element which is protected from the corrodent but which experiences the same temperature. 

Figure 2.10—Thermal conductivity detector. To the left is a schematic showing the path of the carrier gas. To the right is a schematic of the TCD and its operating principle, based on an electrical Wheatstone bridge (equilibrium exists when R /R2 = Ri/Ra)-...

The most common type of combustible gas detectors utilizes the catalytic combustion principle. A platinum wire filament (in a Wheatstone bridge cir-... 

The accuracy of the measurement has been enhanced by the application of compensators . The principle of compensators was elaborated by Feussner in 1890 [iii]. The compensators worked with two or more identical series of resistances, sometimes combined with a - Wheatstone-bridge and a Thomson-bridge. Many variants such as Franke-, Wilsmore-, Raps- or Siemens-Halske-, Lindeck-Rothe- etc. compensators were used. 

Although conductance/resistance meters are available commercially (e.g., YSI, Yellow Springs Instruments, and others), the simple ac Wheatstone bridge of Fig. 1 is adequate and better illustrates some of the principles for accurate conductivity measurements. Here the ionic solution is placed in the conductivity cell, which is part of abridge subjected to a small ac voltage from a 1-kHz oscillator. The condition of balance for the bridge is detected with an oscilloscope and requires that the alternating potential at points B and D be of equal amplitude and exactly in phase. This corresponds to a balance condition... 

Figure 2.14 Thermal conductivity detector. Left, schematic showing the carrier gas passage. Right, the cross-sectional scheme of the metal block with its operating principle, based on an electrical Wheatstone bridge assembly (equilibrated when Rj/Rj = R3/R4).

Although the Nemst method is subject to many minor sources of error Nernst s results on the dielectric constant of liquids are very close to those obtained by later and more precise measurements. The Wheatstone bridge method lias been used by Smyth and associates.6 In principle their bridge is essentially that shown in Fig. 2. However, a vacuum tube oscillator replaces the induction coil, and precision in locating the balance is attained with the aid of vacuum tube amplification. As in precise conductance measurements care must be exercised to avoid errors due to capacity effects between the different arms of the bridge, and to earth. This may be accomplished by appropriately designed electrostatic screening, ... 

The sensor s transducer principle combines a membrane, as a mechanical transducer, with thin-film metal resistors forming a Wheatstone bridge, as an electromechanical transducer. The output voltage of the Wheatstone bridge is given by... 

Figure 1 illustrates the operational principle of hydrogel-based sensors. Pressure sensor chips with a flexible thin silicon bending plate and with an integrated piezoresistive Wheatstone bridge inside this plate have been employed as... 

Principle of measuring setup with two enzyme thermistors in compensation. P = pump I = sample injector T = thermostate C = temperature equilibration coil E, and E = active and inactive enzyme columns, T, and Tj = thermistors B = Wheatstone bridge and amplification R = recorder. 

Information on vacuum controllers for the medium and high vacuum ranges is scarce. Laporte [49] has described an instrument working on the thermal conductivity principle in which the Wheatstone bridge is connected to a signalling device which produces an acoustic signal when the pressure exceeds a given value. 

In the one-electrode sensors, the metal resistor simultaneously acts as the heater and the measuring electrode. The operation principle of the one-electrode sensor is based on the shunting of the Pt electrode by the semiconductor oxide, coating the metal spiral or strip. Typically, one-electrode sensors are incorporated in a Wheatstone Bridge circuit, or in the simplest electrical format the change of voltage drop at the sensor is determined. For... 

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