ELECTRONIC DEVICES AND MEASUREMENTS

The rapid development of solid-state electronic devices in the last two decades has had a profound effect on measurement capabilities in chemistry and other scientific fields. In this chapter we consider some of the physical aspects of the construction and function of electronic components such as resistors, capacitors, inductors, diodes, and transistors. The integration of these into small operational amplifier circuits is discussed, and various measurement applications are described. The use of these circuit elements in analog-to-digital converters and digital multimeters is emphasized in this chapter, but modern integrated circuits (ICs) have also greatly improved the capabilities of oscilloscopes, frequency counters, and other electronic instruments discussed in Chapter XIX. Finally, the use of potentiometers and bridge circuits, employed in a number of experiments in this text, is covered in the present chapter. 

Differential inverter gate AND gate OR gate amplifier 

Precision wire-wound resistors are used for critical applications in which low noise and resistance stability are important, but since they are coils, they have an inductive reactance that must be taken into account for ac circuits. By folding the insulated wire in the middle and carefully forming parallel windings, the inductive component can be reduced by about a factor of 100. Precision standard resistors with an accuracy of about 50 ppm in the range of 1 fl to 10 Mfl are available from Electro Science Industries and other companies. These are often combined in the form of decade resistance boxes to provide a wide range of fixed values. 

Resistors whose values can be varied are termed potentiometers. They are made of the same materials as fixed resistors but have a movable wiper to contact a coil of resistance wire or a strip of resistive film at any point along the resistor. Potentiometers used only occasionally to adjust a circuit are called trimmers, while those employed for high-wattage applications such as control of heating mantles and ovens are called rheostats. Precision potentiometers have played an important role in the measurement of electrical quantities and are considered in detail in a later section. 

Capacitors. A capacitor consists of two conductors separated by an insulator. The capacitance C gives a measure of the charge separation Q produced when a potential V is 

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Applied Sciences, Inc. has, in the past few years, used the fixed catalyst fiber to fabricate and analyze VGCF-reinforced composites which could be candidate materials for thermal management substrates in high density, high power electronic devices and space power system radiator fins and high performance applications such as plasma facing components in experimental nuclear fusion reactors. These composites include carbon/carbon (CC) composites, polymer matrix composites, and metal matrix composites (MMC). Measurements have been made of thermal conductivity, coefficient of thermal expansion (CTE), tensile strength, and tensile modulus. Representative results are described below. 

Electric lights, office machines and other items of a direct energyconsuming nature will liberate all their heat into the conditioned space, and this load maybe measured and taken as part of the total cooling load. Particular care should be taken to check the numbers of office electronic devices, and their probable proliferation within the life of the building. Recent advice on the subject is to take a liberal guess and then double it . 

The term electronics normally refers to the theory and function of the electronic devices and circuits used so universally for measurement, control, and computation. Conventional electronics is an important tool also in the study of supported catalysts. 

The area detector - is an electronic device for measuring many diffracted intensities at one time. It is an electronic substitute for film, and is now used, where possible, for crystals of biological macromolecules. It is a position-sensitive detector, and is coupled to an electronic device for recording the data in computer-readable form. The data so recorded include the intensity of a Bragg reflection (diffracted beam) and its precise direction (as a location on the detector). Both types of information are needed for each Bragg reflection so that I(hkl), and sinO/X can be determined. 

The topic of this chapter may seem like a digression from methods and approaches to reaction mechanisms, but it is not it is an introduction to it. We worked on both topics for some time and there is a basic connection. Think of an electronic device and ask how are the logic functions of this device determined Electronic inputs (voltages and currents) are applied and outputs are measured. A truth table is constructed and from this table the logic functions of the device, and at times some of its components, may be inferred. The device is not subjected to the approach toward a chemical mechanism described in the previous chapter, of taking the device apart and testing its simplest components. (That may have to be done sometimes but is to be avoided if possible.)... 

Now, E(<) is the sum of the electric fields E (t) of the beams impinging on the detector from scattering points 1,2,..., N in the small volume considered. Since these points are moving thermally, the E " (t) and hence E(t) fluctuate from time to time. Techniques using electronic devices to measure the resulting fluctuation of i(t) have been developed in recent years. The QELS method aims at extracting information about the motion of scattering points from the i t) data so obtained. To this end we introduce two functions Ci(r) and C2(r) defined by... 

Pressure can be measured by means of manometers which show the pressure in terms of the different levels of a liquid in a U-tube, by mechanical pressure gauges which record the differential effect of pressure forces on the inside and outside surfaces of a coiled tube, and electronic devices which measure the change of electrical characteristic of an element with pressure. 

PULSE HEIGHT ANALYSER (PHA) An electronic device for measuring the heights of pulses. See SCA and MCA. 

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