Galvanometer System

An absolute method for measuring intensities is to use a calibrated thermopile, bolometer or radiometer and galvanometer system the first of these is the most suitable. Calibration may be carried out at the National Physical Laboratories and The National Bureau of Standards, or by using one of their carbon filament lamps. The thermopiles consist of a bank of interconnected fine-wire thermocouples (Fig. 47). One circuit uses a photoelectric amplifier for the measuring galvanometer (Fig. 48) The image of a tungsten filament F, is projected onto a photo-... 

Like chemical actinometers, photocells have to be calibrated against a thermopile-galvanometer system this has to be done frequently as there tends to be some variation with time. Under these conditions, they can be used to measure the absolute intensity of monochromatic light. The cell best suited for photochemical studies is the photoemissive type, which operates via the photoelectric emission of electrons from an irradiated surface. The metallic cathode is mounted either in a vacuum or in a small pressure of one of the inert gases. The cell may involve a single phototube or a multielement photomultiplier. An amplification of about 10 is achieved with the latter. 

The last quantity is readily determined with a simple phototube-galvanometer system. Frequently, however, a chemical actinometer is employed this involves photolyzing a compound providing a known quantum yield of product. 

Answer Flans are being made to convert the present galvanometer systems at all reactors Into octant-type systems wherein elc t neutron chambers are Installed In comer regions of the reactor block as at the C and K reactors. The older reactors will be provided with special ion chambers of small enou dimensions to be Inserted Into the front and rear ends of four comer process tubes. Placed In the safety circuit system, the flux level trips will be spread about the reactor block rather than-beneath the pile as at present. 

The main drawback of the galvanometer-spot follower system is that the sensitivity level of the line is defined ab initio and cannot be modified except by increasing or decreasing the number of thermoelements in the circuit. A more versatile, but more expensive, system is provided by the addition of a dc amplifier to a recording voltmeter. 

The sensitivity of instruments using low resistance circuits is determined primarily by the sensitivity of the galvanometer (Figure 4.5). Electrode systems that have a high resistance, e.g. glass electrodes, require a high impedance voltmeter, which converts the potential generated into current which can be amplified and measured. Such instruments are commonly known as pH meters but may be used for many potentiometric measurements other than pH. 

Fig. 5.6. A block diagram of an optical coherence tomography/Raman spectroscopy system C, circulator RSOD, rapid scanning optical delay BP, 785 bandpass BSO, beam shaping optics DM1, dichroic mirror at 990 nm DM2, dichroic mirror at 800-950 nm LP, long pass at 808 nm GP, galvanometer pair BD, balanced detector BPF, electronic band-pass filter AI-AO DAQ, analog input-output data acquisition (reprinted with permission from [34]. Copyright 2008 Optical Society of America)...

The balanced bridge circuit (Figure 10) uses a galvanometer to compare the RTD resistance with that of a fixed resistor. The galvanometer uses a pointer that deflects on either side of zero when the resistance of the arms is not equal. The resistance of the slide wire is adjusted until the galvanometer indicates zero. The value of the slide resistance is then used to determine the temperature of the system being monitored. 

In exact work it is necessary to allow for absorption by the reacting system of some of the light which is reflected back from the windows at the rear of the cell and in the thermostat and thermopile, for this is not registered by the galvanometer. The correction is usually negligible or small. 

The potential of an electrode is determined in combination with a second constant electrode which does not belong to the actual electrolytic system. This subsidiary or standard electrode, whose potential is either arbitrarily taken, as zero or has a certain absolute value, is connected by a siphon with the liquid surrounding the experimental electrode. The electromotive force of this galvanic combination is then measured by one of the well-known methods, with a galvanometer or capillary electrometer. If the potential difference of the standard electrode is correctly subtracted from the obtained value, the difference in potential of the reaction electrode, based on the agreed-upon zero) value of the potential, is... 

Cavendish, a century before, had attempted to compare quantitatively the electrical conductivity of rain water with various salt solutions. Possessing no galvanometer to register the strength of the currents, he had bravely converted his own nervous system into one. As he discharged Leyden jars through the different liquids he compared the electric shocks which he received. With this crude, heroic method he obtained a number of surprisingly accurate results. 

One of the earliest commercial DTA systems, a field portable unit, is still available from this manufacturer. It is used extensively in bauxite exploration and provides qualitative data in about 20 min. The complete apparatus is housed in a metal case, 21"L x 8.5"W x 9"H, containing the insulated base supporting the crucibles, pyrometer, galvanometer, and small tools. 

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