Galvanometer amplifier

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. 14. Scheme of the vidicon tube. (1) Scanning beam (cathode potential F = 0 volt) (2) gauze (3) dye film (4) transparent supporting electrode (potential +30 volt) (5) amplifier (6) galvanometer... 

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... 

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-... 

The electronic signal obtained from the photodetector is usually electronically amplified, measured using some sort of galvanometer, and presented in either analog or digital form. The signal may alternatively be recorded on a strip-chart recorder, to supply a permanent record of the spectrum. The limitation of these recorders, however, is the relatively long response time of the pen. This limits the speed at which accurate spectra can be obtained. Oscilloscopes have been developed, however, in which elaborate electrical circuits have been employed to obtain spectra with a much shorter response time. 

The flame photometer consists essentially of an atomizer, a burner, some means of isolating the desired part of the spectrum, a photosensitive detector, sometimes an amplifier and, finally, a method of presenting the desired emission, whether by galvanometer, null meter, or chart recorder. 

Figure 8.4l. Apparatus for the parallel recording of DTA. T, TG. DTG. TGT- and DTGT curves (86). 1. compressed test piece 2. compressed reference substance 2. furnace 4. silica bell 5, inlet tube for carrier gas 6. tube for eas extraction 7. silica tube S. suka tube with stirrup-shaped end 9. thermoelement 10. diaphragms 11. light cell 12- lamps l3. optical slit 14. magnet l5. coil 16. galvanometer 17. photographic paper 18. damns transformer 19, absorber 20. electrodes 2l, amplifier 22. vacuum pump 23. automatic burette 24. potentiometer 25. servomotor.

A variety of read-out devices are used. Frequently a galvanometer with a 0-100% T scale is used. Digital read-out also is used and arrangements for chart recording of line intensities are available with most densitometers. Recently instrumental design has switched to solid state devices to serve as amplifiers of the signal from the photoreceptor. 

The simplest read-out device presently in use for atomic absorption is a visually observed meter with a galvanometer-type movement, usually used with a series resistance to read voltage. By using variable potentiometers in the output circuit of the amplifier, it is simple to adjust the meter to full-scale reading on a sample blank and zero when no signal enters the entrance slit of the monochromator. The signal produced by the sample can then easily be obtained in terms of percentage transmittance if the meter scale is 0-100. 

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