. Electronic apparatus for biological research . Input Output â o Figure There are, however, fields in which much smaller steady potentials have to be measured, usually from low-resistance surface electrodes. A case in point is the study of eye movements by measuring the vertical and horizontal components of electro-oculogram. These are of the order of a few microvolts only. In work on brain or spinal cord activity there is a sense in which an experiment is over in a single traverse of the cathode ray tube trace, perhaps 100 msecs; in this time the preparation has been stimulated and the
. Electronic apparatus for biological research . Input Output â o Figure There are, however, fields in which much smaller steady potentials have to be measured, usually from low-resistance surface electrodes. A case in point is the study of eye movements by measuring the vertical and horizontal components of electro-oculogram. These are of the order of a few microvolts only. In work on brain or spinal cord activity there is a sense in which an experiment is over in a single traverse of the cathode ray tube trace, perhaps 100 msecs; in this time the preparation has been stimulated and the response elicited and recorded. In an eye movement study the time scale is likely to be of the order of minutes. Clearly, then, only a very small drift rate indeed is acceptable. This is the kind of work for which the chopper amplifier is suited. Galvanometer amplifiers are at present rarities and it seems likely that they will remain so. Their freedom from noise gives them a position of supremacy which is probably unassailable in studies of very feeble electrical effects, but their bulk, necessary for elaborate resilient mounting and sound- proofing, coupled with their use of special photocells and galvanometers of extreme sensitivity, neither of which are readily available, may account for their infrequency of appearance. Straightforward amplifiers An enormous amount of work has been directed towards the design of better amplifiers, and descriptions of new circuits are published steadily. Unfortunately most of these are developed for purposes other thanâand are not suitable forâbioelectric studies. The reader who is interested is referred to a very useful study on the subject by Yarwood and Le Croisette^ which includes a very long bibliography of work in this field. 621
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