. Electronic apparatus for biological research . Figure large (2 or 3 mV). In encephalography the electrodes are also of low resis- tance but the signal is much smaller, of the order of ^V. Fortunately the electrodes on the patient's scalp are quite close together and subject to similar interference effects. Thus full advantage can be taken of the proper- ties of differential amplification. In research work on excised preparations, however, electrode resistances are generally much higher and some form of screening cage is frequently essential. The precise form which this takes depends on
. Electronic apparatus for biological research . Figure large (2 or 3 mV). In encephalography the electrodes are also of low resis- tance but the signal is much smaller, of the order of ^V. Fortunately the electrodes on the patient's scalp are quite close together and subject to similar interference effects. Thus full advantage can be taken of the proper- ties of differential amplification. In research work on excised preparations, however, electrode resistances are generally much higher and some form of screening cage is frequently essential. The precise form which this takes depends on the size of the animal and on personal taste. In the Cambridge School of Physiology under Matthews the cage is of sufficient size to contain animal, workbench, experi- menter and battery driven pre-amplifier {Plate ). At the National Insti- tute for Medical Research, Mill Hill, the animal rests on an earthed metal table over which is erected a sort of screening gantry. This provides screening at top, bottom and the two ends, leaving two sides for access by the experi- menter. The sides may be closed, if necessary, by clipping on special panels. The cathode-follower probe units which feed the recording amplifier, and the RF stimulating probe units, are mounted inside the gantry; the whole of the rest of the apparatus is carried on 19 in. racks in a different part of the room. An open-mesh structure, say \ in. wire netting, suitably braced, is quite satisfactory for such cages, but care must be taken, if a number of separate pieces are used, to ensure electrical continuity between them, preferably by soldering. Preparation protection—magnetic fields—The rule here is simple. Keep all single core leads as short as possible and all recording electrodes as close together as the terms of the experiment and working convenience allow. Identifying nature of interfering fields—A quick method of detecting whether 50 cycle pick-up is electrically or magnetically borne is to make
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