LAYOUT AND THE CONTROL OF INTERFERENCE 



work involving human subjects, e.g. cardiography and encephalography, the 

 imprisonment of the patient in some form of wire screening cage is clearly 

 undesirable and it is fortunate that it is also usually unnecessary. In cardio- 

 graphy the electrodes are of extremely low resistance and therefore not 

 subject to serious electric interference; furthermore the signal is relatively 



Amp 



Figure 42.14 



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 42.1). 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 use 

 of the fact that electric pick-up is a function of electrode resistance and 

 magnetic pick-up is usually not. Thus irrigation with Ringer's fluid of an 

 excised nerve lying over a silver-wire recording electrode will reduce the 

 amplitude of the pick-up if it is electric, but not if it is magnetic. If the 



663 



