. Electronic apparatus for biological research. Electronic apparatus and appliances; Biology -- Research. VARIABLE CAPACITORS apparatus using non-thermionic rectifiers, or thermionic rectifiers of the directly heated kind, the HT is available before the valves have warmed up to consume it, and during this transient period the HT voltage 'surges' to an excessive value. The mechanical form of electrolytic capacitors is either tubular, with a wire emerging at each end (see Plate ) or enclosed in a cylindrical can, secured to the apparatus by a single large nut or by a clip, and having all the
. Electronic apparatus for biological research. Electronic apparatus and appliances; Biology -- Research. VARIABLE CAPACITORS apparatus using non-thermionic rectifiers, or thermionic rectifiers of the directly heated kind, the HT is available before the valves have warmed up to consume it, and during this transient period the HT voltage 'surges' to an excessive value. The mechanical form of electrolytic capacitors is either tubular, with a wire emerging at each end (see Plate ) or enclosed in a cylindrical can, secured to the apparatus by a single large nut or by a clip, and having all the connections at one end. Where several capacitors are enclosed in one case the latter construction is usual, and the capacitors often have a common negative pole which is internally connected to the can itself. Future developments At the time of writing there are two further types of capacitor of great promise which are not widely used, probably because at present they are too expensive. Polystyrene film capacitors—These have low loss (tan d = 0-0005) and phenomenally low leakage, about 30 times less than a mica capacitor and 100 times less than impregnated paper and foil. They are made from 100 pF to 4 fxF and should be ideal for coupling purposes in low-level stages. Tantalum electrolytic capacitors—These are even smaller for their size and working voltage than aluminium electrolytic components. One 50 ^i¥ 70 V unit is about the size of a stack of half-a-dozen halfpennies. In addition they have better tan 6, about 0-05, and extremely low leakage, about a hundred times better than conventional electrolytics. VARIABLE CAPACITORS Variable capacitors are of two types: (1) the truly variable type intended for frequent readjustment, employing air as dielectric, and famihar to anyone who has looked inside a wireless set as the 'tuning condenser'; (2) 'preset', screwdriver-adjusted types, otherwise 'trimmers', meant to be initially set up to a correct value and left there. VW^.
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