SUNDRIES 



sizes. A dozen or so lengths of insulated and flexible wire, supplied with a 

 crocodile chp at each end, are extremely useful in electrophysiological work 

 for effecting rapid ad hoc connections. 



FUSES 



It is good practice to provide the power packs of electronic apparatus with 

 at least two fuses : 



(1) A fuse in the mains transformer primary circuit. If this is not included 

 and due to some fault condition the primary current becomes excessive, 

 either the local house fuse blows — which is liable to annoy other people — or 

 the local house fuse does not blow, in which case there is hkelihood of fire. 

 The coming of the fused mains plug removes the necessity for providing 

 primary fuses on the apparatus itself. 



(2) A fuse in the HT circuit, preferably in the connection between the 

 transformer HT secondary winding and earth. This protects the transformer, 

 rectifiers and smoothing choke against short circuits across the HT supply, 

 particularly from faulty electrolytic capacitors. 



Fuses are rated at 60 mA, 150 mA, 250 mA, 500 mA, 1 amp and 3 amp. 

 In choosing an appropriate fuse rating for a particular application the steady 

 current which normally flows may not be of much help because the fuse has 

 to be able to withstand the 'switching-on surge'. Thus in the case of the HT 

 fuse there is a surge to charge the smoothing capacitors which is greatest 

 with non-thermionic rectification, imermediate with directly heated rectifier 

 valves and small with indirectly heated types (that is, where the rectifier 

 comes into action gradually). There is a surge through the primary fuse partly 

 as reflected HT secondary surge current and partly as reflected LT secondary 

 surge current. The latter occurs because, before the valve heaters have warmed 

 up, their resistance is much lower than normally. In view of this, probably 

 the best thing to do is to use the lowest fuse rating which does not persistently 

 blow as soon as the equipment is switched on. Occasional fuse failures are 

 then to be expected, and no fault need be suspected unless the replacement 

 fails instantly also. 



Plate 25.6 shows a neat fuse fitting which fits flush with the front panel 

 of the apparatus. 



INDICATOR LAMPS 



These are useful for informing or reminding the user that circuits are alive. 

 Small filament lamps are obtainable for low-voltage circuits (Plate 25.7) and 

 diminutive neon bulbs may be employed for high tension voltages upwards 

 of 80 or so. These bulbs do not possess built-in stabilizing resistors, and a 

 small carbon component of a few hundred kilohms must be wired in series 

 with the holder; otherwise, disaster is immediate. 



SLOW-MOTION DRIVES 



Where fine control is required of a variable resistor or capacitor and the use 

 of a large knob proves inadequate, a slow-motion drive is indicated. A 

 popular variety — an epicychc gear — is shown in Plate 25.8. The velocity 

 ratio obtained is about 4:1. Where extreme precision is called for, slow 



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