SUNDRIES 



respects. The use of silver plated contacts is now general and the main 

 difference between types lies in the insulating material used. This may be 

 fibre, bakehte, nylon or PTFE, of which the first is the cheapest and least 

 durable and the last the best. Bakelite or nylon are perfectly satisfactory 

 for the general run of electrobiological apparatus. Plate 25.2 shows nylon- 

 insulated valveholders for the octal, miniature noval, and miniature-heptal- 

 based valves which constitute the suggested short-list in the previous chapter. 



KNOBS 



The design of knob used for a particular control is by no means a subject 

 unworthy of thought; correct choice here can make all the difference to the 

 'feel' of an instrument. For example, knob No. 1 in Plate 25.3 is clearly for 

 use with a continuously variable control where fine movements are necessary. 

 The diameter is such that thumb and all four fingers may be arranged com- 

 fortably round it, and accurate operation secured thereby. Not so knob No. 2; 

 this is for use with a rotary switch, where accurate indexing is performed by 

 the switch itself. It can therefore be quite small, being grasped between 

 forefinger and thumb and 'flicked' by an action of the wrist. Knob No. 3 

 may be regarded as a compromise for persons who like to lay out the front 

 of an instrument with all the knobs the same, irrespective of function. 



TAGBOARDS AND TAGSTRIPS 



It is clear from the photographs of the components we have dealt with that 

 whilst the larger items are provided with some fixing device involving bolts 

 or nuts, the smaller rely on their own connections for fixing. At one time it 

 was usual to employ the 'direct' method of wiring, in which such components 

 were suspended by their own connections, but the appearance of the underside 

 of apparatus built in this manner leaves much to be desired. The best that 

 can be said for the method is that it ought to minimize stray capacitances; 

 the worst is better left unuttered. Direct wiring is necessary at very high 

 radio frequencies but is usually not justified below a megacycle. For the 

 bulk of electrobiological apparatus a much neater procedure is to arrange 

 small components in a row on one or more tagboards {Plate 25.4). Alter- 

 natively they may be soldered between two parallel tagstrips. Tagstrip 

 mounting is more satisfactory for components which reach high operating 

 temperatures, such as vitreous resistors, as the ventilation is better. The use 

 of tagboards and tagstrips in equipment makes for much easier construction 

 and subsequent servicing. 



CONNECTORS 



Complete electronic systems frequently comprise a number of discrete units 

 connected together by flexible cables. Connectors are devices which enable 

 these cables to be quickly and conveniently detached from the units in order 

 that they may be separated where necessary. Although this may seem very 

 obvious, it is easy to recall pieces of apparatus which cannot be moved either 

 without much preliminary coiling, or else attendant trains of dragging cable. 



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