25 

 SUNDRIES 



SWITCHES 



Switches fall into two clearly defined types, those for controlling the flow of 

 signal currents and those for controlling the transfer of power. In the former 

 the emphasis is on effecting good contact, by using noble metals for the con- 

 tact faces — platinum, silver, gold — and by the employment of 'wipe'. A 

 wiping action in switches is one in which the contact faces are rubbed 

 together as the switch is operated; in this manner the contacts are in some 

 measure self cleaning. 



Power control switches employ a spring toggle action which ensures that 

 when the switch is opened the contacts are rapidly separated; in this way any 

 arc which may be struck is quickly extinguished. 



It is a mistake to try to use power switches for signal control, and vice 

 versa. The thermal-capacity of the contacts of signal switches is low, and if 

 these are used to break, for example, the primary circuit of a mains trans- 

 former the contacts are rapidly destroyed, even if one remembers to operate 

 the component smartly. Similarly the use of power type toggle switches for 

 signals — at least at low level — is also frequently disappointing. The contacts 

 are of base metal, e.g. brass, and there is often no wipe, and in consequence 

 the contacts are covered by an oxide film. Under the conditions of high 

 electric field which obtain when these switches are properly used this film 

 breaks down and current can flow. The feeble voltages constituting a signal 

 do not affect the necessary breakdown and the result is a high resistance 

 and noisy contact. 



Despite the precautions taken, the performance of signal switches can 

 eventually become unsatisfactory due to contamination of the contacts by 

 air-borne dirt. The elegant solution is to employ sealed switches, but these 

 are not yet common. The popular open variety may be cleaned by brushing 

 the contacts with a grease-dissolving fluid such as trichlorethylene (Scroggie 

 (Radio Laboratory Handbook) is against the use of commercial carbon 

 tetrachloride, he points out that it frequently contains acid and may cause 

 corrosion): a httle lubricant may then be applied. Some typical switches 

 are illustrated in Plate 25.1. No. 1 is a power-control toggle switch; No. 2 

 is a popular type of rotary signal switch, whilst No. 3 is a miniaturized and 

 sealed version; No. 4 is a 'key switch' of the signal genus but, like the power 

 toggle switch, capable of being operated — under conditions of experimenters' 

 duress — by the foot. 



VALVEHOLDERS 



The purposes of a valveholder are, fairly obviously, to retain the valve firmly 

 in place, to make good connection to the valve pins and yet preserve a high 

 resistance between them: modem valveholders are very reliable in all three 



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