CONNECTORS 



As with knobs, choice of appropriate connectors merits a certain amount 

 of thought. A selection of the important types is shown in Plate 25.5. 



No. 1 is the least readily dismantled, requiring as it does the use of a screw- 

 driver. It comprises a row of brass inserts, provided with grub screws, 

 supported in an insulating plastic moulding. The required number of ways, 

 up to a maximum of 12, can be cut off from stock with a pair of scissors. 

 No. 2 is the famiUar wander-plug and socket. Connection can be made 

 extremely rapidly but an inadvertent tug on the wire can easily pull out the 

 plug. A robuster device is the screw terminal and spade; the scheme is 

 suitable for high currents, of the order of amps, as the contact surface is 

 large. No. 4 is a spring terminal; this has the firm grip of the screw terminal, 

 but is quicker to use. No. 5 is a connector for a screened cable carrying 

 signals ; this particular one is for single core cables, but neat fittings may be 

 had for twin- or 3-core cables. Nos. 6 and 7 are multi-way connectors for 

 power supphes. In considering plugs and sockets for handhng supplies where 

 there are dangerous voltages, there is an important safety rule which deter- 

 mines whether the pins should be on the end of the cable and the sockets on 

 the piece of apparatus, or vice versa. This is merely that 'power must always 

 come out of the socket', not out of the plug; otherwise there is a risk of 

 shock. Thus, if a power unit feeds an amphfier via a multicore cable, the 

 power unit will have a fitting possessing sockets, whereas the amplifier fitting 

 will be provided with pins. The cable will of course have pins in the power- 

 pack end and sockets on the amplifier end. 



No. 8 is a mains lead connector. 8a goes on the end of the lead, and 8b 

 on the chassis of the apparatus. Notice that the safety rule is obeyed, but 

 that the pins are recessed for neatness. 



No. 9 is a 'jack' plug and socket. These may be used for similar purposes 

 to number 5, but a more important use of jacks is their 'break-in' facihty. 

 Auxiliary contacts on the socket are operated by the action of inserting the 

 plug. These may be wired up to achieve various things, of which perhaps 

 the most important is that, as the plug is pushed home, a hitherto intact 

 circuit is broken and the plug circuit is connected in, in series (Figure 25.1). 



Meter 



Meter 



Figure 25.1 



This property of jacks is extremely valuable in the metering of currents. To 

 check a number of currents by having a separate meter for each is expensive 

 and usually unnecessary. A better plan is to have one meter connected to a 

 jack plug and break-in sockets in each of the circuits requiring measurement. 

 One meter then serves for many. The same effect can, of course, be had by 

 switching, but if the circuits in question are widely dispersed about the labora- 

 tory the extra wiring required is considerable; the jack system is usually 

 preferable. 



No. 10 is the famihar crocodile clip, in the standard and the miniature 



323 



