20 

 RESISTORS 



FIXED RESISTORS 



In assessing the suitability of a fixed resistor for a particular application in 

 electronics it is necessary to pay attention to a number of factors beside the 

 value in ohms. These are: 



(1) The material of which the resistor is made: the possibilities are carbon 

 composition, cracked carbon, or wirewound. 



(2) The power rating. This is the maximum electrical power which may 

 be dissipated in the component as heat in order that the consequent tempera- 

 ture rise shall not damage it. Strictly speaking the rating has not much 

 meaning unless the ambient temperature is also taken into account. Thus a 

 component sold as a 'half-watt resistor' should not be expected to dissipate 

 half a watt immediately above a 25 watt power pentode in miniatured equip- 

 ment for tropical use. 



(3) The voltage rating. This is the maximum potential difference which 

 may be allowed to occur across the resistor in order that the electrical field 

 within the component shall not damage it. 



Carbon composition resistors 



These are much the commonest resistors in electronic practice; despite 

 their defects they are cheap, extremely reliable if properly used and perfectly 

 satisfactory for the general run of electronic circuitry. The resistive element 

 is a stick composed of a compressed mixture of carbon black and a resin 

 binder. In the 'uninsulated' variety connection is made to the stick by tinning 

 the ends, wrapping round the connecting wires, and soldering the join to 

 make good contact; the wires leave the resistor perpendicularly {Plate 20.1). 

 In the 'insulated' kind the element is encased in a ceramic tube, and the 

 connecting wires enter axially at the ends {Plate 20.2). Modern resistors 

 tend to be of the insulated type : an advantage of the uninsulated kind is 

 the facility for adjusting the value of resistance upward by filing the com- 

 ponent thinner. 



Resistance values of composition resistors are marked by means of a 

 colour code. The meaning of the colours is as follows : 



Insulated resistors are marked by bands of colour which completely encircle 

 them, so that the value may be read from any aspect {Figure 20.1). The 

 first band, identifiable by being nearer one end than the fourth band is to 



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