RESISTORS 



General purpose wirewound resistors — These are used where the power to 

 be dissipated is beyond the capabiUties of a carbon component. They com- 

 prise a winding of resistance wire (usually nickel-chrome) round a ceramic 

 tube, the winding being secured and protected by a coating of a refractory 

 cement or vitreous enamel. By the use of these fire-resistant materials 

 extremely high working temperatures are possible, and resistors of this type 

 are characterized by remarkably high wattage ratings for their size {Plate 

 20.4). The temperature coefficient is about +0-015 per cent per degree C and 

 the voltage rating about 200 V per in. of resistor body length. The tolerance 

 is usually 5 per cent and the stability is comparable with cracked carbon. 

 High value general purpose wirewound resistors (100 kO) are often specified 

 for the anode loads in biological amplifiers. These contain a great deal of 

 very thin wire and in the author's experience are very unreliable and are often 

 faulty even before being used. The solution is to use a number of lower 

 values in series. 



Precision resistors — These are used in instrument-measuring practice where 

 a very low tolerance, +0-1 per cent, and good long-term stabihty, less than 

 1 per cent in value, are required. The winding is of nickel-chrome or nickel- 

 copper alloy, and, like the general purpose type, is often wound on a ceramic 

 tube. They are, however, very definitely not of fireproof construction, con- 

 taining as they do materials such as silk, rubber and paper ; they are essentially 

 'room-temperature' devices and are consequently large for their power ratings. 

 A temperature coefficient of +0-005 per cent per degree C may be regarded 

 as typical. A \-M0. precision resistor is shown in Plate 20.5. 



VARIABLE RESISTORS OR POTENTIOMETERS 



The only difference between a variable resistor and a potentiometer lies in 

 the provision of a terminal at only one end, or at both ends, of the 'track'. 

 In view of the trifling extra cost of providing one at either end this is always 

 done; in practice a variable resistor and a potentiometer are the same thing — 

 the only difference lies in the external connections. 



In this section we are not concerned with the high grade instrument type 

 potentiometer — these are dealt with under Mechanoelectric transducers in 

 Part III — but with the ordinary cheap component used in radio, television 

 and general electronic practice. A group is shown in Plate 20.6. No. 1 is a 

 standard type, and No. 2 a miniature. No. 3 is a 'two-gang' component, two 

 variable resistors controlled together by a single shaft. No. 4 is a 'tandem' 

 potentiometer, in which two units are separately adjustable by a pair of 

 concentric knobs. As with fixed resistors they are of two kinds, carbon or 

 wirewound. 



Carbon track variable resistors 



These are the cheaper variety. They are obtainable in values between 10 

 ohms and 5 megohms, but the 'preferred value' system is not used; the 

 sequence of values being 1, 2-5, 5, 10 . . . The tolerance is about +25 per cent. 

 They may be made in either of two ways : the carbon track may be moulded 

 out of a similar material to that used for making uninsulated fixed resistors, 

 and might in fact be regarded as such a resistor, but horse-shoe shaped 



294 



