CAPACITORS 



the mica by reducing an oxide powder of silver at high temperature {Plate 

 21.3). All mica capacitors have extremely low loss (tan 6 = 0-0003) and the 

 silver mica types have the added advantage that the process lends itself to 

 an extremely close manufacturing tolerance, ±2 per cent. In this way com- 

 ponents whose actual values lie close to the nominal may be bought cheaply, 

 because further sorting and selection is not required. 



Mica capacitors are only made in low capacitance values (10-10,000 pF), 

 otherwise their cost would be prohibitive. This means that they cannot easily 

 be checked by examining the rate of self-discharge, since the leakage resistance 

 (about 10,000 MQ fi¥) of the component is hkely to be so high as to become 

 comparable with that of the measuring device. However, mica capacitors 

 are extremely reliable, and if they have not failed as a result of catastrophic 

 damage such as would show up on a simple ohmmeter test, they are likely 

 to be in good order. The temperature coefficient of foil and mica capacitors 

 is better than ±0-05 per cent per degree C; that of silver mica components 

 better than ±0'01 per cent per degree C. 



Some mica capacitors are colour-coded, using up to 6 dots. The system is 

 as follows: 



\ 2pd sig. fig. of capacitance 



British Mica capacitor colour code 



1st sig.fig.of capacitance 



Type 

 C 



Max ^.-'■'' ^ X — -H. 



working jolirance ^"'^'P"*'' 

 voltage 'o'^'^ance 



Low k ceramic capacitors* {Plate 21.4, left) 



These are made over approximately the same capacitance range as mica 

 capacitors (1-500 pF), and have a similar tan d. Further, Uke the silver mica 



* k'ls the permittivity, or specific inductive capacitance, of the dielectric. 



300 



