358 ELECTRICAL MEASUREMENTS 



As the absorption increases very markedly with the increase 

 in temperature, while the insulation resistance decreases, con- 

 densers are preferably used at low temperatures, about 20. 



The measured capacities of condensers which have large 

 absorption are greatly affected by the time of discharge. 



Condensers on Alternating-current Circuits. If an air con- 

 denser, which is perfectly insulated and the resistance of whose 

 leads is zero, is subjected to an alternating potential difference, 

 the current flowing into the condenser will lead the potential 

 difference across its terminals by 90, there being no expenditure 

 of energy; if the dielectric is solid, energy is expended in the con- 

 denser as is shown by its rise of temperature under continuous 

 operation. If energy is expended, the current flowing into the 

 condenser must have an energy component, or, in other words, 

 the current and potential difference will no longer have a phase 

 difference of 90. The amount of departure from the 90 rela- 

 tion will be denoted by <: the power factor of the condenser is 

 then sin <. The angle <, called the phase angle, is dependent 

 on the quality of the condenser. For a first-class instrument with 

 mica as the dielectric, it may not be more than a few minutes of 

 arc, possibly 5, and may be much below this. If the condenser 

 is of poor quality with a paraffined paper dielectric, this angle 

 may in extreme cases be as much as 20. The power factor of 

 such inferior condensers is very sensitive to changes of frequency. 

 It must not be assumed that mica condensers are of necessity 

 characterized by very small phase angles, for such condensers 

 from well-known makers may occasionally show phase angles of 

 several degrees. Such abnormal values are found most frequently 

 in the small sections GKooo microfarad) and show the condenser 

 to be of poor quality. In a divided condenser, the different 

 sections may have very different phase angles. The measured 

 capacities of condensers which have large phase angles will be 

 found to be very dependent on the frequency. 



Various methods for the measurement of electrostatic capacity 

 by means of alternating currents are to be given and it is of prac- 

 tical importance to be able to apply them to condensers with im- 

 perfect dielectrics such as are met with in practice, that is, to find 

 the equivalent capacity of such condensers. As there is a dissi- 

 pation of energy in the condenser, its equivalent arrangement 



