364 ELECTRICAL MEASUREMENTS 



Paraffined Paper Condensers. 9 On account of the possibility 

 of large absorption effects and frequency errors, paraffined 

 paper condensers should not be employed as standards. No 

 general rule concerning their behavior can be formulated. When 

 used with alternating current, the capacity of a paraffined paper 

 condenser decreases with an increase of frequency and very 

 markedly if the phase angle be large. An increase of tempera- 

 ture usually causes an increase in the capacity; for an exception, 

 see Fig. 213. The phase angle is much larger than that of a good 

 mica condenser. It generally increases with a rise of tempera- 

 ture and more and more rapidly as the temperature becomes 

 higher. The phase angle is very susceptible to changes in fre- 

 quency. Usually an increase of frequency causes a decrease in 

 the angle. Fig. 213 shows the characteristics of a good paraffined 

 paper condenser. Fig. 214 applies to a rolled condenser such as is 

 frequently used in telephony, and an inspection of the curves 

 will show that this is a poor instrument and that in some respects 

 its behavior is the reverse of that of the better condenser. 



The internal resistance of a condenser, that is, the resistance 

 of the connections from the binding posts to the plates and of 

 the plates themselves, may cause an abnormal phase angle. This 

 is the case in that form of telephone condenser which is made by 

 rolling up long strips of tin foil together with the paper dielectric, 

 the electrical connections being made at the ends of the strips. 

 High internal resistance causes excessive heating and an increase 

 of phase angle with an increase of frequency. 



METHODS OF MEASUREMENT 



Determination of Capacity in Absolute Measure. Maxwell in 

 his " Treatise on Electricity and Magnetism"* gives a bridge 

 method for determining the electrostatic capacity of a condenser 

 in electromagnetic measure. This method has been employed 

 in many determinations of "v" and is probably the best yet 

 devised for determining pure capacities in absolute measure. 4 

 The connections are shown in Fig. 215. 



The condenser to be measured is at C; M, N, and P are the 

 resistances of the bridges arms, B is the battery resistance and 



*Art. 776, third edition. 



