278 grover: paraffined paper condensers 



was of paraffined paper, except in two cases; where paper impreg- 

 nated in beeswax was employed. Such condensers were selected, 

 on account of their relatively large and varied absorption effects, 

 as being better fitted for a theoretical study of absorption than 

 mica condensers. 



The measurements were made by means of alternating current 

 bridge methods in use at the Bureau of Standards, 1 using as indi- 

 cating instruments vibration galvanometers, tuned to the fre- 

 quencies of the currents used. This procedure has the advantage, 

 that the balance of the bridge depends on the fundamental com- 

 ponent only of the electromotive force wave employed, thus 

 rendering valid in the theoretical work, the assumption of a sinu- 

 soidal electromotive force. The condensers were compared, by 

 substitution, with standard mica condensers whose constants 

 were known from previous work, and capacity effects between 

 the different parts of the bridge and the earth were shown to 

 have no influence on the results thus obtained. 



The following points of practical importance were established 

 by the measurements: 



1. The temperature coefficients of the capacity of those con- 

 densers with the smaller values of the phase difference (5' to 

 20'), were found to be negative and nearly constant, 2 to 6 in 

 10,000, except at the higher temperatures. This effect is to 

 be ascribed to the expansion of the paraffin, the actual effect 

 of absorption being of relatively small importance, except in those 

 condensers of larger phase difference. In the latter case the 

 temperature coefficient of the capacity is positive and increases 

 rapidly with the temperature, reaching in some instances, values 

 of the order of one per cent per degree. 



2. In all cases, the capacity decreases with increasing fre- 

 quency, at first rapidly and then more slowly, the changes being 

 the larger, at all frequencies, in condensers with the larger phase 

 differences. 



3. While the phase difference of a mica condenser is usually 

 less than 3' or 4' and often is as small as 30" at 1000 cycles, the 



1 Bull. Bureau of Standards, 3: 371. 1907. 



