352 ELECTRICAL MEASUREMENTS 



When the pile has been completed, electrode 1 is firmly clamped 

 between the rings RiRi by means of the nuts M. Electrode 

 2 is clamped between the rings R 2 R 2 . The spaces between the 

 two electrodes are finally adjusted by raising or lowering electrode 

 2 by means of the adjusting screws Q, which are then locked. 

 The top of 2 is firmly held by tightening and then locking the 

 screws N which bear on the ring R z by means of the amber 

 cylinders B. 



The assembled condenser is about 30 cm. high and weighs 

 approximately 37J^ Ib. 



By making the air space 1 mm. instead of 2 mm. and employing 

 107 plates, condensers having a capacity of 0.03 microfarad 

 have been constructed. With this extremely small thickness 

 of the dielectric, trouble was experienced in insulating the two 

 sets of plates, for when voltage was applied fine particles of dust 

 from the air bridged the space between the plates, thus reducing 

 the insulation resistance. It is not possible to remove the 

 dust after the condenser is assembled but the insulation may 

 be improved by placing a drying material in the case of the 

 instrument. 



The breakdown voltage of the condenser with 2 mm. air space 

 is 3,000 volts, and with 1 mm. air space, 900 volts. All sharp 

 edges on the plates and internal fittings must be avoided, in 

 order to prevent brush discharges. 



To render the condenser independent of the surroundings, one 

 set of plates is connected to the case, the other set being con- 

 nected directly to the measuring apparatus. 



Variable capacities are necessary for general laboratory pur- 

 poses, but 'a difficulty presents itself when one attempts to put 

 a number of very small condensers in parallel by the ordinary 

 means, since the connections possess an unknown capacity which 

 may be enough to introduce serious errors. For this reason it 

 is necessary that the design of the small sections from which the 

 larger capacities are built up be such that this uncertainty is 

 eliminated. 6 



In the most refined work the temperature coefficient of an 

 air condenser, due to the change of dimensions and change in 

 the dielectric coefficient of the air, must be considered. It 

 may amount per degree to 2 or 3 parts in 100,000. 



