350 ELECTRICAL MEASUREMENTS 



This result must be corrected for the effects of the gaps between 

 the main cylinder and the guard cylinders and for the different 

 thicknesses of the dielectric on the two sides of the gaps due to 

 differences in the diameters of the cylinders. The deduction of 

 these corrections involves the use of the higher forms of analysis. 

 The numerical value of the net correction for the condenser just 

 mentioned, when the lower gap is 0.6 mm. and the upper gap 

 0.5 mm., is +0.182 electrostatic units. Thus, in the case of this 

 short condenser, the corrections with even these small air gaps 

 amount to over one-fourth of 1 per cent. The longer the central 

 section, the smaller is the percentage correction. 



For the most refined work the parallel plate condenser is 

 inferior to these just referred to, for comparatively large errors 

 may be introduced if the adjustments are not perfect. 



Secondary Air Condensers. Secondary air condensers are 

 useful in experimental work in those cases where the dielectric 

 losses must be reduced to zero; as in determining the phase 

 angles of mica condensers or in the investigation of the dielectric 

 losses occurring in insulating materials. As such condensers 

 must be calibrated, it is possible to use for each electrode a num- 

 ber of plates in parallel and to make the distance between the 

 electrodes only a few millimeters. By this means capacities 

 of a few hundredths of a microfarad may be obtained without 

 undue bulk. 



In any air condenser, the ohmic resistance between the termi- 

 nals and the condenser proper must be kept low in order that 

 there may be no appreciable internal I 2 R losses which would 

 cause an alternating current to lead the applied voltage by less 

 than 90. 



A secondary air condenser, 5 having a capacity of 0.01 

 microfarad, is shown (with the case removed) in Fig. 208, A. 

 The plates (of magnalium) are 20 cm. in diameter, 1 mm. thick, 

 and so spaced that the thickness of the dielectric is 2 mm.; 35 

 plates are used in one electrode and 36 in the other. In order 

 to insure permanence a very solid construction must be employed. 



The plates are supported as shown in Fig. 208, B. The bronze 

 ring, R 1} is firmly screwed to the base of the instrument; through 

 it pass four adjusting screws of fine pitch, Q, which support a 

 second ring, R 2 , by means of the little amber cylinders, B, which 



PROPERTY OF ELECTRICAL LAGORATORY, 



FACULTY OF APPLIED SCIENCE. 



