Mr. R. Appleyard on Dielectrics. 



157 



here m + n, and the " guard " wire is supposed to come 



between m and n 



Let &, q, and r denote the resistances of 



Fi* 2. 



Ill 



the battery, galvanometer, and the dielectric resistance of the 

 condenser, respectively ; let E be the E.M.F. of the testing 

 battery, and G the current through the galvanometer when 

 the "guard" wire is applied. It is clear that m shunts the 

 galvanometer, and n shunts the battery ; and we have to find 

 the error which this introduces into the results. From fig. 2, 

 by KirchhofFs laws, we have 



G = 



E 



(b +g + r) H (nrg + brm + brg + bmg + nbg) 



TYlTi, 



Let 



k= — (nrg + brm + brg + bmg + nbg) , 



(i) 



(2) 



then k may be regarded as equivalent to a resistance added 

 to the simple circuit b + g + r. 



To evaluate k, we will put m = n = r ; which means that 

 the whole leakage, m + n, is assumed to be only twice the 

 resistance of the dielectric of the condenser, and corresponds 

 to a worse lead than any I have employed throughout these 

 tests. In this case, (2) reduces to 



7 %bq 

 fc=g + b + —^ > 



* m 7 



which has now to be compared with (b+g + r) in (1). 



Taking the very lowest observed value of r, which was 

 5xl0 8 ohms, and putting # = 8000, and 6 = 200 ohms, we 

 have K i 



b + g + r = 50^000' 



so that k is quite negligible in equation (1), and the error 

 introduced by the " guard " wire is practically nil. 

 Phil. Mag. S3. 5. Vol. 42. No. 255. Aug. 1896. N 



