WILLIAM SIEMENS, F.R.S. 53 



M and N arc substituted for m and w, the following equation is 

 established : 



z 



or by introducing for & its value from above, 





N 



The chief advantage of this arrangement consists in the un- 

 changed sensibility of the instrument, since the whole strength of 

 the insulation current acts upon the needle, which nevertheless is 

 brought back always to zero. 



In measuring the insulation resistance of short cables the 

 resistance of the galvanometer coils W and w may in practice 

 be neglected, and the following more simple formula may be 

 adopted : 



M V 



The value of k is independent of the sensibility of the needle, and 

 need only be determined once for all. 



The tests are thus reduced to a very simple and easy method. 



In order to calculate the insulation resistance of insulated wires 

 from the specific conductivity of the material used, and vice versa, 

 we employ the following formula : 



C. log. ? 



(III.) 



The derivation of this formula has been given by Mr. Werner 

 Siemens in " Poggendorff s Annalen " of 1857, vol. 102, and will be 

 found in Appendix 2 of this paper. 



The foregoing methods suffice to ascertain insulation and 

 copper resistances of cables of all lengths and forms ; they do not 

 comprise, however, the test necessary to determine their inductive 

 capacities. 



