1892.] Method of measuring High Electrical Resistances. 343 



an ordinary measuring tape, 1400 megohms per incb, &c., down to a 

 piece of wet tape, which gave 64,000 ohms per inch. 



These resistances are, to some extent, affected by the degree of 

 humidity of the air, but, when necessary, they can be rapidly standard- 

 ised with sufficient accuracy by determining one of the lowest by the 

 usual method]; or, as a check, when time allows, a highly insulated 

 condenser can be shunted by a length of silk, and the loss of charge, 

 in a given time, measured. 



Unsymmetrical Insulation of Apparatus. If, from any cause which 

 cannot be discovered or removed, the insulation resistance of the 

 apparatus is unsymmetrical, indicated by the needle taking up a false 

 zero when connection is made between the battery and quadrants, 

 symmetry can be always secured by connecting a length of silk, found 

 by trial, between the more highly insulated pole and earth. 



Limits of Accuracy. The accuracy attainable by this method, 

 depends on the sensibility of the electrometer and the potential 

 difference employed. 



With an ordinary suspension, however, it has been found that 

 with a battery giving about 350 volts the difference in reading be- 

 tween that with the centre of the battery earthed and that with the 

 earth connection made at 1 volt from the centre amounted to 1 2 scale 

 divisions. 



This sensibility should, therefore, be ample to secure an accuracy 

 within 1 per cent., which, for resistances of several thousand 

 megohms, is generally sufficient. 



Leakage Indicator. The same principle of balance may be usefully 

 adapted as a leakage indicator for electric supply circuits worked at 

 high pressure. 



For this purpose a special pattern of electrometer is requisite. 



The quadrants are connected, respectively, to the two mains con- 

 stituting a circuit, and th'e needle to earth. If the insulation of the 

 entire circuit is good, the potential from earth of the two mains will 

 probably be nearly equal, and the needle will remain at zero ; any 

 leakage taking place will disturb the balance to one side or the 

 other. 



By temporarily switching a small leak first on one side and then 

 on the other, and noting the effect, the absolute value of the insula- 

 tion may be approximately assessed. 



The arrangement is not applicable, however, to a concentric system 

 of mains with alternating currents, or in any case where the capacities 

 are large and seriously unequal. 



