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BELL SYSTEM TECHNICAL JOURNAL 



treme, at least with respect to mechanical strength. This design under 

 test gives excellent performance for direct surface leakage, having 

 under various weather conditions from one to 25 per cent as much 

 leakage as the more reasonable design shown in Fig. 6. 



0.01 



0.000001 



Fig. 4— Illustrates large variation of d.-c. surface leakage in dry weather. 



Another, but somewhat less effective, way of increasing the length 

 of path is the use of one or more petticoats. This method has a me- 

 chanical advantage in keeping the overall insulator length small but 

 the effectiveness of the longer path is partly lost due to the necessary 

 increase in diameter. An example of this method is the design shown 

 in Fig. 1 which gives about half as much leakage as that of Fig. 2. 



