106 On the Detection of bad Insulators on Telegraph Lines. 



No. of insulator. 



Resistance in millims. 

 S. units. 



Strength of magneto-electric currents 

 (as indicated by the human body) 

 through the resistance of the insu- 

 lator under test. 



1. 

 2. 



3. 

 4. 

 5. 



6. 



7. 



8. 



9. 

 10. 

 11. 

 12. 

 13. 

 14. 



Oil 



013 



0145 



019 



075 



2-30 



570 



71 



8-2 



82-0 



1890 



615-0 



25200 



GO 



Strong shocks felt by fingers. 



do. do. 



do. do. 



do. do. 

 Slight shocks felt by fingers. 

 J No shocks felt by fingers, but good 

 \ shocks through tongue. 

 J No shocks felt by tongue, but a 

 X strong acid taste. 

 Distinct but slight acid taste 



do. do. 

 Nothing felt by tongue. 



do. do. 



do. do. 



do. do. 



do. do. 



Prom these experiments it follows that all insulators offering 

 a resistance up to about 1 millim. S. unit can be detected by 

 the fingers, and those above 1 millim. and under 8 millims. can 

 be unmistakably detected by the tongue. It appeared also that 

 tongues of different persons were equally sensitive, since several 

 persons (Europeans and natives) acknowledged the known acid 

 taste, even through the insulator No. 9, having 8*2 millims. S. 

 units resistance. 



The highest limit of the method could, of course,, be increased 

 by filling the revolving bobbin of the magneto-electric machine 

 with much finer wire and increasing the number of permanent 

 magnets; however this will be scarcely necessary, because it 

 seems to be a fact that if an insulator has more than about 8 

 millims., the resistance is generally so high as to be practically 

 infinite, and therefore a greater sensitiveness of the instrument 

 would only complicate the method. 



As it is intended that the tester himself should turn the handle 

 of the magneto-electric machine, he has it entirely in his power 

 to regulate the strength of the induction-currents by turning 

 faster or slower ; and as, besides this, he always begins the test- 

 ing by at first sending the currents through his fingers, no 

 severe shocks can occur to him in the subsequent operation. 



The method has also a safeguard in itself against carelessly 

 rejecting good insulators, because the tester will certainly be 

 careful in having the insulator properly cleaned before testing it, 

 in order to avoid severe shocks. 



There can also be scarcely any doubt that the tongue is the 



