for detecting bad Insulators on Telegraph Lines, 105. 



t' is in permanent connexion with the clamp c, to which is 

 fixed a small platinum knob m ; and both the clamps c and d are 

 permanently connected with each other. A well-insulated lead- 

 ing wire, II, which is to be hooked on to the bracket of the insu- 

 lator under test, is in contact with the moveable platinum knob 

 m' } which, however, is insulated from c' when pressed down, but 

 which in its position of rest (or when not pressed down short) 

 closes the circuit between c and c' at/. 



The whole arrangement is constructed light but strong, pro- 

 tected from rain, and can be carried along the line by one man 

 only. 



The tester proceeds as follows : — 



After having cleaned the insulator carefully, he removes tem- 

 porarily the line from the insulator and hooks the leading wire 

 No. I to the iron hood, and leading wire No. II to the bracket 

 of the insulator. He then turns the handle of the magneto- 

 electric machine with one hand, while one finger of the other is 

 resting on the knob m of clamp c. 



As soon as he touches with the other finger the knob m' of 

 clamp c ! , at the same time pressing it down, the metallic circuit 

 between c and m! is opened, and the positive and negative mag- 

 neto-electric currents have to pass from one finger to the other, 

 and consequently, if strong enough, will give the tester sensible 

 shocks, by which he is at once informed that the insulator under 

 test is defective, and much under the fixed standard of insulation. 



If the tester does not feel any current through his fingers (a 

 comparatively rough galvanoscope), he has only to repeat the 

 experiment by placing his tongue on the knob m while his hand 

 still presses the knob m f down. If no current is felt by the 

 tester through this most delicate galvanoscope (the tongue), he 

 can rest assured that the insulator is perfect for all practical 

 purposes. 



By opening and closing the circuit alternately at the knob m' 

 the tester has it in his power to allow at short intervals currents 

 to pass through his tongue, and consequently will be able to de- 

 tect the slightest induction-currents. 



The following experiments were made with insulators of known 

 resistance, to ascertain the highest limit by which the tongue is 

 still able to detect induction-currents. 



The currents in these experiments were produced by one of 

 Siemens's well-known dial instruments, the revolving bobbin of 

 which had a resistance =1577 S. units. 



The absolute resistance of each insulator was first carefully 

 measured in the ordinary manner without water in the porcelain 

 cups, and the insulator afterwards tested by the method above 

 described. 



