Dr. Andrews on the Conducting Poxioer, S,-c. 1 77 



able electrical properties that have yet been discovered, it ap- 

 peared to be deserving of new investigation. 



To detect the passage of an electrical current, the test which 

 I employed in the following experiments was the solution of 

 the iodide of potassium, the extreme delicacy of which was 

 first, I believe, pointed out by Faraday*. A slip of bibulous 

 paper moistened with this solution was placed on a platina 

 plate, supported upon an insulating stand of glass. The ne- 

 gative pole of the battery was brought into contact with the 

 platina plate, while a wire of platina attached to the positive 

 pole rested on the moistened paper. The existence of the 

 current was inferred from the tlepo.sition of iodine beneath the 

 positive pole; and when no iodine was deposited I have de- 

 scribed the current as being interrupted. By this expression 

 it is only to be understood that if a current did pass it was 

 too feeble to produce any sensible decomposition of the solu- 

 tion of iodide of potassium. The extreme delicacy of this test, 

 and the precautions which must be employed, in using it, will 

 be evident from this simple experiment. If the positive pole 

 of a battery composed of a single pair of zinc and platina 

 plates be placed upon the paper of the decomposing appa- 

 ratus, and the platina plate touched by the moistened finger, 

 no effect will occur, provided the negative pole be insulated ; 

 but if that pole be brought into contact with the ground, a 

 deposition of iodine will immediately take place beneath the 

 positive pole. Here the feeble current produced by a single 

 pair of plates after traversing a long extent of imperfect con- 

 ductors, is still capable of being easily detected by the decom- 

 position of the solution of iodide of potassium. 



A battery of 20 pairs of plates charged with common pump, 

 water was carefully insulated, the poles were terminated by 

 platina wires which were introduced into the flame of a spirit 

 lamp, and a decomposing apparatus was interposed in the 

 course of the circuit. The passage of the current was joroved 

 by the deposition of iodine at the positive pole. 



This experiment was varied by placing the wires in different 

 positions in the flame, but the result was the same, even when 

 they were at the distance of an inch and a half from each other. 

 When very fine wires were employed and brought only into 

 contact with the flame, the effect was diminished, although it 

 was still distinct; but by substituting slips of platina foil for 

 the wires, and augmenting the surfaces of contact, the effect 

 was greatly increased. 



A battery consisting of a single pair of plates of platina and 

 amalgamated zinc charged with dilute sulphuric acid was now 



[• See Loiul. & Edinl). Phil. M;if^., vol. iii. p. 255. — Edit.] 

 77ihd Series. Vol. 9. No. 53. Sept. 1836. U 



