238 INNERVATION. [CHAP. ix. 



perfectly clean surfaces is sufficient to excite a state of polarity 

 in each. 



In the development of the galvanic current in the battery, one 

 plate or metal may be regarded in the light of the generator of 

 force, the other as its propagator or conductor. The former has 

 therefore been called the positive, the latter the negative pole. The 

 absolute contact of the metallic plates themselves is not necessary. 

 It will suffice if they be connected by any material which is itself 

 capable of serving as a conductor. A piece of platinum wire, for 

 instance, extended between the two plates, although it actually 

 connects only a very small portion of the surface of each, will 

 answer the purpose. From such an arrangement it may be con- 

 cluded that, during the development of the galvanic current, the 

 conducting metal is in a state similar to that of the generating plate, 

 for the temperature of the conducting wire is raised considerably ; 

 and, when there is much energy of action, the wire is melted. 



The existence of a galvanic current is readily detected, even when 

 of feeble intensity, by certain phenomena, which are now familiar 

 to those who conduct such investigations. If the poles of a battery 

 be connected by conducting wires with a delicate galvanometer 

 (electro-dynamic multiplier), the needle is obviously deflected during 

 the passage of the current, and returns to its previous position 

 whenever the current is interrupted. By making and breaking the 

 connexion, in rapid succession, the needle moves to and fro with 

 corresponding rapidity and energy. So delicate is this test of gal- 

 vanic action, that it will detect even the very feeble current which 

 results from the heating of two dissimilar metals, or from the par- 

 tial heating of a coil of platinum wire. As this is the most delicate 

 test, so is it also the most constant, and it has the additional ad- 

 vantage of enabling the observer to judge of the direction of the 

 current, from the position which the needle assumes under the 

 electric influence. 



When a galvanic current is made to pass through certain liquids, 

 as dilute sulphuric acid, solution of iodide of potassium, of sulphate 

 of copper, &c., it induces such an amount of disturbance of the 

 attractions existing in them as to cause their decomposition, and 

 give rise to chemical actions of a similar kind to those which take 

 place in the generation qf the current (electrolysis) . This, there- 

 fore, becomes a test of the presence of galvanic action. The decom- 

 position of iodide of potassium will detect the existence of a current 

 developed by a single pair of plates, and iodine will be set free at 

 the positive pole. 



